Astronomy

Hydrogen Frost Line? Where, if anywhere, is it cold enough for Oort Cloud objects or rogue planets to have solid hydrogen on their surfaces?

Hydrogen Frost Line? Where, if anywhere, is it cold enough for Oort Cloud objects or rogue planets to have solid hydrogen on their surfaces?


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I like the idea of Oort Cloud objects all being crusted with a thin layer of hydrogen snow, though what little information I've managed to find on the topic seems to imply that that is unlikely, although I'm really not sure, since I can't tell what information is definitely applicable, and I haven't found a direct analysis of this issue. That being said, even if that isn't true, it does seem possible that hydrogen ice might exist on the surface of some objects.

I know that 2.725 Kelvin is the theoretical coldest temperature without refrigeration, since that is the temperature at which the black-body radiation from an object balances out the Cosmic (Microwave) Background radiation (CMB). This paper ( https://arxiv.org/pdf/2005.12932.pdf , pg. 4) about 'Oumuamua mentions the sublimation temperature of H2 (in a vacuum?) as 6K. From other sources I could only glean that H2 freezes at around 10K at pressures 1% that of Earth, which is down from 14.01K at one atmosphere. If the 6K number is correct, then one would expect it to be possible for solid hydrogen to be stable on the surfaces of objects without those objects being refrigerated, if maybe only in cosmic voids.

That being said, anywhere inside a galaxy (or between galaxies for that matter) is also inundated with starlight. I've read that interstellar gas clouds only get down to 10~20K, and I even have one source specifically saying, "No interstellar cloud in a galaxy can every [sic] get cold enough for Hydrogen or Helium to go through a phase change from gas to solid (or liquid). Light from stars in our and other galaxies, and even light from the Big Bang itself, is sufficient to keep the H and He warm enough to stay in the gas phase." ( https://www.vanderbilt.edu/AnS/physics/astrocourses/ast201/snowline.html ) Despite this, I've also read claims on several pop-sci and news sites that the object 'Oumuamua (MPC#: 1I/2017 U1) might have been made of frozen hydrogen, which is possible because "such temperatures exist in the coldest cores of giant molecular clouds… said Laughlin and lead author Daryl Seligman, who was a Yale grad student but is now at the University of Chicago." ( https://www.space.com/oumuamua-interstellar-visitor-hydrogen-ice.html ) The paper the people they're quoting wrote speaks of temperatures of 4K and 3K being detected along "filaments" in the cores of some "Giant Molecular Clouds" ( https://arxiv.org/pdf/2005.12932.pdf , pg. 6 and 7). It describes how 3K in such dense environments is cold enough for tiny grains of carbon and silicon compounds to act as seeds for large objects of mostly solid dihydrogen to form in tens of thousands of years, which can last for millions of years, at least, in galactic orbits. This discussion implies that normal interstellar space is too hot for such things and that it would even slowly evaporate such objects, but not so fast that they couldn't be spread out across much of the galaxy.

Here is a related thread on this This Site, though it deals mostly with the temperature of the interstellar medium itself, not of large objects in interstellar space, and those that did broach the topic only considered the CMB, ignoring starlight: How cold is interstellar space?

This all seems to indicate that the interstellar medium is usually too hot for mostly-hydrogen objects to form, except maybe deep inside nebulae, though I would like to know numbers if anyone knows where to find them. I'm also not sure if that means hydrogen couldn't accrete on the surfaces of large objects made from other substances at more normal interstellar temperatures (either from normal interstellar medium or from nebulae if the object were inside one), since I don't know whether size affects the thermodynamics. Based on the vanderbilt.edu source, I'm guessing that it is em radiation that's heating the gas, but it did cross my mind that there might also be other processes that wouldn't affect large solid bodies. There also is the possibility of chemical reactions or surface features (of certain geometry?) on solid objects that could trap hydrogen, although that might not really be hydrogen forming on the surface then, but rather hydrogen compounds.

More importantly, theoretically, a large enough object would would be able accrete a non-trivial amount of gaseous hydrogen, which would increase the pressure and thus raise the freezing point of Hydrogen. This would be much easier in interstellar space, since it would be much colder and other gravity sources would be much further away. I know that the interstellar medium is very thin, so I wouldn't be surprised if any accretion that did happen would be totally insignificant over the lifetime of the universe at densities like those in our Oort cloud, even if the theoretical equilibrium point was significant (though it should be be noted that even a layer just a few molecules thick could significantly change an object's appearance, including it's albedo, and might form on bodies with almost no craters or other geological activity). However, this could obviously be different inside of molecular clouds, since even things as big as stars can accrete in those, and such objects might easily leave these clouds after forming and might even be present in our Oort cloud or elsewhere in interstellar space.

Now, even if the interstellar medium is hot enough that any solid hydrogen will evaporate over time, if it's very cold, I would think that evaporation might be rather slow (as the 'Oumuamua paper suggests). Since the one place we're really sure planets and other solid objects form is near protostars, it does seem reasonable to discuss ejected planets and planetoids in this issue, and what state hydrogen on their surfaces might be like. Larger objects are more likely to form with more hydrogen and to lose it slower, but they also tend to generate more internal heat from things like gravitational compression and radioactive decay, even if we assume lone objects with no tidal heating. This means it's difficult to be sure what type of object would be most likely to have hydrogen ice on it's surface if it were kicked out into interstellar space.

I think that all the hydrogen in our gas giants (close to the sun as they are) is in some kind of fluid form, be that H2 or metallic, but even if there is some solid hydrogen deep inside, that is presumably under lots and lots of supercritical and/or liquid hydrogen, so I won't count that as the "surface". I know that gas giants create a lot of internal heat - enough that we would expect to be able to see them in infrared even if they weren't reflecting sunlight - and since those heating processes have obviously been going since the solar system formed, I find it difficult to believe that gas giants could have gotten all that cold by now, even if they were as old as the first stars, but they do have as much hydrogen atmosphere as you could want, so maybe small ones that formed with few radioisotopes could get cold enough that liquid and even solid hydrogen might exist on the surface without above any supercritical fluid. One problem I see with this is that most, if not all, gas giants probably have enough helium to reach supercritical pressures if it were separated and covered the surface, and such separation might happen if the temperature got cold enough for hydrogen to start condensing out of the air in the upper atmosphere. I also don't know if there are any electromagnetic refrigeration effects that might cool down gas giants (or, conversely, effects that would likely heat them up).

Perhaps a more likely candidate for solid hydrogen on its surface would be a smaller, mostly solid, world, that had only a small amount of hydrogen, either from accretion in formation, which would also imply helium, or perhaps mostly from some chemical reactions and/or volcanism. If such a planet (or smaller object?) had few enough radioisotopes-per-unit-surface-area, maybe it could get cold enough for everything to freeze except for a modest hydrogen-helium atmosphere, just thick enough to allow some of the hydrogen to freeze at interstellar temperatures (not that I'm really sure what those temperatures are in this context). I know some estimates say that there are likely to be many more rogue planets than planets in star systems, so it's possible that objects like this could be very common in the universe.

As I said near the beginning, even if all or some of this is impossible for objects in or near a galaxy, it is also worth considering whether or not they might exist in intergalactic space, including even in cosmic voids, if only for curiosity's sake. All such places ought to have at least some planet- and asteroid-sized objects, if only ones ejected from star systems ejected from galaxies.

One big issue with all of these objects from an observation standpoint is that, by their very nature, they would be very dim, since their black-body radiation would be so cold and they would too far from any stars to reflect much light. Another is that any that did happen to pass by the sun (or any other star) would likely loose much or all of their hydrogen before they were close enough for us to notice them, especially if they had only thin layers of hydrogen; although, as I have pointed out, 'Oumuamua may be an exception to this rule.

I'd love to hear people's thoughts about these ideas and their likelihoods, especially if those thoughts include more exact and/or better sourced information and/or math than the various hand-wavy musings I put here.


Solar System

The Solar System or solar system comprises the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons, three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.

The principal component of the Solar System is the Sun or Sol ( astronomical symbol ) a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally. The Sun's large mass gives it an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation including visible light. Jupiter and Saturn are the Sun's two largest orbiting bodies and account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed).

In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets and an outer belt of small icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury (), Venus (), Earth (), Mars (), Jupiter (), Saturn (), Uranus (), and Neptune (). All planets but two are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon) and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets other than Earth are named after gods and goddesses from Greco-Roman mythology.

From 1930 to 2006, Pluto (), the largest known Kuiper belt object, was considered the Solar System's ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term "planet". Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres ( ), the largest object in the asteroid belt, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.

For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all the newly discovered systems. Technically, however, it should strictly refer to Earth's system only, as the word " solar" is derived from the Sun's Latin name, Sol. Other such systems are usually referred to by the names of their parent star: "the Alpha Centauri system" or "the 51 Pegasi system".


Mind The Gap: Scientists Use Stellar Mass to Link Exoplanets to Planet-forming Disks (Planetary Science)

New survey reveals that the presence of gaps in planet-forming disks is more common to higher mass stars and to the development of large, gaseous exoplanets

Using data for more than 500 young stars observed with the Atacama Large Millimeter/Submillimeter Array (ALMA), scientists have uncovered a direct link between protoplanetary disk structures–the planet-forming disks that surround stars–and planet demographics. The survey proves that higher mass stars are more likely to be surrounded by disks with “gaps” in them and that these gaps directly correlate to the high occurrence of observed giant exoplanets around such stars. These results provide scientists with a window back through time, allowing them to predict what exoplanetary systems looked like through each stage of their formation.

“We found a strong correlation between gaps in protoplanetary disks and stellar mass, which can be linked to the presence of large, gaseous exoplanets,” said Nienke van der Marel, a Banting fellow in the Department of Physics and Astronomy at the University of Victoria in British Columbia, and the primary author on the research. “Higher mass stars have relatively more disks with gaps than lower mass stars, consistent with the already known correlations in exoplanets, where higher mass stars more often host gas-giant exoplanets. These correlations directly tell us that gaps in planet-forming disks are most likely caused by giant planets of Neptune mass and above.”

Gaps in protoplanetary disks have long been considered as overall evidence of planet formation. However, there has been some skepticism due to the observed orbital distance between exoplanets and their stars. “One of the primary reasons that scientists have been skeptical about the link between gaps and planets before is that exoplanets at wide orbits of tens of astronomical units are rare. However, exoplanets at smaller orbits, between one and ten astronomical units, are much more common,” said Gijs Mulders, assistant professor of astronomy at Universidad Adolfo Ibáñez in Santiago, Chile, and co-author on the research. “We believe that planets that clear the gaps will migrate inwards later on.”

The new study is the first to show that the number of gapped disks in these regions matches the number of giant exoplanets in a star system. “Previous studies indicated that there were many more gapped disks than detected giant exoplanets,” said Mulders. “Our study shows that there are enough exoplanets to explain the observed frequency of the gapped disks at different stellar masses.”

The correlation also applies to star systems with low-mass stars, where scientists are more likely to find massive rocky exoplanets, also known as Super-Earths. Van der Marel, who will become an assistant professor at Leiden University in the Netherlands beginning September 2021 said, “Lower mass stars have more rocky Super-Earths–between an Earth mass and a Neptune mass. Disks without gaps, which are more compact, lead to the formation of Super-Earths.”

This link between stellar mass and planetary demographics could help scientists identify which stars to target in the search for rocky planets throughout the Milky Way. “This new understanding of stellar mass dependencies will help to guide the search for small, rocky planets like Earth in the solar neighborhood,” said Mulders, who is also a part of the NASA-funded Alien Earths team. “We can use the stellar mass to connect the planet-forming disks around young stars to exoplanets around mature stars. When an exoplanet is detected, the planet-forming material is usually gone. So the stellar mass is a ‘tag’ that tells us what the planet-forming environment might have looked like for these exoplanets.”

And what it all comes down to is dust. “An important element of planet formation is the influence of dust evolution,” said van der Marel. “Without giant planets, dust will always drift inwards, creating the optimal conditions for the formation of smaller, rocky planets close to the star.”

The current research was conducted using data for more than 500 objects observed in prior studies using ALMA’s high-resolution Band 6 and Band 7 antennas. At present, ALMA is the only telescope that can image the distribution of millimeter-dust at high enough angular resolution to resolve the dust disks and reveal its substructure, or lack thereof. “Over the past five years, ALMA has produced many snapshot surveys of nearby star-forming regions resulting in hundreds of measurements of disk dust mass, size, and morphology,” said van der Marel. “The large number of observed disk properties has allowed us to make a statistical comparison of protoplanetary disks to the thousands of discovered exoplanets. This is the first time that a stellar mass dependency of gapped disks and compact disks has been successfully demonstrated using the ALMA telescope.”

“Our new findings link the beautiful gap structures in disks observed with ALMA directly to the properties of the thousands of exoplanets detected by the NASA Kepler mission and other exoplanet surveys,” said Mulders. “Exoplanets and their formation help us place the origins of the Earth and the Solar System in the context of what we see happening around other stars.”

Featured image: Protoplanetary disks are classified into three main categories: transition, ring, or extended. These false-color images from the Atacama Large Millimeter/submillimeter Array (ALMA) show these classifications in stark contrast. On left: the ring disk of RU Lup is characterized by narrow gaps thought to be carved by giant planets with masses ranging between a Neptune mass and a Jupiter mass. Middle: the transition disk of J1604.3-2130 is characterized by a large inner cavity thought to be carved by planets more massive than Jupiter, also known as Super-Jovian planets. On right: the compact disk of Sz104 is believed not to contain giant planets, as it lacks the telltale gaps and cavities associated with the presence of giant planets. © ALMA (ESO/NAOJ/NRAO), S. Dagnello (NRAO)

“A stellar mass dependence of structured disks: A possible link with exoplanet demographics,” N. van der Marel and G. Mulders, ApJ, DOI: 10.3847/1538-3881/ac0255, preview [https://arxiv.org/pdf/2104.06838.pdf]


Contents

An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4). [24] [ clarification needed ]

The concept of a circumstellar habitable zone was first introduced [25] in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The relevant quotations are given in . [26] The concept was later discussed in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars, coined the term "ecosphere" and referred to various "zones" in which life could emerge. [7] [27] In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. [28] Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. [29] [30] A major early contributor to habitable zone concept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems. [31]

The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million. [2] At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization. [32] The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase. [33] In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone. [29] Kasting was the first to present a detailed model for the habitable zone for exoplanets. [3] [34]

An update to habitable zone concept came in 2000, when astronomers Peter Ward and Donald Brownlee, introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez. [35] [36] The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers. [35]

Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry. [23] In 2013, further developments in habitable zone concepts were made with the proposal of a circum planetary habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away. [37]

It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment. [38] [39] However, surface conditions are dependent on a host of different individual properties of that planet. [38] [39] This misunderstanding is reflected in excited reports of 'habitable planets'. [40] [41] [42] Since it is completely unknown whether conditions on these distant CHZ worlds could host life, different terminology is needed. [39] [41] [43] [44]

Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface. [46]

Solar System estimates Edit

Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, [47] [48] [49] [50] though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapour. [51] The entire orbits of the Moon, [52] Mars, [53] and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. [54] At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year. [54] Despite indirect evidence in the form of seasonal flows on warm Martian slopes, [55] [56] [57] [58] no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres [59] is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, [60] Mars, [61] [62] [63] Vesta [64] and Ceres, [65] [66] suggesting a more common phenomena than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.

According to extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources.

For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU Venus, 0.72 AU Earth, 1.00 AU Mars, 1.52 AU Vesta, 2.36 AU Ceres, 2.77 AU Jupiter, 5.20 AU Saturn, 9.58 AU.

Estimates of the circumstellar habitable zone boundaries of the Solar System
Inner edge (AU) Outer edge (AU) Year Notes
0.725 1.24 1964, Dole [2] Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
1.005–1.008 1969, Budyko [67] Based on studies of ice albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969 [68] and North 1975. [69]
0.92-0.96 1970, Rasool and De Bergh [70] Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
0.958 1.004 1979, Hart et al. [71] Based on computer modelling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
3.0 1992, Fogg [45] Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
0.95 1.37 1993, Kasting et al. [29] Founded the most common working definition of the habitable zone used today. Assumes that CO2 and H2O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate–silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
2.0 2010, Spiegel et al. [72] Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
0.75 2011, Abe et al. [73] Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
10 2011, Pierrehumbert and Gaidos [48] Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in the Solar System.
0.77–0.87 1.02–1.18 2013, Vladilo et al. [74] Inner edge of circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures determined minimum atmospheric pressure required to be 15 mbar.
0.99 1.70 2013, Kopparapu et al. [4] [75] Revised estimates of the Kasting et al. (1993) formulation using updated moist greenhouse and water loss algorithms. According to this measure Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapour. Once the stratosphere becomes wet, water vapour photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a CO
2 dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in CO
2 will not create enough warming to prevent CO
2 catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.70 AU. This definition does not take into account possible radiative warming by CO
2 clouds.
0.38 2013, Zsom et al.
[47]
Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.
0.95 2013, Leconte et al. [76] Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System.
0.95 2.4 2017, Ramirez and Kaltenegger
[49]
An expansion of the classical carbon dioxide-water vapor habitable zone [29] assuming a volcanic hydrogen atmospheric concentration of 50%.
0.93–0.91 2019, Gomez-Leal et al.
[77]
Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of the climate sensitivity in both cases.
0.99 1.004 Tightest bounded estimate from above
0.38 10 Most relaxed estimate from above

Extrasolar extrapolation Edit

Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun, [4] a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at 0.25 >> , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the CHZ concept is more complex.

Spectral types and star-system characteristics Edit

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. [78] If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU. [79] [80]

With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions. [81] Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones. [82] Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment to life. [83]

Others maintain that circumstellar habitable zones are more common, and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking. [84] Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration. [85] At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs. [83] Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales. [86]

Stellar evolution Edit

Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years, [87] would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve. [88] [89] Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out our Sun, for example, was 75% as bright in the Archaean as it is now, [90] and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase. [91] In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region. [91]

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes [92] and huge starspots which can cover 20% of the star's surface area, [93] have the potential to strip an otherwise habitable planet of its atmosphere and water. [94] As with more massive stars, though, stellar evolution changes their nature and energy flux, [95] so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life. [94] [96]

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. [97] For example, the Sun is expected to engulf the previously-habitable Earth as a red giant. [98] [99] However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU. [100] At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. [101] Given that this new equilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. [100] However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. [102] Moreover, as Ramirez and Kaltenegger (2016) [99] showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant. [99] Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage. [99]

Desert planets Edit

A planet's atmospheric conditions influence its ability to retain heat, so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out. [103] [104]

Other considerations Edit

A planet cannot have a hydrosphere—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis. [105] [106] For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep [107] such as GJ 1214 b [108] [109] or Kepler-22b may be. [110]

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing. [111] Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape. [112] In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure. [74] Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained [74] because even a small shift in pressure or temperature could render water unable to form as a liquid. [113]

Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth), [29] a study [49] led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos [48] had eliminated the CO2-H2O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens-of-millions of years.

In the case of planets orbiting in the CHZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between the two sides. [84]

Planetary-mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. [37] More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io, [37] but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet. [114] Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability. [37]

A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the CHZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation (cryptobiosis) and/or hyperthermostability. Tardigrades, for example, can survive in a dehydrated state temperatures between 0.150 K (−273 °C) [115] and 424 K (151 °C). [116] Life on a planetary object orbiting outside CHZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm. [117]

Among exoplanets, a review in 2015 came to the conclusion that Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. [118] These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e is 11.9 light-years away. It is in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F). [119]

Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put ηe, the fraction of stars with planets in the CHZ, at 0.48, [4] meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. [120] However, this is merely a statistical prediction only a small fraction of these possible planets have yet been discovered. [121]

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. [122] NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their CHZs. [123] [124]

Early findings Edit

The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. However these early detections were all gas giant sized, and many in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water. [125] One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. [126] 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the CHZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round. [127]

Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons. [128] Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone [129] and has a low orbital eccentricity, comparable to that of Mars in the Solar System. [130] Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, [131] though it is unclear whether such satellites could form in the first place. [132]

HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth. [133] The following year, 55 Cancri f was discovered within the CHZ of its host star 55 Cancri A. [134] [135] Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces. [136]

Though, in theory, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.

Habitable super-Earths Edit

The 2007 discovery of Gliese 581 c, the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. [137] Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014, [138] and astronomers are divided about its existence.

Discovered in August 2011, HD 85512 b was initially speculated to be habitable, [139] but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone. [121]

Kepler-22 b, discovered in December 2011 by the Kepler space probe, [140] is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. [141] Gliese 667 Cc, discovered in 2011 but announced in 2012, [142] is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planet known.

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163 [143] is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C. [144] [145] [146] HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307. [147] In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away. [148] Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone [149] however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. [150] It is now considered as uninhabitable.

Near Earth-sized planets and Solar analogs Edit

Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses likewise, sub-Earths range up to the size of Venus (

0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5 R the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. [151] A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass such properties are not observable using current technology.

A solar analog (or "solar twin") is a star that resembles the Sun. To date, no solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. [152] Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation. [153] [154] [155]

Using data collected by NASA's Kepler Space observatory and the W. M. Keck Observatory, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone. [156]

On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to our Sun, in the CHZ and expected to offer habitable conditions. [157] [158] [159] [160] The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively. [159] [160] [161]

With a radius estimated at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method [162] [163] [164] though its mass remains unknown and its parent star is not a Solar analog.

Kapteyn b, discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 earth radii was found orbiting the habitable zone of the red subdwarf Kapteyn's Star, 12.8 light-years away. [165]

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky the third, Kepler-440b, is a super-Earth. [166] However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth. [167]

Kepler-452b, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452. [168] [169]

The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016. [170] The discovery is considered significant because it dramatically increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.

Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf K2-72 around 227 light year from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation. [171]

Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs. [172] The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.

Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away. [173]

At 11 light-years away, a second closest planet, Ross 128 b, was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 Earth's mass is it roughly Earth-sized and likely rocky in composition. [174]

Discovered in March 2018, K2-155d is about 1.64 time the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away. [175] [176] [177]

One of the earliest discoveries by the Transiting Exoplanet Survey Satellite (TESS) announced July 31, 2019 is a Super Earth planet GJ 357 d orbiting the outer edge of a red dwarf 31 light years away. [178]

K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapour found in its atmosphere this was announced on September 17, 2019.

In September 2020, astronomers identified 24 superhabitable planet (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth. [179]

Notable exoplanets – Kepler Space Telescope
Confirmed small exoplanets in habitable zones.
(Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b)
(Kepler Space Telescope January 6, 2015). [166]

Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans. [180]

Outside the CHZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water. [16] [17] Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice. [19]

With some theorising that life on Earth may have actually originated in stable, subsurface habitats, [181] [182] it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'. [183] Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface. [184]

Another possibility is that outside the CHZ organisms may use alternative biochemistries that do not require water at all. Astrobiologist Christopher McKay, has suggested that methane ( CH
4 ) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on 1,610,000,000 km (1.0 × 10 9 mi 11 AU) from the star. [23] This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife. [23] In addition, testing of a number of organisms has found some are capable of surviving in extra-CHZ conditions. [185]

The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the CHZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a CHZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics) [35] and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation). [186] But others, such as Ian Stewart and Jack Cohen in their 2002 book Evolving the Alien argue that complex intelligent life may arise outside the CHZ. [187] Intelligent life outside the CHZ may have evolved in subsurface environments, from alternative biochemistries [187] or even from nuclear reactions. [188]

On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvental ecosystems, supporting large complex life forms such as Riftia pachyptila. [189] Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone. [190] Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. [191] In addition, the lichens Rhizocarpon geographicum and Xanthoria elegans have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. [192] [193] [194] The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions. [194]

Species, including humans, known to possess animal cognition require large amounts of energy, [195] and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earth analogs in the CHZ are most likely to provide the closest natural habitat this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits or space habitat analogues on the surface may be eliminated, and complex Earth life can thrive. [2]

Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe. [196] The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter ne , which is the average number of planetary-mass objects orbiting within the CHZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe. [35] A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence [197] [198] that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterhole searches [199] [200] and the Allen Telescope Array now extends Project Phoenix to such candidates. [201]

Because the CHZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and the 2003 Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the CHZ. [202] [203] [204] [205] The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its CHZ. [134] A Message from Earth in 2008, [206] and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.


International Space Station life support

Oxygen generators on board the International Space Station produce oxygen from water using electrolysis the hydrogen produced was previously discarded into space. As astronauts consume oxygen, carbon dioxide is produced, which must then be removed from the air and discarded as well. This approach required copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long-duration missions beyond low Earth orbit.

NASA is using the Sabatier reaction to recover water from exhaled carbon dioxide and the hydrogen previously discarded from electrolysis on the International Space Station and possibly for future missions. The other resulting chemical, methane, is released into space. As half of the input hydrogen becomes wasted as methane, additional hydrogen is supplied from Earth to make up the difference. However, this creates a nearly-closed cycle between water, oxygen, and carbon dioxide which only requires a relatively modest amount of imported hydrogen to maintain.

Ignoring other results of respiration, this cycle looks like:

The loop could be further closed if the waste methane was separated into its component parts by pyrolysis:

The released hydrogen would then be recycled back into the Sabatier reactor, leaving an easily removed deposit of pyrolytic graphite. The reactor would be little more than a steel pipe, and could be periodically serviced by an astronaut where the deposit is chiselled out.

Alternatively, the loop could be partially closed (75% of H2 from CH4 recovered) by incomplete pyrolysis of the waste methane while keeping the carbon locked up in gaseous form:

The Bosch reaction is also being investigated by NASA for this purpose and is:

CO2 + 2H2 → C + 2H2O

The Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste. However, difficulties maintaining its temperature of up to 600°C and properly handling carbon deposits mean significantly more research will be required before a Bosch reactor could become a reality. One problem is that the production of elemental carbon tends to foul the catalyst's surface, which is detrimental to the reaction's efficiency.


Hydrogen Frost Line? Where, if anywhere, is it cold enough for Oort Cloud objects or rogue planets to have solid hydrogen on their surfaces? - Astronomy

The following information is offered for your REVIEW:: It is not presented as "truth" etched in stone. please use discernment.

If you would like to add comments/information to this feature please click HERE

"For more than 10 years plasma physicists have had an electrical model of galaxies. It works with real-world physics. The model is able to successfully account for the observed shapes and dynamics of galaxies without recourse to invisible dark matter and central black holes. It explains simply the powerful electric jets seen issuing along the spin axis from the cores of active galaxies. Recent results from mapping the magnetic field of a spiral galaxy confirm the electric model.

"On the other hand, cosmologists cannot explain why spiral shapes are so common and they have only ad-hoc explanations for galactic magnetic fields. More recently, inter-galactic magnetic fields have been discovered which is the final straw to break the camel's back. Incredible gravitational models involving invisible 'black holes' have had to be invented in a desperate attempt to explain how the attractive force of gravity can result in matter being ejected in a narrow jet at relativistic speeds.

"Why do we accept such science fiction as fact when an Electric Universe predicts spiral shapes, magnetic fields and jets? The cosmic magnetic fields simply delineate the electric currents that create, move and light the galaxies."

"Plasma physicists argue that stars are formed by an electromagnetic 'pinch' effect on widely dispersed gas and dust. The 'pinch' is created by the magnetic force between parallel current filaments that are part of the huge electric currents flowing inside a galaxy. It is far more effective than gravity in concentrating matter and, unlike gravity, it can remove excess angular momentum that tends to prevent collapse. Stars will form like beads on a wire until gravity takes over. The late Ralph Juergens, an engineer from Flagstaff, Arizona, in the 1970's took the next mental leap to suggest that the electrical input doesn't stop there and that stars are not thermonuclear engines! This is obvious when the Sun is looked at from an electrical discharge perspective. The galactic currents that create the stars persist to power them. Stars behave as electrodes in a galactic glow discharge. Bright stars like our Sun are great concentrated balls of lightning! The matter inside stars becomes positively charged as electrons drift toward the surface. The resulting internal electrostatic forces prevent stars from collapsing gravitationally and occasionally cause them to 'give birth' by electrical fissioning to form companion stars and gas giant planets. Sudden brightening, or a nova outburst marks such an event. That elucidates why stars commonly have partners and why most of the giant planets so far detected closely orbit their parent star. Stellar evolution theory and the age of stars is an elaborate fiction. The appearance of a star is determined largely by its electrical environment and can change suddenly. Plasma physicists and electrical engineers are best able to recognize plasma discharge phenomena. Stellar physics is in the wrong hands."

"Earth-like planets and moons are similarly 'born' by electrical expulsion of part of the positively charged cores of dwarf stars and gas giants. That explains the dichotomy between the dense rocky planets and moons and the gaseous giant planets. In the Electric Universe model, gravity itself is simply an electrostatic dipolar force. So planetary orbits are stabilized against gravitational chaos by exchange of electric charge through their plasma tails (Venus is still doing so strongly, judging by its 'cometary' magnetotail, and it has the most circular orbit of any planet) and consequent modification of the gravity of each body. Planets will quickly assume orbits that ensure the least electrical interaction. Impacts between large bodies are avoided and capture rendered more probable by exchange of electric charge between them. Capture of our Moon becomes the only option, it cannot have been created from the Earth. Evidence of past planetary instabilities is written large on the surfaces of all solid bodies in the solar system. That evidence is in the form of electric arc cratering."

(more on planet origins here) http://www.holoscience.com/news.php?article=rbkq9dj2
(and here) http://www.holoscience.com/news.php?article=pca22stj
(more on the Electric Universe here) http://www.holoscience.com/index.php
(and here) http://www.thunderbolts.info/default.htm

Okay, so you may well ask, "So what?" Well, here's Wal Thornhill's response to that:

"The consequences and possibilities in an Electric Universe are far-reaching. First we must acknowledge our profound ignorance! We know nothing of the origin of the universe. There was no Big Bang. The visible universe is static and much smaller than we thought. We have no idea of the age or extent of the universe. We don't know the ultimate source of the electrical energy or matter that forms the universe. Galaxies are shaped by electrical forces and form plasma focuses at their centers, which periodically eject quasars and jets of electrons. Quasars evolve into companion galaxies. Galaxies form families with identifiable 'parents' and 'children'. Stars are electrical 'transformers' not thermonuclear devices. There are no neutron stars or Black Holes. We don't know the age of stars because the thermonuclear evolution theory does not apply to them. Supernovae are totally inadequate as a source of heavy elements. We do not know the age of the Earth because radioactive clocks can be upset by powerful electric discharges.

"The powerful electric discharges that form a stellar photosphere create the heavy elements that appear in their spectra. Stars 'give birth' electrically to companion stars and gas giant planets. Life is most likely to form inside the radiant plasma envelope of a brown dwarf star! Our Sun has gained new planets, including the Earth. That accounts for the 'fruit-salad' of their characteristics. It is not the most hospitable place for life since small changes in the distant Sun could freeze or sterilize the Earth. Planetary surfaces and atmospheres are deposited during their birth from a larger body and during electrical encounters with other planets. Planetary surfaces bear the electrical scars of such cosmic events. The speed of light is not a barrier. Real-time communication over galactic distances may be possible. Therefore time is universal and time travel is impossible. Anti-gravity is possible. Space has no extra dimensions in which to warp or where parallel universes may exist. There is no "zero-point" vacuum energy. The invisible energy source in space is electrical. Clean nuclear power is available from resonant catalytic nuclear systems. Higher energy is available from resonant catalytic chemical systems than in the usual chemical reactions. Biological enzymes are capable of utilizing resonant nuclear catalysis to transmute elements. Biological systems show evidence of communicating via resonant chemical systems, which may lend a physical explanation to the work of Rupert Sheldrake. DNA does not hold the key to life but is more like a blueprint for a set of components and tools in a factory. We may never be able to read the human genome and tell whether it represents a creature with two legs or six because the information that controls the assembly line is external to the DNA. There is more to life than chemistry.

"We are not hopelessly isolated in time and space on a tiny rock, orbiting an insignificant star in an insignificant galaxy. We are hopefully connected with the power and intelligence of the universe. [emphasis added]

"The future in an Electric Universe looks very exciting indeed!"

NOTE: the above comments transferred from the radiOrbit forum by BlueSojourn ::

UPDATE 09-12-06

MILKY WAY IN THE LABORATORY? A plasma with a spiral-shaped pattern of particle density, similar to that of the Milky Way galaxy, has been created stably in the laboratory, supporting the possibility that fluid dynamics effects rather than gravitational ones may be responsible for our home galaxy's structure. Injecting a hot argon plasma (rotating at supersonic speeds) into a cold, stationary argon gas, researchers in Japan (Takashi Ikehata,Ibariki University, [email protected]) observed a spiral-armed structure (with low-density halos of charged particles) that persisted for as long as they kept rotating the plasma. The vortices that typically appear in such hot plasmas became spirals because of the outward "centrifugal" forces introduced by the rotation. Curiously, the spiral structure was not observed to form in the absence of the stationary gas, suggesting that the fluid dynamics interactions between the gas and plasma are central to the spiral formation process. This experiment intensifies the fascinating (and still undecided) question of whether similar interactions occur between hot, bright stars (corresponding to the plasma) and gas clouds (analogous to the stationary gas) to form spiral galaxies.

The theories concerning the origin and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. Over the centuries, many theories have been advanced as to its creation, but it was not until the eighteenth century that the beginnings of the modern theory took shape. With the dawn of the space age, the images and structures of the other worlds in our solar system refined our understanding, while advances in nuclear physics gave us our first glimpses of the processes which underpinned stars, and led ultimately to our first theories of their creation and ultimate destruction.

The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace. The nebular theory holds that the Solar System was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2𔃁 times the mass of the Sun. Over time, a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.

From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. Farther out still, beyond the frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds.

The gas giants were massive enough to retain a “primary atmosphere” of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own "secondary atmospheres" via volcanism, comet impacts, and, in Earth's case, the evolution of life.

After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.

Problems with the solar nebula model

One problem with this hypothesis is that of angular momentum. With the vast majority of the system's mass accumulating at the center of the rotating cloud, the hypothesis predicts that the vast majority of the system's angular momentum should accumulate there as well. However, the Sun's rotation is far slower than expected, and the planets, despite accounting for less than 1 percent of the system's mass, thus account for more than 90 percent of its angular momentum. One resolution of this problem is that dust grains in the original disc created drag which slowed down the rotation in the center.

Planets in the "wrong place" are a problem for the solar nebula model. Uranus and Neptune exist in a region where their formation is highly implausible due to the reduced denisty of the solar nebula and the longer orbital times in their region. Furthermore, the hot Jupiters now observed around other stars cannot have formed in their current positions if they formed from a "solar nebula" too. These issues are dealt with by assuming that interactions with the nebula itself and leftover planetesimals can result in planetary migrations.

The detailed features of the planets are another problem. The solar nebula hypothesis predicts that all planets will form exacltly in the ecliptic plane. Instead, the orbits of the classical planets have various (but admitedly small) inclinations with respect to the ecliptic. Furthermore, for the gas giants it is predicted that their rotations and moon systems will also not be inclined with respect to the ecliptic plane. However most gas giants have substantial axial tilts with respect to the ecliptic, with Uranus having a 98° tilt! The Moon being relatively large with respect to the Earth and other moons which are in irregular orbits with respect to their planet is yet another issue. It is now believed these observations are explained by events which happened after the initial formation of the solar system.

The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek (Galaxias), sometimes referred to simply as "the Galaxy"), is a barred spiral galaxy which forms part of the Local Group. Although the Milky Way is but one of billions of galaxies in the universe, the Galaxy has special significance to humanity as it is the home of our solar system. Democritus (450 BC� BC) was the first known person to claim that the Milky Way consists of distant stars.

There are numerous legends in many traditions around the world regarding the creation of the Milky Way. In particular, there are two similar ancient Greek stories, that explain the etymology of the name Galaxias and its association with milk. One legend describes the Milky Way as a smear of milk, created when the baby Heracles suckled from the goddess Hera. Zeus was particularly fond of this illegitimate son of his with a mortal woman, Alcmene, and devised to have the baby suckle on Hera's milk when she was asleep, an act which would endow the baby with godlike qualities. When Hera woke up and realized that she was breastfeeding an unknown infant, she pushed him away and the spurting milk became the Milky Way.

Another story tells that the milk came from the goddess Rhea, the wife of Cronus, and the suckling infant was Zeus himself. Cronus swallowed his children to ensure his position as head of the Pantheon and sky god, and so Rhea conceived a plan to save her newborn son Zeus: She wrapped a stone in infant's clothes and gave it to Cronus to swallow. Cronus asked her to nurse the child once more before he swallowed it, and the milk that spurted when she pressed her nipple against the rock eventually became the Milky Way.

Older mythology associates the constellation with a herd of dairy cows/cattle, whose milk gives the blue glow, and where each cow is a star. As such, it is intimately associated with legends concerning the constellation of Gemini, with which it is in contact. Firstly, with Gemini, it may form the origin of the myth of Castor and Polydeuces, concerning cattle raiding. Secondly, again with Gemini, but also with other features of the Zodiac sign of Gemini (i.e. Canis Major, Orion, Auriga, and the deserted area now regarded as Camelopardalis), it may form the origin of the myth of the Cattle of Geryon, one of The Twelve Labours of Herakles.

The term "milky" originates from the hazy band of white light appearing across the celestial sphere visible from Earth, which comprises stars and other material lying within the galactic plane. The galaxy appears brightest in the direction of Sagittarius, towards the galactic center.

Relative to the celestial equator, the Milky Way passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the solar system lies close to the galactic plane.

The main disk of the Milky Way Galaxy is about 80,000 to 100,000 light years in diameter, about 250-300 thousand light years in circumference, and outside the Galactic core, about 1,000 light years in thickness. It is composed of 200 to 400 billion stars. As a guide to the relative physical scale of the Milky Way, if the galaxy were reduced to 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 in) in width. The Galactic Halo extends out to 250,000 to 400,000 light years in diameter. As detailed in the Structure section below, new discoveries indicate that the disk extends much farther than previously thought.

The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars (more recent estimates have given numbers around 400 billion) and their planets, and thousands of clusters and nebulae, including at least almost all objects of Messier's catalog which are not galaxies on their own (one might consider two globular clusters as possible exceptions, as probably they are just being, or have recently been, incorporated or imported into our Galaxy from dwarf galaxies which are currently in close encounters with the Milky Way: M54 from SagDEG, and possibly M79 from the Canis Major Dwarf). All the objects in the Milky Way Galaxy orbit their common center of mass, called the Galactic Center.

As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Radio astronomial investigations of the distribution of hydrogen clouds have revealed that the Milky Way is a spiral galaxy of Hubble type Sb or Sc. Therefore, our galaxy has both a pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component. Decade-long observations have brought up more and more evidence that the Milky Way may also have a bar structure (so that it would be type SB), so that it may look like M61 or M83, and is perhaps best classified as SABbc. Recent investigations have brought up support for the assumption that the Milky Way may even have a pronounced central bar like barred spiral galaxies M58, M91, M95, or M109, and thus be of Hubble type SBb or SBc.

The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The two closest neighbors, both already mentioned, have only recently been discovered: The nearest of all, discovered in 2003, is an already almost disrupted dwarf galaxy, the Canis Major Dwarf, the nucleus of which is about 25,000 light-years away from us and about 45,000 light-years from the Galactic Center. Second comes SagDEG at about 88,000 light years from us and some 50,000 light years from the Galactic Center. These two dwarfs are currently in close encounters with our Galaxy and in sections of their orbits situated well within the volume ocupied by our Milky Way. They are followed in distance by the more conspicuous Large and Small Magellanic Cloud, at 179,000 and 210,000 light years, respectively.

The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. On the other hand, the bulge component consists of old stars and contains the globular star clusters our galaxy has probably about 200 globulars, of which we know about 150. These globular clusters are strongly concentrated toward the Galactic Center: From their apparent distribution in the sky, Harlow Shapley has concluded that this center of the Milky Way lies at a considerable distance (which he overestimated by factors) in the direction of Sagittarius and not rather close to us, as had been thought previously.

Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years "above" the equatorial symmetry plane (to the direction of the Galactic North Pole, see below), but about 28,000 light years from the Galactic Center. Therefore, the Milky Way shows up as luminous band spanning all around the sky along this symmetry plane, which is also called the "Galactic Equator". Its center lies in the direction of the constellation Sagittarius, but very close to the border of both neighbor constellations Scorpius and Ophiuchus. The distance of 28,000 light years has recently (1997) been confirmed by the data of ESA's astrometric satellite Hipparcos. Other investigations published consequently have disputed this value and propose a smaller value of some 25,000 light years, based on stellar dynamics a recent investigation (McNamara et.al 2000, based on RR Lyrae variables) yields roughly 26,000 light years. These data, if of significance, wouldn't immediately effect values for distances of particular objects in the Milky Way or beyond.

The solar system is situated within a smaller spiral arm, called the Local or Orion Arm, which is merely connection between the inner and outer next more massive arms, the Sagittarius Arm and the Perseus Arm.

Similar to other galaxies, there occur supernovae in the Milky Way at irregular intervals of time. If they are not too heavily obscurred by interstellar matter, they can be, and have been seen as spectacular events from Earth. Unfortunately, none has yet appeared since the invention of the telescope (the last well observed supernova was studied by Johannes Kepler in 1604).

The Milky Way, the Andromeda Galaxy and the Triangulum Galaxy are the major members of the Local Group, a group of some 35 closely bound galaxies The Local Group is part of the Virgo Supercluster.

The Milky Way is orbited by a number of dwarf galaxies in the Local Group. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light years. The smallest, Carina Dwarf, Draco Dwarf, and Leo II Dwarf are only 500 light years in diameter. The other dwarfs orbiting our galaxy are the Small Magellanic Cloud Canis Major Dwarf, the closest Sagittarius Dwarf Elliptical Galaxy, previously thought to be the closest Ursa Minor Dwarf Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the Milky Way, causing vibrations at certain frequencies when they pass through the edges of our Galaxy. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twenty-fold increase in mass for the Milky Way. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. In this model, the dark matter is spreading out from the Milky Way disk with the known gas layer. As a result, the model predicts that the gravitational impact of the Magellanic Clouds is amplified as they pass through the Milky Way.

Barring some unforeseeable accident, such as the arrival of a rogue black hole or star into its territory, it is estimated that the solar system as we know it today will last another billion years or so, whereupon the Sun will claim its first casualty, the Earth. As the Sun brightens a further ten percent beyond today's levels, its radiation output will increase, gradually searing the Earth until its land surface becomes uninhabitable, though life could still survive in the deeper oceans. Within 3.5 billion years, Earth will attain surface conditions similar to Venus's today the oceans will boil, and all life (in known forms) will be impossible.
Artist's conception of the remains of artificial structures on the Earth after the Sun enters its red giant phase and swells to roughly 100 times its current size.

With the hydrogen reserves within its core spent, the Sun will begin to use those in its less dense upper layers. This will require it to expand to eighty times its current diameter, and, about 7.5 billion years from now, to become a red giant, cooled and dulled by its vastly increased surface area. As the Sun expands, it will swallow the planet Mercury. Earth and Venus, however, are expected to survive, since the Sun will lose about 28 percent of its mass, and its lower gravity will send them into higher orbits. Earth will be left a scorched cinder, its land surface reduced to the consistency of hot clay by sunlight a thousand times more powerful than today's, and its atmosphere stripped away by a now-ferocious solar wind. The Sun is expected to remain in a red giant phase for about a hundred million years.

During this time, it is possible that the watery worlds around Jupiter and Saturn, such as Titan and Europa, might achieve conditions similar to those required for current human life.

Eventually, the helium produced in the shell will fall back into the core, increasing the density until it reaches the unimaginable levels needed to fuse helium into carbon. The Sun will then shrink to slightly larger than its original radius, as its energy source has fallen back to its core, however, due to the relative rarity of helium as opposed to hydrogen, the helium-fusing stage will only last about 100 million years. Eventually it will have to again resort to its reserves in its outer layers, and will regain its red giant form. This phase lasts only 100 million years, after which, over the course of a further 100,000 years, the Sun's outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula.

This is a relatively peaceful event nothing akin to a supernova, which our Sun is too small to ever undergo. Earthlings, if we are still alive to witness this occurrence, would observe a massive increase in the speed of the solar wind, but not enough to destroy the Earth completely.

Eventually, all that will remain of the Sun is a white dwarf, a hot, dim and extraordinarily dense object half its original mass but only the size of the Earth. Were it viewed from Earth's surface, it would be a point of light the size of Venus with the brightness of a hundred current Suns.

As the Sun dies, its gravitational pull on the orbiting planets, comets and asteroids will weaken. Earth and the other planets' orbits will expand. When the sun becomes a white dwarf, the solar system's final configuration will be reached: Mercury will have long since ceased to exist Venus will lie roughly a third again farther out than Earth is now, and Earth's orbit will roughly equal that of Mars today. Two billion years farther on, the carbon in the Sun's core will crystallize, transforming it into a giant diamond. Eventually, after trillions more years, it will fade and die, finally ceasing to shine altogether.


Construction

Ancient accounts, which differ to some degree, describe the structure as being built with iron tie bars to which brass plates were fixed to form the skin. The interior of the structure, which stood on a 15-meter- (50-foot-) high white marble pedestal near the Mandraki harbor entrance, was then filled with stone blocks as construction progressed. [4] Other sources place the Colossus on a breakwater in the harbor. The statue itself was over 30 meters (107 ft) tall. Much of the iron and bronze was reforged from the various weapons Demetrius’s army left behind, and the abandoned second siege tower was used for scaffolding around the lower levels during construction. Upper portions were built with the use of a large earthen ramp. During the building, workers would pile mounds of dirt on the sides of the colossus. Upon completion all of the dirt was removed and the colossus was left to stand alone. After twelve years, in 280 BC, the statue was completed. Preserved in Greek anthologies of poetry is what is believed to be the genuine dedication text for the Colossus. [5]

To you, o Sun, the people of Dorian Rhodes set up this bronze statue reaching to Olympus, when they had pacified the waves of war and crowned their city with the spoils taken from the enemy. Not only over the seas but also on land did they kindle the lovely torch of freedom and independence. For to the descendants of Herakles belongs dominion over sea and land.

Possible construction method

Modern engineers have put forward a plausible hypothesis for the statue construction, based on the technology of those days (which was not based on the modern principles ofearthquake engineering), and the accounts of Philo and Pliny who both saw and described the remains. [6]

The base pedestal was at least 60 feet (18 m) in diameter and either circular or octagonal. The feet were carved in stone and covered with thin bronze plates riveted together. Eight forged iron bars set in a radiating horizontal position formed the ankles and turned up to follow the lines of the legs while becoming progressively smaller. Individually cast curved bronze plates 60 inches (1,500 mm) square with turned in edges were joined together by rivets through holes formed during casting to form a series of rings. The lower plates were 1-inch (25 mm) in thickness to the knee and 3/4 inch thick from knee to abdomen, while the upper plates were 1/4 to 1/2 inch thick except where additional strength was required at joints such as the shoulder, neck, etc. The legs would need to be filled at least to the knees with stones for stability. Accounts described earthen mounds used to aid construction however, to reach the top of the statue would have required a mound 300 feet (91 m) in diameter, which exceeded the available land area, so modern engineers have proposed that the abandoned siege towers stripped down would have made efficient scaffolding.

A computer simulation of this construction indicated that an earthquake would have caused a cascading failure of the rivets, causing the statue to break up at the joints while still standing instead of breaking after falling to the ground, as described in second hand accounts. The arms would have been first to separate, followed by the legs. The knees were less likely to break and the ankles’ survival would have depended on the quality of the workmanship.


Rheotaxis in the Garden of the Ediacaran

The “Garden of the Ediacaran” was a period in the ancient past when Earth’s shallow seas were populated with a bewildering variety of enigmatic, soft-bodied creatures.

Scientists traditionally have pictured it as a tranquil, almost idyllic interlude that lasted from 635 to 540 million years ago. But new interdisciplinary studies suggests that the organisms living at the time may have been much more dynamic than experts have thought.

Scientists have found It extremely difficult to fit these Precambrian species into the tree of life. That is because they lived in a time before organisms developed the ability to make shells or bones. As a result, they didn’t leave much fossil evidence of their existence behind, and even less evidence that they moved around.

So, experts have generally concluded that virtually all of the Ediacarans—with the possible exception of a few organisms similar to jellyfish that floated about—were stationary and lived out their adult lives fixed in one place on the sea floor.

The new findings concern one of the most enigmatic of the Ediacaran genera, a penny-sized organism called Parvancorina, which ischaracterized by a series of ridges on its back that form the shape of a tiny anchor.

By analyzing the way in which water flows around Parvancorina’s body, an international team of researchers has concluded that these ancient creatures must have been mobile: specifically, they must have had the ability to orient themselves to face into the current flowing around them.

That would make them the oldest species known to possess this capability, which scientists call rheotaxis.

The analysis, which used a technique borrowed from engineering called computational fluid dynamics (CFD), also showed that when Parvancorina faced into the current, its shape created eddy currents that were directed to several specific locations on its body.

The absence of fast-moving animals allowed microbes to colonize the surface of the ocean floor, then create a layer of secretion wherever they grow. Such a sticky layer allowed the sediment to stabilize and acted as a mold when the animals died on top of them. This age was the Time of the Slime, where the ocean floor was filled with sticky substances. Such a slow-paced life, combined with the lack of predators, is a feature unique to this period. As a nod to the biblical Garden of Eden, some people have referred to this peaceful early Earth as the Garden of Ediacara.

Extraterrestrial Occupation

At this time, the universe was already mature.

So even though our solar system was still rather youthful, the rest of the universe was quite old.

In fact, the universe was already 11 billion years old when the Ediacaran period began.

What this means is that there were entire life cycles of stars that were born, grew into maturity, and died well before our solar system was even formed.

In fact, there is evidence, from the spectral composition of our sun, that at least four generations of previous stars came before our solar system was berthed. This means that it completely realistic to expect the presence of extremely advanced galactic-wide extraterrestrial civilizations with interstellar transport technology in our region of space.

The Ediacaran period saw the presence of the very first humanoid extraterrestrial bases on the earth.

These facilities were short duration affairs. Mostly used for scientific inquiry. To imagine what these facilities were like, one should consider what the current human research stations look like in Antarctica.

Scout. Scan. Visit. Sample. Leave.

I am quite confident that the extraterrestrial bases were very similar to those facilities in both form and function.

Essentially,we should realistically consider the base facilities at this time and place to be similar to that consisting of a small cluster of habitats around a secured landing area for the associative vehicles.

None of the bases or communities during this entire huge swath of time (during the Ediacaran period) were ever very large.

Typically, the species operated out of their spacecraft, which at that time, tended to be (comparatively) huge. (Not all, and not the “critical” visits. Just the ones that made the greatest disruption in the quantum envelope that is recorded.) They would then send excursions to the surface and form “base camps” which typically tended to consist of rudimentary structures and facilities.

Typically planetary excursions were very very short lived affairs. Often lasting less than one month in duration.

Although there were a number which lasted for much longer perhaps as long as two years in duration. However, in all cases, they could just be considered to be scientific excursions, which were there for the purposes of scientific investigation and inquiry.

Typically, one might expect (or more accurately, assume) the base facilities to lie close to the equator for reasons of avoiding the gravity sink of the earth. Nevertheless, when one studies the map of the Earth at that time, one can clearly see a problem with the base placement.

There weren’t too many dry land locations near the equator at this time.

That severely limited the location of the bases of operation around a water world swimming full of proto-jellyfish like creatures. In any event, none were involved in any type of colonization or industrial facilities.

It is entirely possible that contamination of the native ecosystem by extraterrestrial races contributed to the emergence of life on the Earth at this time.

Contamination refers to any extraterrestrial influence on the biology of the earth ecosystem at that time.

We can be assured that there was some degree of contamination.

This is both physical, spiritual and in all ways quantum. But, no one knows for sure the impact it had, if any.

Nothing (physical) remains of whatever visitors occupied the earth at this time.

The only evidence remaining for (supplemented) human observation are the tell-tale quantum level signatures of early visitations in the (local regional) quantum cloud.

Unfortunately, we as humans, do not possess the ability to read and interpret these signatures.

We only know what is told to us by those whom have this ability.

What they tell us is quite simplistic.

They tell us that the planet was visited and explored by humanoid bipedal entities at this time. We also know that they traveled through various methods, not limited to physical transport. Indeed dimensional transport seemed to be the most common method.

Their past, history, appearance, and other traits that we might find interesting are shrouded in the mists of time.

That includes what happened to the various species whom visited this planet and where they are today.

This is the full extent of what I know about this time.

Summary

Around 650 million years ago, the first extraterrestrial life set foot on the earth and investigated it. Over time there were numerous subsequent visits. During some of these visits a small number of bases or facilities were constructed for various scientific and investigative purposes.

The solar system at that time was still very young, being only three billion years old. There were many comets and orbiting rocky bodies that yet had to be absorbed or collided with the larger planetary bodies.

Mars was not habitable, but both Mars and Venus were more habitable to ambulatory humanoids than they are today.

To this end, this solar system was of interest because of the three possible marginally desirable planets in the system. The Earth, Venus and Mars. Additionally, since the gas giants were closer to the sun than they are now, and hotter, a number of Jupiter moons possessed atmosphere in a gaseous state, and some even had oceans that held water in a liquid state.

This entire solar system held promise.

The earth at that time was mostly bare rock with oceans teeming with soft-shell creatures.

At that time there was no galactic federation that would claim administration for our solar system.

For the Ediacaran Period of nearly 89 million years, the situation was pretty much a stable one. Our solar system was mapped, explored, and systematically ignored by other species.

They actually found our solar neighbors far more interesting for a host of reasons, and thus at this time just mostly ignored our solar system.

The solar system was still evolving and there were various comets and rogue asteroids that would and did present a threat to any native life in the solar system. This system was considered to be moderately interesting but not worthy of colonization by any of the species who visited it.

It was noted explored in a more or less cursory manner, and archived.

Very little happened on the earth in the regard to extraterrestrial involvement of a substantive nature during this time period.

Those MM readers who might wonder what life might resemble around planets in the habitual zone of stars around three billion years old, might well learn from this narrative and explanation here.


Outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large that they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of "ices," such as frozen water, ammonia, and methane. For this reason some astronomers have suggested putting them in a separate category—"Uranian planets" or "ice giants." All four outer planets exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates all planets that lie outside Earth's orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Three of its 63 satellites—Ganymede, Io, and Europa—share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede has a larger diameter than Mercury.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury it is also the only satellite in the solar system with a substantial atmosphere.

Uranus

Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side its axial tilt lies at over 90 degrees to the ecliptic. Its core is remarkably cooler than that of the other gas giants (though it is still several thousand degrees Celsius) and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel, and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is denser and slightly more massive, at 17 Earth masses. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen.


Wednesday, 13 February 2013

Another Earth Nasa The Milky Way And Universe Is Awash In Water

10:04 Unknown

The Daily Galaxy via http://www.nasa.gov

APRIL 8, 2015 - SPACE - As NASA missions explore our solar system and search for new worlds, they are finding water in surprising places. Water is but one piece of our search for habitable planets and life beyond Earth, yet it links many seemingly unrelated worlds in surprising ways. "NASA science activities have provided a wave of amazing findings related to water in recent years that inspire us to continue investigating our origins and the fascinating possibilities for other worlds, and life, in the universe," said Ellen Stofan, chief scientist for the agency. "In our lifetime, we may very well finally answer whether we are alone in the solar system and beyond."

The chemical elements in water, hydrogen and oxygen, are some of the most abundant elements in the universe. Astronomers see the signature of water in giant molecular clouds between the stars, in disks of material that represent newborn planetary systems, and in the atmospheres of giant planets orbiting other stars.

There are several worlds thought to possess liquid water beneath their surfaces, and many more that have water in the form of ice or vapor. Water is found in primitive bodies like comets and asteroids, and dwarf planets like Ceres. The atmospheres and interiors of the four giant planets -- Jupiter, Saturn, Uranus and Neptune -- are thought to contain enormous quantities of the wet stuff, and their moons and rings have substantial water ice.

Perhaps the most surprising water worlds are the five icy moons of Jupiter and Saturn that show strong evidence of oceans beneath their surfaces: Ganymede, Europa and Callisto at Jupiter, and Enceladus and Titan at Saturn.

Scientists using NASA's Hubble Space Telescope recently provided powerful evidence that Ganymede has a saltwater, sub-surface ocean, likely sandwiched between two layers of ice.

Europa and Enceladus are thought to have an ocean of liquid water beneath their surface in contact with mineral-rich rock, and may have the three ingredients needed for life as we know it: liquid water, essential chemical elements for biological processes, and sources of energy that could be used by living things. NASA's Cassini mission has revealed Enceladus as an active world of icy geysers. Recent research suggests it may have hydrothermal activity on its ocean floor, an environment potentially suitable for living organisms.

NASA spacecraft have also found signs of water in permanently shadowed craters on Mercury and our moon, which hold a record of icy impacts across the ages like cryogenic keepsakes.

While our solar system may seem drenched in some places, others seem to have lost large amounts of water.

On Mars, NASA spacecraft have found clear evidence that the Red Planet had water on its surface for long periods in the distant past. NASA's Curiosity Mars Rover discovered an ancient streambed that existed amidst conditions favorable for life as we know it.

More recently, NASA scientists using ground-based telescopes were able to estimate the amount of water Mars has lost over the eons. They concluded the planet once had enough liquid water to form an ocean occupying almost half of Mars' northern hemisphere, in some regions reaching depths greater than a mile (1.6 kilometers). But where did the water go?

It's clear some of it is in the Martian polar ice caps and below the surface. We also think much of Mars' early atmosphere was stripped away by the wind of charged particles that streams from the sun, causing the planet to dry out. NASA's MAVEN mission is hard at work following this lead from its orbit around Mars.

The story of how Mars dried out is intimately connected to how the Red Planet's atmosphere interacts with the solar wind. Data from the agency's solar missions -- including STEREO, Solar Dynamics Observatory and the planned Solar Probe Plus -- are vital to helping us better understand what happened.

Understanding the distribution of water in our solar system tells us a great deal about how the planets, moons, comets and other bodies formed 4.5 billion years ago from the disk of gas and dust that surrounded our sun. The space closer to the sun was hotter and drier than the space farther from the sun, which was cold enough for water to condense. The dividing line, called the "frost line," sat around Jupiter's present-day orbit. Even today, this is the approximate distance from the sun at which the ice on most comets begins to melt and become "active." Their brilliant spray releases water ice, vapor, dust and other chemicals, which are thought to form the bedrock of most worlds of the frigid outer solar system.

Scientists think it was too hot in the solar system's early days for water to condense into liquid or ice on the inner planets, so it had to be delivered -- possibly by comets and water-bearing asteroids. NASA's Dawn mission is currently studying Ceres, which is the largest body in the asteroid belt between Mars and Jupiter. Researchers think Ceres might have a water-rich composition similar to some of the bodies that brought water to the three rocky, inner planets, including Earth.

The amount of water in the giant planet Jupiter holds a critical missing piece to the puzzle of our solar system's formation. Jupiter was likely the first planet to form, and it contains most of the material that wasn't incorporated into the sun. The leading theories about its formation rest on the amount of water the planet soaked up. To help solve this mystery, NASA's Juno mission will measure this important quantity beginning in mid-2016.

Looking further afield, observing other planetary systems as they form is like getting a glimpse of our own solar system's baby pictures, and water is a big part of that story. For example, NASA's Spitzer Space Telescope has observed signs of a hail of water-rich comets raining down on a young solar system, much like the bombardment planets in our solar system endured in their youth.

With the study of exoplanets -- planets that orbit other stars -- we are closer than ever to finding out if other water-rich worlds like ours exist. In fact, our basic concept of what makes planets suitable for life is closely tied to water: Every star has a habitable zone, or a range of distances around it in which temperatures are neither too hot nor too cold for liquid water to exist. NASA's planet-hunting Kepler mission was designed with this in mind. Kepler looks for planets in the habitable zone around many types of stars.

Recently verifying its thousandth exoplanet, Kepler data confirm that the most common planet sizes are worlds just slightly larger than Earth. Astronomers think many of those worlds could be entirely covered by deep oceans. Kepler's successor, K2, continues to watch for dips in starlight to uncover new worlds.

The agency's upcoming TESS mission will search nearby, bright stars in the solar neighborhood for Earth- and super-Earth-sized exoplanets. Some of the planets TESS discovers may have water, and NASA's next great space observatory, the James Webb Space Telescope, will examine the atmospheres of those special worlds in great detail.

It's easy to forget that the story of Earth's water, from gentle rains to raging rivers, is intimately connected to the larger story of our solar system and beyond. But our water came from somewhere -- every world in our solar system got its water from the same shared source. So it's worth considering that the next glass of water you drink could easily have been part of a comet, or an ocean moon, or a long-vanished sea on the surface of Mars. And note that the night sky may be full of exoplanets formed by similar processes to our home world, where gentle waves wash against the shores of alien seas. - DAILY GALAXY.

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