Astronomy

How is the size of a distant planet determined?

How is the size of a distant planet determined?


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How is an object's size determined from other solar systems than our own? How are new large bodies being found in our solar system currently?


One method of finding planets orbiting stars is to analyze the transit effect. This is when a planet crosses the face of a star between the star and observer. This will block some of the light reaching the observer. By measuring this decrease in light from the star we can determine how much of the surface area of star is occluded.

The speed of a planet across the face of a star will let us calculate the orbital radius considering a circular orbit. The temperature of the star can be determined from its brightness and location on HR diagram and spectroscopic data. Combining this information lets us determine the size and of the planet.

Using space-based or ground-based telescopes and automated data collection and analysis methods large regions of the sky can be viewed for these transits. This method is highly accurate and will probably surpass the radial velocity method as most common method to discover new planets.

Another method for finding planets is known as the radial velocity method. When a planet orbits a star it is really the planet and star rotating about the center-of-mass of the system. Hence the star wobbles due to the bodies orbiting the star. It wobbles because usually the center-of-mass is inside the star and this results in the star rotating about an axis running thru the star but offset from the center. By doing a doppler analysis it can be determined whether the star is revolving or not and the period of revolution. The radial velocity method is the most effective method. The transit method is very limited by the viewing angle, the planet must pass between observer and star.

A good source to better learn about these methods is here.


The size of a planet is determined by

  1. The amount of gas & dust available to build it.
  2. The type of star it orbits.
  3. Its distance from the star.
  4. Whether or not the sort of resonances we see in Bode's Law come into play. We don't know for sure whether Bode's Law operates for all stars, some stars, or only our star.

How is the size of a distant planet determined? - Astronomy

The Kepler Space Telescope Spacecraft

You may have seen recent news stating that scientists have discovered several new planets that may support thanks to the Keplar Space Telescope, a spacecraft that is currently traveling through space and observing distant planets, stars, and solar systems. Thanks largely in part to the work being done by the Keplar Space Telescope, scientists have discovered around 2,300 planets that may be capable of supporting extraterrestrial life. However, this raises the question – what exactly is the Habitable Zone for planets, and what does this have to do with alien life?

One of the main features that many scientists believe can lead to discovery of life outside our own planet is finding another planet with life-supporting qualities similar to Earth. While this may seem to be a slightly close-minded approach regarding the types of life that may exist outside of the conditions we are familiar with, it does potentially provide better focus on what we should be looking for when we try to find extraterrestrial life, and ultimately increases our odds of finding it.

While many scientists don’t focus on finding a planet exactly like our own, one of the general conditions that most look for when trying to discover life in outer space is that the potentially life supporting planet supports the conditions necessary for liquid water. The requirements for having available liquid water on a planet depend on many factors, but one of the first indicators that scientists look for in determining whether or not a planet may have liquid water on it is its position to a star. As you know, Earth orbits the sun at a specific distance. This distance in relation to our own Sun provides for the ability of our own planet to harbor liquid water. Scientists are able to observe other solar systems and determine if any planets in those solar systems share a similar relationship with their own central star. While the central star, the planet in orbit, and the distance between the two do not need to be exactly like earth, the proper relationship between all three of those factors can determine whether or not a planet will support liquid water.

All stars have differing properties, including size, gravitational pull, and energy output. By taking into account all of these properties of a star, scientists can determine at what distance in orbit a planet would need to fall in order to have similar life-sustaining conditions as Earth, especially in regards to the ability to harbor liquid water. This determined distance generally covers a certain range, and this range is called the Habitable Zone. Once scientists determine the Habitable Zone for a star, they look and see if any planets orbit within that Habitable Zone. If a planet does, it is considered capable of harboring liquid water and the promise of it supporting life (at least life as we know it) is much greater.


The Search for Earth-like Planets

Problem 492: Alpha Centauri Bb - a nearby extrasolar planet? Students plot data for the orbiting planet and determine its orbit period. They use this in a simple formula to determine its distance, then they estimate its surface temperature at this distance. [Grade: 9-12 | Topics: graphing periodic data finding periods evaluating simple formulae ] [Click here]

Problem 465: Comparing Planets Orbiting other Stars Students use simple fraction arithmetic to determine the relative sizes of several new planets recently discovered by the Kepler mission, and compare these sizes to that of Jupiter and Earth. [Grade: 3-5 | Topics: scale models proportions fractions] [Click here]

Problem 458: Playing Baseball on the Earth-like Planet Kepler-22b! The recently-confirmed Earth-like planet Kepler-22b by the Kepler Observatory is a massive planet orbiting its star in the temperature zone suitable for liquid water. This problem explores the gravity and mass of this planet, and some implications for playing baseball on its surface! [Grade: 8-10 | Topics: scale models proportions scientific notation metric math Evaluating equations] [Click here]

Problem 441: Exploring the new planet Kepler 16b called 'Tatooine' Using the tangent function, students estimate the angular diameter and separation of the two stars in the Kepler 16 binary system as viewed from the planet's surface. if it had one!! [Grade: 8-10 | Topics: angle measure tangent] [Click here]

Problem 405: Discovering Earth-like Worlds by their Color Students use recent measurements of the reflected light from solar system bodies to graph their colors and to use this in classifying new planets as Earth-like, moon-like or Jupiter-liike [Grade: 6-8 | Topics: graphing tabular data interpreting graphical data] [Click here]

Problem 402: Kepler- Earth-like planets by the score! II Students use recent Kepler satellite data summarized in tabular form to estimate the number of planets in the Milky Way galaxy that are about the same size as our Earth, and located in their Habitable Zones were liquid water may exist. [Grade: 6-8 | Topics: Percentage re-scaling sample sizes] [Click here]

Problem 401: Kepler - Earth-like planets by the score! I Students use recent Kepler satellite data to estimate the number of Earth-like planets in the Milky Way galaxy. [Grade: 6-8 | Topics: Percentage histograms Re-scaling sample sizes] [Click here]

Problem 396: Kepler 10b - A matter of gravity Students use the measured properties of the Earth-like planet Kepler 10b to estimate the weight of a human on its surface. [Grade: 8-10 | Topics: Evaluating formulas mass = density x volume volume of a sphere scientific notation] [Click here]

Problem 376: The Earth-like Planet Gliese 518g Students use data for the Gliese 581 planetary system to draw a scaled model of the locations and sizes of the discovered planets. They also identify the location and span of the Habitable Zone for this planetary system. [Grade: 3-5 | Topics: scale models measurement] [Click here]

Problem 360: Kepler's First Look at 700 Transiting Planets A statistical study of the 700 transits seen during the first 43 days of the mission. [Grade: 6-8 | Topics: Percentages area of circle] [Click here]

Problem 333: Hubble: Seeing a Dwarf Planet Clearly Based on a recent press release, students use the published photos to determine the sizes of the smallest discernible features and compare them to the sizes of the 48-states in the USA. They also estimate the density of Pluto and compare this to densities of familiar substances to create a 'model' of Pluto's composition. A supplementary Inquiry Problem asks students to model the interior in terms of two components and estimate what fraction of Pluto is composed of rock or ice. [Grade: 8-12 | Topics: scales and ratios volume of sphere density=mass/volume] [Click here]

Problem 331: Webb Space Telescope: Detecting dwarf planets The 'JWST' will be launched some time in 2021-22. One of its research goals will be to detect new dwarf planets beyond the orbit of Pluto. In this problem, students use three functions to predict how far from the sun a body such as Pluto could be detected, by calculating its temperature and the amount of infrared light it emits. [Grade: 9-12 | Topics: Evaluating square-roots and base-e exponentials] [Click here]

Problem 325: Kepler Spies Five New Planets Students count squares on a Bizarro Star to study the transit of a planet, and determine the diameter of the planet. This demonstrates the basic principle used by NASA's Kepler satellite to search for Earth-sized planets orbiting distant stars. [Grade: 4-6 | Topics: Counting graphing area of a square] [Click here]

Problem 213: Kepler: The hunt for Earth-like planets Students compare the area of a star with the area of a planet to determine how the star's light is dimmed when the planet passes across the star as viewed from Earth. This is the basis for the 'transit' method used by NASA's Kepler satellite to detect new planets. [Grade: 6-8 | Topics: Area of circle ratios percents.] [Click here]

Problem 197: Hubble Sees a Distant Planet Students study an image of the dust disk around the star Fomalhaunt and determine the orbit period and distance of a newly-discovered planet orbiting this young star. [Grade: 6-10| Topics: Calculating image scales Circle circumferences Unit conversions distance-speed-time] [Click here]

Problem 168: Fitting Periodic Functions - Distant Planets Students work with data from a newly-discovered extra-solar planet to determine its orbit period and other parameters of a mathematical model. [Grade: 9-12 | Topics: trigonometry functions algebra] [Click here]

Problem 160: The Relative Sizes of the Sun and Stars Students work through a series of comparisons of the relative sizes of the sun compared to other stars, to create a scale model of stellar sizes using simple fractional relationships. ( e.g if Star A is 6 times larger than Star B, and Star C is 1/2 the size of Star B, how big is Star C in terms of Star A?) [Grade: 4-6 | Topics: working with fractions scale models] [Click here]

Problem 156: Spectral Classification of Stars Students use actual star spectra to classify them into specific spectral types according to a standard ruberic. [Grade: 5-8 | Topics: Working with patterns in data simple sorting logic] [Click here]

Problem 155: Tidal Forces: Let 'er rip! Students explore tidal forces and how satellites are destroyed by coming too close to their planet. [Grade: 7-10| Topics: Algebra number substitution] [Click here]

Problem 141: Exploring a Dusty Young Star Students use Spitzer Space Telescope data to learn about how dust emits infrared light and calculate the mass of dust grains from a young star in the nebula NGC-7129. [Grade: 4 - 7 | Topics: Algebra I multiplication, division scientific notation] [Click here]


EXPRES looks to the skies of a scorching, distant planet

The planet MASCARA-2 b, a Jupiter-like gas giant roughly 2.68 quadrillion miles from Earth. Credit: Sam Cabot

Yale technology is giving astronomers a closer look at the atmosphere of a distant planet where it's so hot the air contains vaporized metals.

The planet, MASCARA-2 b, is 140 parsecs from Earth—or roughly 2.68 quadrillion miles. It's a gas giant, like Jupiter. However, its orbit is 100 times closer to its star than Jupiter's orbit is to our sun.

The atmosphere of MASCARA-2 b reaches temperatures of more than 3,140 degrees Fahrenheit, putting it on the extreme end of a class of planets known as hot Jupiters. Astronomers are keenly interested in hot Jupiters because their existence had been unknown until 25 years ago and they may offer new information about the formation of planetary systems.

"Hot Jupiters provide the best laboratories for developing analysis techniques that will one day be used to search for biosignatures on potentially habitable worlds," said Yale astronomer Debra Fischer, the Eugene Higgins Professor of Astronomy and co-author of a new study that has been accepted by the journal Astronomy and Astrophysics.

Fischer is the guiding force behind the instrument that made the discovery possible: the Extreme PREcision Spectrometer (EXPRES), which was built at Yale and installed on the 4.3-meter Lowell Discovery Telescope near Flagstaff, Ariz.

The primary mission of EXPRES is finding Earth-like planets based on the slight gravitational influence they have on their stars. This precision also comes in handy when looking for atmospheric details of far-away planets, the researchers said.

Credit: Yale University

Here's how it works. As MASCARA-2 b crosses the direct line of sight between its host star and Earth, elements in the planet's atmosphere absorb starlight at specific wavelengths—leaving a chemical fingerprint. EXPRES is able to pick up those fingerprints.

Using EXPRES, Yale astronomers and colleagues from the Geneva Observatory and Bern University in Switzerland, as well as the Technical University of Denmark, found gaseous iron, magnesium, and chromium in MASCARA-2 b's atmosphere.

"Atmospheric signatures are very faint and difficult to detect," said co-author Sam Cabot, a graduate student in astronomy at Yale and leader of the study's data analysis. "Serendipitously, EXPRES offers this capability, since you need very high-fidelity instruments to find planets outside our own solar system."

The study's lead author, astronomer Jens Hoeijmakers of the Geneva Observatory, said EXPRES also found evidence of different chemistry between the "morning" and "evening" sides of MASCARA-2 b.

"These chemical detections may not only teach us about the elemental composition of the atmosphere, but also about the efficiency of atmospheric circulation patterns," Hoeijmakers said.

Along with other advanced spectrometers such as ESPRESSO, built by Swiss astronomers in Chile, EXPRES is expected to collect a wealth of new data that may dramatically advance the search for exoplanets—planets orbiting stars other than our own sun.

"The detection of vaporized metals in the atmosphere of MASCARA-2 b is one of the first exciting science results to emerge from EXPRES," Fischer said. "More results are on the way."


Seeing a Distant Planet

After eight years and repeated photographs of a nearby star in hopes of finding planets, University of California, Berkeley, astronomer Paul Kalas finally has his prize: the first visible-light snapshot of a planet outside our solar system.

Only 25 light years from Earth, the planet &ndash probably close to the mass of Jupiter – orbits the star Fomalhaut at a distance about four times that between Neptune and the sun. Formally known as Fomalhaut b, the planet could have a ring system about the dimension of Jupiter’s early rings, before the dust and debris coalesced into the four Galilean moons.

The planet’s existence was suspected in 2005, when images Kalas took with the Hubble Space Telescope’s Advanced Camera for Surveys showed a sharply defined inner edge to the dust belt around Fomalhaut, in the southern constellation Piscus Austrinus (southern fish). The sharp edge and off-center belt suggested to Kalas that a planet in an elliptical orbit around the star was shaping the inner edge of the belt, much like Saturn’s moons groom the edges of its rings.

"The gravity of Fomalhaut b is the key reason that the vast dust belt surrounding Fomalhaut is cleanly sculpted into a ring and offset from the star," Kalas said. "We predicted this in 2005, and now we have the direct proof."

"It will be hard to argue that a Jupiter-mass object orbiting an A star like Fomalhaut is anything other than a planet," said coauthor James R. Graham, professor of astronomy at UC Berkeley. "That doesn’t mean it’s exactly what we expected when we went hunting for planets in this system."

The discovery was reported Nov. 14 on Science Express, an online site that posts articles in advance of their print publication in the journal Science.

The Science paper is complemented by an article to appear in The Astrophysical Journal (ApJ) that analyzes the interaction between the planet and the dust belt surrounding Fomalhaut and cinches the estimation of the planet’s mass.

"Every planet has a chaotic zone, which is basically a swath of space that encloses the planet’s orbit and from which the planet ejects all particles," said Eugene Chiang, a UC Berkeley associate professor of astronomy and of earth and planetary science, and first author of the ApJ paper. "This zone increases with the mass of the planet, so, given the size of the chaotic zone around Fomalhaut b, we can estimate that its likely mass is in the vicinity of one Jupiter mass."

Kalas, Graham, Chiang and UC Berkeley graduate student Edwin S. Kite are coauthors of both papers, along with Mark Clampin of the Goddard Space Flight Center in Greenbelt, Md. The other authors of the Science paper are Michael P. Fitzgerald of the Institute of Geophysics & Planetary Science at Lawrence Livermore National Laboratory, and John Krist and Karl Stapelfeldt of the Jet Propulsion Laboratory in Pasadena, Calif.

Kalas has focused on the star Fomalhaut since his days 15 years ago as a graduate student, when he used early optical CCD technology to look for dust surrounding Fomalhaut. In 1998, submillimeter-wavelength radio observations of the disk showed that cold dust was distributed in a ring around the central star, much like the ring of comets called the Kuiper Belt in our solar system.

In 2004, Kalas began using Hubble’s Advanced Camera for Surveys to probe the dust belt and, three years ago, he showed that the disk’s inner edge was sharply delineated, suggesting that an unseen planet orbits Fomalhaut and shapes the edge.

He now has two photographs of the planet, taken in 2004 and 2006, which show that its movement over a 21-month period exactly fits what would be expected from a planet orbiting Fomalhaut every 872 years at a distance of 119 astronomical units, or 11 billion miles. One astronomical unit (AU) is the average distance between the Earth and the sun, or 93 million miles.

"I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star," Kalas said. "It’s a profound and overwhelming experience to lay eyes on a planet never before seen."

Because of the relatively low mass and distant orbit of the planet, it cannot be detected by today’s standard technique, which is to measure the tiny wobble a planet induces in its star. Astronomers hoping to photograph extrasolar planets have taken a different approach, looking in the infrared at young stars in hopes of detecting the warm glow from cooling planets. Fomalhaut b is not seen in the infrared, however. Instead, Kalas snapped photos in visible light, which for most planets would be impossible because of the bright glare of the star. Even with Hubble’s coronagraph to block the star’s light, the new planet would not have been detected if it had been much closer to its star, or much dimmer.

"To make this discovery at optical wavelengths is a complete surprise," he said. "If we’re seeing light in reflection, then it must be because Fomalhaut b is surrounded by a planetary ring system so vast it would make Saturn’s rings look pocket-sized by comparison. Fomalhaut b may actually show us what Jupiter and Saturn resembled when the solar system was about a hundred million years old."

Kalas noted that all other reported images of extrasolar planets have either been disproven or the planets have masses so uncertain they could be brown dwarfs, which are failed stars with masses above 13 Jupiter masses and that glow bright in the infrared. The Fomalhaut science team, on the other hand, used not only the observed brightness of Fomalhaut b to estimate the mass, but also the fact that a massive planet or brown dwarf would push Fomalhaut’s dust belt farther away than what is observed. Chiang thinks he can narrow the mass to between 0.3 and 2 Jupiter masses, most likely one Jupiter mass.

"Any more massive than that and its gravity would destroy the vast dust belt encircling the star," Kalas said.

The belt extends from 133 to about 200 AU from the star, and Chiang estimates that, in dust alone, it has a mass that exceeds three Earths.

"There’s so much solid material in the belt that it could easily form the basis of a core for a Jupiter-mass planet. This is consistent with Fomalhaut b having formed in situ, at an unprecedentedly large distance from its host star," Chiang said.

Fomalhaut is about 200 million years old and will burn out in about a billion years, making it a short-lived star compared to our sun, which is now about 4.5 billion years old and expected to burn another 5 billion years.

Its short life is a result of being 16 times brighter than the sun, but this also makes the star appear from the planet’s surface about as bright as our sun appears from Neptune, despite the fact that it lies four times farther from Fomalhaut than Neptune does from the sun, Kalas said. "Fomalhaut b sits in a frigid location, but it’s not too different from that of Neptune in our solar system," he said.

Interestingly, the planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations. Fomalhaut b could have a hot outer atmosphere heated by bubbling convection cells, or the brightness change could indicate there is hot gas at the inner boundary of the ring around the planet.

"This is not your theorist’s ideal planet, it is obviously more complicated than we thought," Graham said.

The researchers are awaiting repair of Hubble’s Advanced Camera for Surveys and the Near Infrared Camera to resume their observations to confirm the planet’s orbit, discover the source of its unusual brightness in the visible, and perhaps find other planets.

"There is plenty of empty space between Fomalhaut b and the star for other planets to happily reside in stable orbits," Kalas said. "We’ll probably have to wait for the James Webb Space Telescope to give us a clear view of the region closer to the star where a planet could host liquid water on the surface."

The research was supported by National Aeronautics and Space Administration and the National Science Foundation.


Age of Distant Planets in the Milky Way

In recent years, astronomers have discovered hundreds of new planets from distant parts of our galaxy, the Milky Way.

Our knowledge of most of the new planets is sparse, but now scientists have dug into the planetary past to identify the age of many of these so-called exoplanets (see fact box).

"It is the first time we have determined the age of such a large group of planets. So far, we had only measured the exact age of a handful of exoplanets, but now we have done it for 33 stars and their planets,” says Victor Silva Aguirre, assistant professor at the Stellar Astrophysics Centre at Aarhus University in Denmark.

“Some of these stars are much older than the Sun, whilst others are a bit younger. This tells us that planets have been created throughout the galaxy's history," he says.

Aguirre was the lead scientist of the new research, published in the scientific journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Researcher: Great Work

At the University of Copenhagen, associate professor Uffe Jørgensen Gråe, is excited about the new study.

"Their work is great because it gives us a fundamental insight into how our own solar system has evolved compared to others. They have measured the age of the planets with the best possible method," says Jorgensen, who studies planets at the Niels Bohr Institute at the University of Copenhagen.

According to Jorgensen, who was not involved with the study himself, the new results also provide insights into the size of the stars and their planets.

"They have determined the radius, density, and age of the planetary stars. This is absolutely fundamental to understand each of the individual stars and planets," says Jørgensen.

New Results Hint at Planets' Potential for Life

According to Aguirre, the new study also provides insight of the potential for life to exist on any of the planets.

"When you are searching for life in space, you look for planets resembling Earth. Therefore, among other things you have to know how old a planet is. We know that the earth was created about 4.5 billion years ago, but life arose only later," says Aguirre.

He points out that some of the 33 stars identified in the new study, have more than one planet in orbit around them, so a total of 56 exoplanets are included in the study. Some of the planets are only half as old as the Earth, while others are up to two and half times older.

"We find stars and planets of all ages," says Aguirre.

In terms of size, previous research showed that whilst some of the 56 planets are smaller than Earth, some are up to 10 times as large.

Based on Data from Kepler

The new study is based on data collected by the Kepler telescope, currently flying around in space.

Kepler cannot directly observe exoplanets. Unlike stars, they are not illuminated in space. So, scientists can only detect them as they orbit around their star and cast a shadow as they pass by.

But when you cannot even see the planets directly, how do you measure their size and age?

According to Aguirre you do this using a method called astroseismology.

Looking at Starquakes

Astroseismology is the study of stellar vibrations -- so-called starquakes, resembling the more familiar earthquakes.

"Space telescopes like Kepler can measure these starquakes because the light from the star changes when the star's surface is moving back and forth," says Aguirre.

Measuring the light emitted by the star gives scientists information about the vibrations at its surface, and this can provide clues about the star’s age.

Aguirre explains that once you have found the stellar age, then you also know the age of the planets orbiting it.

"It is estimated that the planets have the same age as the stars they orbit around. Earth, for example, is the same age as its star, the Sun, because they formed from the same cloud of gas," says Aguirre.

Starquakes Can Tell Us the Age of Stars

The fusion processes that are continuously active inside stars help to calculate a star’s age.

During this process, hydrogen is converted to helium. So, the older the star, the more helium it contains.

Furthermore, the chemical composition of a star will determine how the star's surface shakes. In other words, starquakes tell us not only about the star’s chemical composition, but also how old it is.

"The researchers in Aarhus are leading the world in determining stellar ages using astroseismology. And it is the most accurate method we have to determine the age of the individual stars," says Jørgensen.

Far Greater Precision

"It is very difficult to determine stellar ages. There are other ways to do it, but they are not as precise as astroseismology," he says.

"With astroseismology we can achieve a margin of error of 15 per cent, but other methods have much larger uncertainties where the margin of error can easily reach 50 per cent or more," says Aguirre.


How is the size of a distant planet determined? - Astronomy

Eris is a dwarf planet, only slightly smaller than Pluto. Mike Brown, Chad Trujillo and David Rabinowitz discovered it at Palomar Observatory. They had already found other trans-Neptunian objects (TNOs), i.e., objects orbiting beyond Neptune. But it caused a sensation when Brown and NASA announced the new discovery as a tenth planet, on the basis that it was bigger than Pluto. This seemed to be true at the time.

Discovery
Eris was discovered in 2005 after a new analysis of 2003 images. The Minor Planet Center of the International Astronomical Union (IAU) gave the new object the provisional designation 2003 UB313.

Although they found 2003 UB313 in January 2005, there was more to come. In October Mike Brown and a team at Keck Observatory in Hawaii used adaptive optics to image Eris. This technique corrects some of the image blurring that our atmosphere causes, and it showed that Eris had a moon.

Yet if being the tenth planet was a sensation, an even bigger one was the IAU deciding to define what a planet was. Some people are still annoyed by their decision, because planets ended up as something for which neither Pluto nor Eris qualified. They both became dwarf planets when the definitions were approved in August 2006.

What's in a Name?
The two new discoveries couldn't get names until the IAU sorted out its definitions, because different categories of object have different naming conventions. Meanwhile the discovery team called the new “planet” Xena after an American TV series Xena Warrior Princess. Xena's sidekick was Gabrielle, so that's what they called the moon.

When Eris was officially a dwarf planet, the Minor Planet Center gave it the number 136199, and the discovery team was allowed to propose names for it and its moon. From Greek mythology they offered the name Eris for the dwarf planet and Dysnomia (Eris's daughter) for the moon. Appropriately, after all the fuss that followed their discovery, Eris was the goddess of strife and discord, and Dysnomia the spirit of lawlessness.

Dysnomia
Dysnomia orbits 37,350 km (23,200 mi) from Eris in a nearly circular orbit taking almost sixteen days. No one knows how big the moon is, but there are several estimates based on how much light it reflects. Unfortunately, no one knows that either. Mike Brown says the moon is 500 times fainter than Eris and suggests that therefore it may be 100 km (60 mi) in diameter. Based on different assumptions, it could be as big as 250 km (150 mi) in diameter.

Without knowing its size, nonetheless the moon was useful. In you want the mass of a distant planet, you can't put it on a giant weighing scale. But if it has a moon, scientists can calculate the mass based on the orbit of the moon. As it is, we have a good value for Eris's mass, and that's how we know that it's more massive than Pluto.

Eris's orbit
The Kuiper Belt is a broad belt of icy bodies orbiting beyond Neptune at about 30-50 AU. (The astronomical unit (AU) is equal to the Earth-Sun distance.) Pluto is there, but Eris is in the scattered disk. Objects in the scattered disk were scattered by the gravitational influence of Neptune in the early Solar System. They were thrown into highly tilted and elongated orbits.

It takes 558 Earth years for Eris to orbit the Sun on a path that's inclined 44° to the ecliptic plane. The ecliptic is the plane in which most objects in the Solar System orbit. Here is a comparison between the orbits of Eris, Pluto and three outer planets. The parts of orbits below the ecliptic are shown in darker colors. The diagram on the left is a polar view.

At its closest to the Sun, Eris is 38 AU away. It's sometimes closer in than Pluto is. But its most distant point is 98 AU from the Sun. Although there are over three dozen trans-Neptunian objects whose average distance from the Sun is greater than that of Eris, at the time Eris and Dysnomia were discovered, they were the most distant known natural Solar System objects, except for long-period comets.

A year is centuries long on Eris, but a month is scarcely sixteen days, and a day is only two hours longer than an Earth day, 26 hours.

Size, mass, density, composition
Eris's diameter is 2326 km (1445 mi), no more than 50 km (30 mi) smaller than Pluto's. Since Eris's mass is 27% greater than that of Pluto, we know that Eris is denser.

From Eris's density, scientists conclude that it's a rocky body surrounded by an ice mantle about 100 km (60 mi) thick. Topping it off is a thin (maybe less than 1 mm) surface layer, which is a mixture of frozen nitrogen and methane. According to models of planetary formation, Eris should have a thicker ice layer, but it may have been lost in a large impact. An impact may also have formed its moon as one did our own Moon.

Temperature and atmosphere
There are a number of estimates for Eris's varying temperature. One calculation suggests that during its year the temperature ranges from -243°C to -217°C (-405°F to -359°F).

Contrary to appearances, Eris probably has an atmosphere, but in an unexpected place: on the ground. Eris is so far from the sun that the atmosphere has collapsed and frozen, and is glazing the dwarf planet's surface. It makes Eris almost as bright as Saturn's moon Enceladus, and 96% of the sunlight it receives is reflected back into space. In about 250 years, Eris will be at its closest to the Sun, with enough heat for its frozen gases to sublimate, i.e., turn directly from a solid to a gas. Then it should have an atmosphere again.

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Raissa Estrela (JPL): "Exoplanets atmospheres: evolution and habitability"

Abstract: Most of the small exoplanets detected have a size that is unusual in our Solar System (SS). They also seem to have an evolution that in some cases might differ from that of the terrestrial planets in our SS. Evidence of that is seen in the observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a 'gap'. This gap appears to define a transition region in which sub-Neptune planets are believed to have lost their primordial H/He envelope. These small planets could have developed a secondary atmosphere and should attract attention in the next years. Here we investigate the evolution of the atmosphere of observed small close-in planets by looking into the relationships between their radius, insolation, and density, and by tracking the evolution of their envelope due to photoevaporation. We also show that secondary atmospheres in small exoplanets can be detected using haze and we show that haze can be widely characterized using observations taken with HST/STIS. Although the presence of a secondary atmosphere can be one of the key factors for the habitability of terrestrial planets, other factors can also have a major influence, such as the activity of the host star. Here we determine the habitability of terrestrial planets under the environment of a flaring star M dwarf and G-type stars.


Curious Tilt of the Sun Traced to Undiscovered Planet

Planet Nine—the undiscovered planet at the edge of the solar system that was predicted by the work of Caltech's Konstantin Batygin and Mike Brown in January 2016—appears to be responsible for the unusual tilt of the sun, according to a new study.

The large and distant planet may be adding a wobble to the solar system, giving the appearance that the sun is tilted slightly.

"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," says Elizabeth Bailey, a graduate student at Caltech and lead author of a study announcing the discovery.

All of the planets orbit in a flat plane with respect to the sun, roughly within a couple degrees of each other. That plane, however, rotates at a six-degree tilt with respect to the sun—giving the appearance that the sun itself is cocked off at an angle. Until now, no one had found a compelling explanation to produce such an effect. "It's such a deep-rooted mystery and so difficult to explain that people just don't talk about it," says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy.

Brown and Batygin's discovery of evidence that the sun is orbited by an as-yet-unseen planet—that is about 10 times the size of Earth with an orbit that is about 20 times farther from the sun on average than Neptune's—changes the physics. Planet Nine, based on their calculations, appears to orbit at about 30 degrees off from the other planets' orbital plane—in the process, influencing the orbit of a large population of objects in the Kuiper Belt, which is how Brown and Batygin came to suspect a planet existed there in the first place.

"It continues to amaze us every time we look carefully we continue to find that Planet Nine explains something about the solar system that had long been a mystery," says Batygin, an assistant professor of planetary science.

Their findings have been accepted for publication in an upcoming issue of the Astronomical Journal, and will be presented on October 18 at the American Astronomical Society's Division for Planetary Sciences annual meeting, held in Pasadena.

The tilt of the solar system's orbital plane has long befuddled astronomers because of the way the planets formed: as a spinning cloud slowly collapsing first into a disk and then into objects orbiting a central star.

Planet Nine's angular momentum is having an outsized impact on the solar system based on its location and size. A planet's angular momentum equals the mass of an object multiplied by its distance from the sun, and corresponds with the force that the planet exerts on the overall system's spin. Because the other planets in the solar system all exist along a flat plane, their angular momentum works to keep the whole disk spinning smoothly.

Planet Nine's unusual orbit, however, adds a multi-billion-year wobble to that system. Mathematically, given the hypothesized size and distance of Planet Nine, a six-degree tilt fits perfectly, Brown says.

The next question, then, is how did Planet Nine achieve its unusual orbit? Though that remains to be determined, Batygin suggests that the planet may have been ejected from the neighborhood of the gas giants by Jupiter, or perhaps may have been influenced by the gravitational pull of other stellar bodies in the solar system's extreme past.

For now, Brown and Batygin continue to work with colleagues throughout the world to search the night sky for signs of Planet Nine along the path they predicted in January. That search, Brown says, may take three years or more.