Astronomy

Would Venus have any significant effects on Earth if its orbit were entirely within the habitable zone?

Would Venus have any significant effects on Earth if its orbit were entirely within the habitable zone?


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If Venus was entirely inside the habitable zone, or life zone, would its proximity to the Earth provoke any remarkable changes to the Earth?


There are several scenarios: An Earth-Venus binary would be possible, or coplanar prograde or inclined up to retrograde on resonant or non-resonant orbits, elliptic or almost circular.

One option would be a co-orbital scenario, which is described in more detail here. The latter would allow Earth and Venus to stay both in the habitable zone. But the effects for Earth could be very remarkable in some cases: Seasons would change over time, since eccentricity of Earth's orbit would change. If Earth and Venus would follow a horseshoe orbit, we would get variations in the length of a year.

Co-orbital systems can become instable by the gravity of other planets.

In the long run tidal forces may lead to a collision of the two co-orbiting planets.


Changes on Earth:

Except for possible tides and possible eclipses, if it were near enough, no.

Changes on Earth's orbit:

Venus would establish some resonance with Earth's orbit, if it were near enough or in one of the exact possible spots. This will not have a major impact except on year's lenght and Sun illumination on Earth.


'Venus zone' narrows search for habitable planets

An artistic representation of the exoplanet Gliese 832c as compared with Earth. The large planet may be Earth-like, or it could have a dense atmosphere and a closer relationship to Venus. Credit: PHL, University of Puerto Rico, Arecibo

Long before the hunt began to find Earth lookalikes around other stars, one planet in the Solar System had already been named Earth's twin. With its similar size and mass, Venus measures very close to Earth, with one major yet significant difference. Its thick atmosphere makes temperatures on the planet hot enough to melt lead, and therefore most certainly too hot to sustain life.

In order to weed out Venus-like planets from those that would be more habitable, several scientists, including planetary scientist Stephen Kane of San Francisco State University, proposed the establishment of a "Venus zone" around stars, a region where the atmosphere could be consumed by a runaway greenhouse effect that superheats its planets.

"We're specifically trying to make it clear that size is no indication of habitability," Kane told Astrobiology Magazine.

In other words, just because a planet is roughly the size of Earth, instead of, say, Jupiter, doesn't guarantee the conditions are right for life to evolve.

Defining the Venus Zone

The region around a star where liquid water can form on a planet is known as the habitable zone. But just because water can form doesn't mean that it does. Finding out the conditions on a planet often requires follow-up observations to the discoveries that first pinpointed it, but limitations on observation time and equipment mean prioritizing which planets should be the first to be studied in-depth.

"The primary purpose of the habitable zone is target selection," said Kane.

Kane serves as the chair of NASA's Kepler Telescope's Habitable Zone working group, which seeks to utilize all available data from NASA's Kepler mission, along with any follow-up observations, to provide the most robust list of habitable zone planets discovered by the telescope. The aim is to better understand how common Earth-sized planets are in the habitable zones of other stars. To date, the telescope has identified more than 4,100 planetary candidates.

The Venus zone would similarly serve as a target selection tool. Scientists hoping to find the next Earth-like planet perform follow-up searches on planetary candidates in the habitable zone the establishment of a Venus zone would narrow down the inner edge of potential habitability.

Despite being similar sizes, Earth (right half) and Venus (left half) have different surface conditions, a fact that has implications in the search for an Earth-like exoplanet. Credit: NASA/JPL-Caltech/Ames

A planet within the Venus zone may form an ocean at some point in its history. Like Earth, Venus was thought to contain water on its surface until approximately one billion years ago, at which point it lost its liquid.

Kane and his team labeled the point at which a planet would lose its oceans due to energy from its star as the outer edge of the Venus zone, and the inner boundary of the habitable zone. Losing liquid water would inhibit the carbon cycle of a planet, allowing more to build up in the atmosphere. Rising carbon levels would kick off a runaway greenhouse effect that would heat the planet.

The runaway greenhouse effect for a planet can be avoided if it experiences significant atmospheric loss. As the atmosphere escapes into space, it prevents the carbon from building up and superheating the planet. This loss of atmosphere establishes the inner edge of the Venus zone.

Kane presented his research at the January meeting of the American Astronomical Society in Seattle, Washington. The work was also published in the scientific journal, Astrophysical Journal Letters.

The majority of the new planetary candidates discovered in recent years has come from NASA's Kepler telescope. Studying planetary atmospheres, however, continues to be a challenge, one that requires advanced telescopes and the right kind of stars, a situation that may change in the future.

"At the moment, we lack enough planets around bright stars, and we lack the resources," Kane said. "Resources means James Webb."

Set to launch in 2018, the James Webb Space Telescope will be able to search for and study planets around distant stars. At the same time, the Transiting Exoplanet Survey Satellite, or TESS, will map exoplanets around the brightest stars in the sky after its 2017 launch.

This graphic shows the location of the ‘Venus Zone’, the area around a star in which planets are likely to have an atmosphere more like Venus than Earth. Credit: Chester Harman, Pennsylvania State University

"James Webb combined with TESS will really change the game," Kane said.

Because TESS searches for transiting planets—planets that are observed as they cross between Earth and their star—it will be more sensitive to those that orbit closer to their sun.

"TESS will see a lot more exo-Venuses than it will exo-Earths," planetary atmospheric scientist James Kasting, of Penn State University, told Astrobiology Magazine in an email. "These are the planets to rule out in the search of the more interesting exo-Earths."

At the same time, studying more exo-Venuses will help to narrow down the line between the Venus zone and the habitable zone, helping scientists to pinpoint which Earth-size planets are Earth-twins, and which bear a stronger resemblance to Venus.

"Once we can observe these exo-Venuses and exo-Earths, we'll be able to determine more accurately the boundary between them," Kasting said. "Right now, that boundary is based entirely on theoretical climate models, which may not be very accurate under these distinctly non-Earth-like conditions."

Until then, scientists may have to deal with Venus-twins posing as Earth-analogues in the samples obtained by Kepler. Kane and his team identified 43 potential Venus analogs, and think that even more exist.


29 January 2021

Nasa’s Space Launch System. (Nasa)
No-one has visited the Moon since 1972. But with the advent of commercial human spaceflight , the urge to return is resurgent and generating a new space race . Nasa has selected the private company SpaceX to be part of its commercial spaceflight operations, but the firm is also pursuing its own space exploration agenda.

To enable flights to the Moon and beyond, both Nasa and SpaceX are developing new heavy lift rockets: SpaceX’s Starship and Nasa’s Space Launch System.

But how do they differ and which one is more powerful?

Starship

Rockets go through multiple stages to get into orbit. By discarding spent fuel tanks while in flight, the rocket becomes lighter and therefore easier to accelerate. Once in operation, SpaceX’s launch system will be comprised of two stages: the launch vehicle known as Super Heavy and the Starship.

Super Heavy is powered by the Raptor rocket engine, burning a combination of liquid methane and liquid oxygen. The basic principle of a liquid fuel rocket engine is that two propellants, – a fuel such as kerosene and an oxidiser such as liquid oxygen – are brought together in a combustion chamber and ignited. The flame produces hot gas under high pressure which is expelled at high speed through the engine nozzle to produce thrust.

The rocket will provide 15 million pounds of thrust at launch, which is approximately twice as much as the rockets of the Apollo era. Atop the launcher sits the Starship, itself powered by another six Raptor engines and equipped with a large mission bay for accommodating satellites, compartments for up to 100 crew and even extra fuel tanks for refuelling in space, which is critical to long duration interplanetary human spaceflight.

Super Heavy separating from Starship. (wikipedia, CC BY-SA)

The Starship is designed to operate both in the vacuum of space and within the atmospheres of Earth and Mars, using small moveable wings to glide to a desired landing zone.

Once over the landing area, the Starship flips into a vertical position and uses its on-board Raptor engines to make a powered descent and landing. It will have sufficient thrust to lift itself off the surface of Mars or the Moon, overcoming the weaker gravity of these worlds, and return to Earth – again making a powered soft landing. The Starship and Super Heavy are both fully reuseable and the entire system is designed to lift more than 100 tons of payload to the surface of the Moon or Mars.

The spacecraft is maturing rapidly. A recent test flight of the Starship prototype, the SN8, successfully demonstrated a number of the manoeuvres required to make this work. Unfortunately, there was a malfunction in one of the Raptor engines and the SN8 crashed on landing. Another test flight is expected in the coming days.

Nasa’s Space Launch System

The Space Launch System (SLS) from Nasa will be taking the crown from the discontinued Saturn V as the most powerful rocket the agency has ever used. The current incarnation (SLS block 1) stands at almost 100 metres tall.

The SLS core stage, containing more than 3.3 million litres of liquid hydrogen and liquid oxygen (equivalent to one-and-a-half Olympic size swimming pools), is powered by four RS-25 engines, three of which were used on the previous Space Shuttle. Their main difference from the Raptors is that they burn liquid hydrogen instead of methane.

Stages of the SLS. (Nasa)

The core stage of the rocket is augmented by two solid rocket boosters, attached to its sides, providing a total combined thrust of 8.2 million pounds at launch - about 5% more than the Saturn V at launch. This will lift the spacecraft to low Earth orbit. The upper stage is intended to lift the attached payload – the astronaut capsule – out of Earth’s orbit and is a smaller liquid fuel stage powered by a single RL-10 engine (already in use by ATLAS and DELTA rockets) which is smaller and lighter than the RS-25.

The Space Launch System will send the Orion crew capsule, which can support up to six crew for 21 days, to the Moon as part of the Artemis-1 mission – a task that current Nasa rockets are currently not capable of performing.

It is intended to have large acrylic windows so astronauts can watch the journey. It will also have its own engine and fuel supply, as well as secondary propulsion systems for returning to the Earth. Future space stations, such as the Lunar Gateway, will serve as a logistical hub, which may include refuelling.

The core stage and booster rockets are unlikely to be reusable (instead of landing they will drop in the ocean), so there is a higher cost with the SLS system, both in materials and environmentally. It is designed to evolve to larger stages capable of carrying crew or cargo weighing up to 120 tonnes, which is potentially more than Starship.

NASA’s SLS and SpaceX’s Starship, on the right, could both get us to the Moon and beyond. (Ian Whittaker/NASA/SpaceX, Author provided)

A lot of the technology being used in SLS is so-called “legacy equipment” in that it is adapted from previous missions, cutting down the research and development time. However, earlier this month, a test fire of the SLS core stage was stopped a minute into the eight-minute test due to a suspected component failure. No significant damage occurred, and the SLS program manager, John Honeycutt, stated: “I don’t think we’re looking at a significant design change.”

And the winner is…

So which spacecraft likely to reach carry a crew to the Moon first? Artemis 2 is planned as the first crewed mission using SLS to perform a flyby of the Moon and is expected to launch in August 2023. Whereas SpaceX has no specific date planned for crewed launch, they are running #dearMoon – a project involving lunar space tourism planned for 2023. Musk has also stated that a crewed Martian mission could take place as early as 2024, also using Starship.

Ultimately it is a competition between an agency that has had years of testing and experience but is limited by a fluctuating taxpayer budget and administration policy changes, and a company relatively new to the game but which has already launched 109 Falcon 9 rockets with a 98% success rate and has a dedicated long-term cash flow.

Whoever reaches the Moon first will inaugurate a new era of exploration of a world which still has much scientific value.

About Today's Contributors:

This article is republished from The Conversation under a Creative Commons license.


Would Venus have any significant effects on Earth if its orbit were entirely within the habitable zone? - Astronomy

3. The first observation was in 1956 by Mayer, McCullough, and Sloanaker. Mayer (1983), p. 271. BACK

4. The actual temperature is around 750°K. Wildt predicted roughly 400, "higher than the terrestrial boiling-point." Wildt (1940) something other than CO 2 would be necessary to get to 600 according to Kuiper in Kuiper (1952), ch. 12. BACK

5. Sagan, interview by Ron Doel, Aug. 27, 1991, AIP, tape 4 side 1. BACK

6. Sagan (1960b) Sagan (1960a), "efficient" p. vii, "lifeless" p. 20 see also Sagan (1961) arguments against a waterless Venus were developed by Gold (1964) see Davidson (1999), pp. 101-106. BACK

8. Rasool and de Bergh (1970) calculated that water would always have boiled on Venus, but Pollack (1971) was the first to deploy enough computer power to calculate a reasonable "runaway greenhouse" atmosphere. The ever prescient Tommy Gold had already speculated in a 1963 symposium about a "runaway process" when water boiled away, Gold (1964), p. 250. Others continued to speculate about a Venus that had once had a "clement" climate, e.g., Wang et al. (1976). BACK

9. Hart (1979). More accurate calculations in the 1990 found that for our Sun, a considerably larger zone should be habitable despite the gradually increasing brightness of the Sun itself. BACK

11. Newell (1980), ch. 20. Newell also says that "study of the role of halogens in the atmosphere of Venus. led to the suspicion that chlorine produced in Earth's stratosphere from the exhausts of Space Shuttle launches or from Freon used at the ground in aerosol sprays might dangerously deplete the ozone layer.". BACK

12. The first publication was Sill (1972). The idea that sulfuric acid was "the most probable constituent of the Venus clouds" was independently suggested by Louise Young, whose husband credited her in his publication, Young (1973), p. 564. Young relied especially on measurements by Hansen, who had also identified the acid but was dissuaded from publishing the idea. Hansen, interview by Weart, Oct. 2000, AIP Hansen’s contribution to the identification was noted by Prather (2002). Confirmation from an infrared telescope carried in an aircraft was reported by Pollack et al. (1975). BACK

13. He thought "a great deal stands to be gained" by studying other planets' climates alongside the Earth's. Hansen et al. (1978), p. 1067. For these matters and NASA contributions in general see Conway (2008). BACK

14. Hitchcock and Lovelock (1967) see Lovelock (2000), pp. 228ff. the absence of detectable oxygen on Mars was long considered no definitive argument against the presence of vegetation, and in the early 1960s, NASA's ideas on detecting life through atmospheric analysis centered on a search for complex organic molecules. Dick (1998), pp. 48, 175. BACK

17. Prediction (hoping that Martian life was only hibernating through the winter of a 50,000-year cycle): Sagan (1971) Sagan et al. (1973), quotes pp. 1045, 1048. CO 2 ice: Forget and Pierrehumbert (1997). BACK


The once and future Earth

And as that atmosphere grew thicker, the conditions on the surface grew even more hellish.

The atmosphere might even have had enough drag to literally slow down the rotation of Venus itself, giving it its present-day sluggish rates.

Once this process was complete, which probably took 100 million years or so, the potential for any life on Venus was snuffed out.

And here's the worst part about the story of Earth's twisted sister. This is our fate, too. Our sun isn't done aging, and as it grows older, it grows brighter, with the habitable zone steadily and inexorably moving outward. At some point within the next few hundred million years, the Earth itself will approach the inner edge of the habitable zone. Our oceans will evaporate. Temperatures will spiral upward. Plate tectonics will shut off. Carbon dioxide will dump into the atmosphere.

And by that time, our solar system will be home to not just one hell but two.


Researchers Discover Earth-Size, Habitable Zone Planet Found Hidden in Early NASA Kepler Data

NASA hosting 'Reddit Ask Me Anything' Friday, April 17, at 2 pm

A team of transatlantic scientists, using reanalyzed data from NASA’s Kepler space telescope, has discovered an Earth-size exoplanet orbiting in its star’s habitable zone, the area around a star where a rocky planet could support liquid water. (NASA Image)

(NASA) – A team of transatlantic scientists, using reanalyzed data from NASA’s Kepler space telescope, has discovered an Earth-size exoplanet orbiting in its star’s habitable zone, the area around a star where a rocky planet could support liquid water.

Scientists discovered this planet, called Kepler-1649c, when looking through old observations from Kepler, which the agency retired in 2018.

While previous searches with a computer algorithm misidentified it, researchers reviewing Kepler data took a second look at the signature and recognized it as a planet.

Out of all the exoplanets found by Kepler, this distant world – located 300 light-years from Earth – is most similar to Earth in size and estimated temperature.

This newly revealed world is only 1.06 times larger than our own planet. Also, the amount of starlight it receives from its host star is 75% of the amount of light Earth receives from our Sun – meaning the exoplanet’s temperature may be similar to our planet’s, as well.

But unlike Earth, it orbits a red dwarf. Though none have been observed in this system, this type of star is known for stellar flare-ups that may make a planet’s environment challenging for any potential life.

“This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.

“The data gathered by missions like Kepler and our Transiting Exoplanet Survey Satellite (TESS) will continue to yield amazing discoveries as the science community refines its abilities to look for promising planets year after year.”

There is still much that is unknown about Kepler-1649c, including its atmosphere, which could affect the planet’s temperature.

Current calculations of the planet’s size have significant margins of error, as do all values in astronomy when studying objects so far away. But based on what is known, Kepler-1649c is especially intriguing for scientists looking for worlds with potentially habitable conditions.

There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c. Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d.

But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system.

The system has another rocky planet of about the same size, but it orbits the star at about half the distance of Kepler-1649c, similar to how Venus orbits our Sun at about half the distance that Earth does. (NASA Image)

“Out of all the mislabeled planets we’ve recovered, this one’s particularly exciting – not just because it’s in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet,” said Andrew Vanderburg, a researcher at the University of Texas at Austin and first author on the paper released today in The Astrophysical Journal Letters.

“If we hadn’t looked over the algorithm’s work by hand, we would have missed it.”

Kepler-1649c orbits its small red dwarf star so closely that a year on Kepler-1649c is equivalent to only 19.5 Earth days.

The system has another rocky planet of about the same size, but it orbits the star at about half the distance of Kepler-1649c, similar to how Venus orbits our Sun at about half the distance that Earth does.

Red dwarf stars are among the most common in the galaxy, meaning planets like this one could be more common than we previously thought.

Looking for False Positives

Previously, scientists on the Kepler mission developed an algorithm called Robovetter to help sort through the massive amounts of data produced by the Kepler spacecraft, managed by NASA’s Ames Research Center in California’s Silicon Valley.

Kepler searched for planets using the transit method, staring at stars, looking for dips in brightness as planets passed in front of their host stars.

Most of the time, those dips come from phenomena other than planets – ranging from natural changes in a star’s brightness to other cosmic objects passing by – making it look like a planet is there when it’s not. Robovetter’s job was to distinguish the 12% of dips that were real planets.

Those signatures Robovetter determined to be from other sources were labeled “false positives,” the term for a test result mistakenly classified as positive.

With an enormous number of tricky signals, astronomers knew the algorithm would make mistakes and would need to be double-checked – a perfect job for the Kepler False Positive Working Group.

That team reviews Robovetter’s work, going through all false positives to ensure they are truly errors and not exoplanets, ensuring fewer potential discoveries are overlooked. As it turns out, Robovetter had mislabeled Kepler-1649c.

Even as scientists work to further automate analysis processes to get the most science as possible out of any given dataset, this discovery shows the value of double-checking automated work.

Even six years after Kepler stopped collecting data from the original Kepler field – a patch of sky it stared at from 2009 to 2013, before going on to study many more regions – this rigorous analysis uncovered one of the most unique Earth-analogs discovered yet.

A Possible Third Planet

These small and dim stars require planets to orbit extremely close to be within that zone – not too warm and not too cold – for life as we know it to potentially exist. (NASA Image)

Kepler-1649c not only is one of the best matches to Earth in terms of size and energy received from its star, but it provides an entirely new look at its home system. For every nine times the outer planet in the system orbits the host star, the inner planet orbits almost exactly four times. The fact that their orbits match up in such a stable ratio indicates the system itself is extremely stable, and likely to survive for a long time.

Nearly perfect period ratios are often caused by a phenomenon called orbital resonance, but a nine-to-four ratio is relatively unique among planetary systems. Usually resonances take the form of ratios such as two-to-one or three-to-two. Though unconfirmed, the rarity of this ratio could hint to the presence of a middle planet with which both the inner and outer planets revolve in synchronicity, creating a pair of three-to-two resonances.

The team looked for evidence of such a mystery third planet, with no results. However, that could be because the planet is too small to see or at an orbital tilt that makes it impossible to find using Kepler’s transit method.

Either way, this system provides yet another example of an Earth-size planet in the habitable zone of a red dwarf star. These small and dim stars require planets to orbit extremely close to be within that zone – not too warm and not too cold – for life as we know it to potentially exist. Though this single example is only one among many, there is increasing evidence that such planets are common around red dwarfs.

“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-size exoplanets are common around these kinds of stars,” said Vanderburg. “With red dwarfs almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them, the chance one of them isn’t too different than our Earth looks a bit brighter.”

NASA is holding a Reddit Ask Me Anything on this result on Friday, April 17, from 2:00 to 3:30 p.m. EDT.


Views of giant planet in wild orbit would be unparalleled

Contrary to previous thought, a gigantic planet in wild orbit does not preclude the presence of an Earth-like planet in the same solar system -- or life on that planet.

What's more, the view from that Earth-like planet as its giant neighbor moves past would be unlike anything it is possible to view in our own night skies on Earth, according to new research led by Stephen Kane, associate professor of planetary astrophysics at UC Riverside.

The research was carried out on planets in a planetary system called HR 5183, which is about 103 light years away in the constellation of Virgo. It was there that an eccentric giant planet was discovered earlier this year.

Normally, planets orbit their stars on a trajectory that is more or less circular. Astronomers believe large planets in stable, circular orbits around our sun, like Jupiter, shield us from space objects that would otherwise slam into Earth.

Sometimes, planets pass too close to each other and knock one another off course. This can result in a planet with an elliptical or "eccentric" orbit. Conventional wisdom says that a giant planet in eccentric orbit is like a wrecking ball for its planetary neighbors, making them unstable, upsetting weather systems, and reducing or eliminating the likelihood of life existing on them.

Questioning this assumption, Kane and Caltech astronomer Sarah Blunt tested the stability of an Earth-like planet in the HR 5183 solar system. Their modeling work is documented in a paper newly published in the Astronomical Journal.

Kane and Blunt calculated the giant planet's gravitational pull on an Earth analog as they both orbited their star. "In these simulations, the giant planet often had a catastrophic effect on the Earth twin, in many cases throwing it out of the solar system entirely," Kane said.

"But in certain parts of the planetary system, the gravitational effect of the giant planet is remarkably small enough to allow the Earth-like planet to remain in a stable orbit."

The team found that the smaller, terrestrial planet has the best chance of remaining stable within an area of the solar system called the habitable zone -- which is the territory around a star that is warm enough to allow for liquid-water oceans on a planet.

These findings not only increase the number of places where life might exist in the solar system described in this study -- they increase the number of places in the universe that could potentially host life as we know it.

This is also an exciting development for people who simply love stargazing. HR 5813b, the eccentric giant in Kane's most recent study, takes nearly 75 years to orbit its star. But the moment this giant finally swings past its smaller neighbor would be a breathtaking, once-in-a-lifetime event.

"When the giant is at its closest approach to the Earth-like planet, it would be fifteen times brighter than Venus -- one of the brightest objects visible with the naked eye," said Kane. "It would dominate the night sky."

Going forward, Kane and his colleagues will continue studying planetary systems like HR 5183. They're currently using data from NASA's Transiting Exoplanet Survey Satellite and the Keck Observatories in Hawaii to discover new planets, and examine the diversity of conditions under which potentially habitable planets could exist and thrive.


The Habitable Zone in the Solar System

The habitable zone around a star is the range of orbital distances where a planet can support liquid water. This implies that water is indispensable for life to exist, which is not necessarily correct.

The habitable zone depends mostly on two factors: the star’s mass and its age. As it evolves, a star changes its spectral type (i.e. its color, which is connected with its surface temperature) and luminosity. The lower limit of the habitable zone is estimated from the photodissociation of water. In other words, when the solar radiation is so intense that water breaks down into its basic elements (oxygen and hydrogen), and hydrogen leaves the plant since it cannot be retained by the Earth’s gravitational field.

To a large extent arbitrarily, it is estimated that the required radiation is 1.1 times the solar constant (1.1×1366 Watts/m^2). In the Solar System, this is equivalent to 0.95 astronomical units. The upper limit of the habitable zone is determined by the condensation of carbon dioxide (CO2). A conservative estimate indicates that this happens at 0.53 times the solar constant. Again, in the Solar System, this is equivalent to 1.37 astronomical units.

Stars evolve and their luminosity changes. For this reason, the concept of continued habitable zone (CHZ) has been created. It represents the range of orbital distances for which the solar constant stays within these limits (1.1. to 0.53) during a significant portion of the star’s history. Since the Sun’s luminosity increases slowly, the CHZ in the Solar System is between 0.95 and 1.15 astronomical units. Consequently, liquid water and, as a result, life should be expected within this range of orbital distances. At least, life as we know it.

Nevertheless, it should be noted that the following factors may play a crucial role in the development and continuity of biological activity: greenhouse effect (the Earth’s average temperature would be several degrees below its current value without the impact of this effect caused by the presence of gases such as CO2 and methane in the atmosphere), geological activity (plate teutonics and the subsequent release of gases to the atmosphere), presence or absence of global magnetic fields (they protect us from the burst of high-energy particles coming from the Sun), or albedo (the amount of energy from a star which is reflected back into space).

Diagram with several exoplanets orbiting their stars in the habitable or comfort zone, where water (if it exists) could be in liquid state.

So far, several super-Earths have been found in orbit around stars that are colder than the Sun. The star Kepler-452 is a solar analog its surface temperature is almost identical even though it could be a lot older. As for the planet, it is 60% bigger than the Earth. We have no information about its mass, average density or possible composition.


Mars retreats

On the first of these two requirements, the history of methane on Mars provides a cautionary tale. In 2004 scientists using three Earth-based telescopes and a spacecraft orbiting Mars all thought they had detected what appeared to be the spectral signature of methane in the planet’s atmosphere. It was a classic Lovelock anomaly. Chemical models insist that methane does not last all that long in the Martian atmosphere, so these observations suggested there had to be a continuous source of the gas. And on Earth most, though not all, methane is produced by microbes. What was more, there was an increasingly widespread belief that, although there is now only a smidgen of water on the surface of Mars, there might be plenty more below it, perhaps in deep aquifers. On the Earth microbes—including microbes that produce methane—are found many kilometres below the surface. Maybe Mars had a similar “deep biosphere”?

Maybe. But if so, there is currently no persuasive evidence that it is producing methane. In 2018 the European Space Agency’s ExoMars Orbiter started to look at trace gases in Mars’s atmosphere with much more sensitive instruments than had been used before. It has seen no evidence of methane at anything like the level previously claimed, which makes it hard to credit the earlier observations. It is true that NASA’s Curiosity rover has detected methane more recently but with ExoMars coming up empty, many see that as the way to bet.

This tale of woe makes it very clear that looking through the Earth’s thick atmosphere for signs of a tiny amount of gas in the atmosphere of another planet is an exacting and error-prone undertaking. Hence the need for observations of phosphine over Venus from other groups using other instruments. At the same time, though, the chain of reasoning which made a deep Martian biosphere plausible applies, mutatis mutandis, to theories about life above Venus, too.

Mars appears always to have been a pretty cold, dry place. But in the distant past, when it had a thicker atmosphere, it clearly had running and standing water at its surface, at least sporadically Curiosity is currently studying mudstones laid down in an ancient lake. As Mars lost its atmosphere its surface became ever more arid and frigid. That put evolutionary pressure on any microbes previously living in those surface waters to migrate deeper and deeper into the still warm and moist subsurface.

The surface of Venus, too, has dried out over its history: but through heating, not cooling. For billions of years the Sun has been growing brighter, thus changing the boundaries of its habitable zone. In the case of Mars, this warming was not enough to offset the cooling effect of losing most of the atmosphere. On Venus, though, it prompted what atmospheric scientists call a “runaway greenhouse effect”, boiling away the seas which many scientists believe to have graced the planet’s youth. If there had been microbes in the surface waters of Venus before this catastrophe, evolution would have urged them not into the depths, as it did on Mars, but into the skies, where even today the temperature remains bearable and water remains liquid, though admittedly in droplets not oceans.

This idea has been much further from the mainstream than that of subsurface life on Mars. One reason may be that, though the existence of Earth’s deep biosphere is quite widely appreciated, beyond some recherché microbiological circles the fact that there are also bacteria busily metabolising up in the sky is widely ignored. And to be fair, the high-biosphere analogy is not perfect. Though bacteria live in Earth’s cloud droplets there is as yet no evidence that they reproduce there. That may be because the experiment is hard to do, but it may also be because they have no particular need to do so the Earth’s surface, and the creatures that roam across it, provide bacteria with all the locales for reproduction they could possibly want.

It is a beguiling story of life finding a way. But it remains very speculative. If the phosphine is indeed present as described, there needs to be a strenuous effort to find, or rule out, non-biological sources. The team behind the detection has done some of this it argues convincingly that the phosphine cannot come up from volcanoes, drift down from comets, or be made in mid-air through photochemistry. But the chemistry that happens on surfaces can be very different to what happens in mid air, and Venus’s atmosphere, as well as offering extremes of temperature, pressure and acidity, has surfaces to spare, both in its cloud decks and in the hazes that float above and below them. Imaginative chemists should have a field day working through ever more abstruse possibilities—and may make some very interesting discoveries of their own on the way.

Then there is the possibility of going to take a closer look. NASA has not launched a mission to Venus since the 1980s, though some of its spacecraft have swung past it on their way elsewhere. But two Venus missions have reached the final stage of the selection process for the next round of its “Discovery” program of small planetary missions. One, VERITAS, is an orbiter mainly intended to map the surface in more detail the other, DAVINCI+, features a small chemistry lab that would descend through the atmosphere beneath a parachute. If it can be made capable of detecting phosphine at a few parts per billion, the case for sending it would become even stronger than it already is. The next mission to Venus, though, is not American but Indian: the Shukrayaan-1 orbiter is currently pencilled in for launch in 2023, which should be enough time to put on a phosphine-optimised instrument. Meanwhile, Dr Seager has secured a grant from Breakthrough Initiatives, a research programme funded by Yuri Milner, a Russian billionaire, to investigate the scientific case for life on Venus and the technical challenges of a potential exploratory mission.


Colonies on small moons

One word: don't. Humans live at 1 g of gravity. Gas giants don't have surfaces, Venus has .9 g at the surface, Mars and Mercury have 0.38g. Our anatomy and physiology is designed for 1 g, it helps our bones calcify properly, helps our blood circulate, and does countless other things we may not even have discovered yet. Humans maybe could live a lifetime on Mars and Mercury. It is very unlikely they could live a lifetime at an even lower gravity. Luna, Titan, and the 4 Jovian moons have gravity of 0.12–0.18 g. Pluto, Triton, and various Kuiper belt objects are below 0.10.

A medium sized moon (like Rhea of Saturn, say) might have 0.027 g. This is way not enough for a human to live, but annoyingly much if you want to, say, launch things into space. Humans are better off living in space colonies that generate artificial gravity by rotating. If you want to exploit a moon, you are much better off with one that has a much lower surface gravity. Phobos, for example, has a radius of about 11 km, and a surface gravity of .0006 g, but still has $10^<16>$ kg of usable materials to mine.