Gallery: Super-Earth Atmosphere May Be Mostly Water
01gj-1214b-bsar
The first direct measurement of a super-Earth exoplanet's atmosphere finds the world is either shrouded in steam or covered in clouds. "This is the first probe of an atmosphere of a super-Earth planet," said exoplanet observer [Jacob Bean](https://www.cfa.harvard.edu/~jbean/) of the Harvard-Smithsonian Center for Astrophysics, lead author of a paper describing the cloudy world in the Dec. 2 *Nature.* "It's a real big step in the direction of doing this kind of work for a planet that's potentially habitable." The planet, called [GJ 1214b](http://stag-komodo.wired.com/wiredscience/2009/12/super-earth/), is the smallest planet yet to have its atmosphere examined -- but it's just the latest in nearly a decade of probing exoplanet atmospheres. The others have all been gas giants. When the first exoplanet atmosphere was measured in 2002, many astronomers dismissed it as a one-time success. Now, just 8 years later, exo-atmosphere studies are a thriving field. Astronomers hope eventually to find true twins of Earth: small rocky planets with liquid water and atmospheres that could support life. Teasing out which molecules make up exo-atmospheres will be crucial to that search. "Ultimately the goal is to try to look for biosignatures," Bean said. "This work is another sort of milestone on this road. We're going directly towards that." This gallery traces the history of the study of exoplanet atmospheres, and looks forward to how astronomers plan to search for the real exo-Earth. GJ 1214b -------- This planet was hailed as the most Earth-like exoplanet yet when announced almost exactly a year ago. It was only the second super-Earth -- a planet with a mass between about 2 and 10 times Earth's -- found to pass in front of its star, or transit. The amount of light the planet blocked as it eclipsed its star told astronomers how big the planet was, about 2.7 times as wide as Earth. Follow-up measurements of the planet's gravitational tug on the star showed it was 6.5 times Earth's mass. Taken together, these two numbers tantalizingly suggested the planet could be one big, hot ocean world. But it could also be a kind of mini-Neptune, with a solid core and an atmosphere of hydrogen and helium, or a rocky planet with a huge atmosphere made of hydrogen. Bean and colleagues measured the color of the starlight as it filtered through the thin ring of GJ 1214b's atmosphere. They made 197 separate observations using the [Very Large Telescope](http://www.eso.org/public/teles-instr/vlt.html) in Chile, 88 of which caught the planet passing in front of the star. "The chemical components of the atmosphere imprint their fingerprints on that light, and we can measure that," Bean said. Surprisingly, the light that reached the ground-based telescope was almost featureless -- it didn't appear to have interacted with any interesting molecules at all. Rather than suggesting there's no atmosphere, Bean says, the lack of spectral features rules out a puffy hydrogen atmosphere. Atmospheres made mostly of hydrogen extend high above the planet's surface, because hydrogen is so light. Starlight passing through the planet's atmosphere has a high chance of interacting with these molecules. But if the atmosphere is mostly composed of something heavier than hydrogen, the planet's gravity will scrunch the atmosphere closer to the surface. That means most of the starlight misses the molecules -- which, ironically, suggests these heavy molecules are actually there. "I'm jealous of his data," said NASA exoplanet observer [Drake Deming](http://ssed.gsfc.nasa.gov/code693/vitae/drakedeming.html), who was not involved in the new work. "His data are really of superb quality." Based on planetary formation theories, the atmosphere is most likely to be a thick veil of water vapor. But the planet could still have a hydrogen atmosphere full of clouds, which can block starlight and make a puffy hydrogen atmosphere look a lot like a dense water atmosphere. Either way, the planet is "unequivocally not habitable," Bean said. "It's much too hot. But this is the coolest (in terms of temperature, not "coolest") planet we've done this observation on. You can see the progression toward a planet that really will be potentially habitable." "I love this new planet, because it's so mysterious," said exoplanet expert [Sara Seager](http://seagerexoplanets.mit.edu/) of MIT, who laid some of the theoretical groundwork for studying atmospheres of other worlds. "It's definitely a milestone in exoplanet history." *Image: Paul A. Kempton*
02corot-7b-bsar
COROT-7b -------- Another mile marker in studying what exoplanets are made of was [COROT-7b](http://stag-komodo.wired.com/wiredscience/2009/09/rockyexoplanet/). Discovered in February 2009, it was the first super-Earth found transiting in front of its star, which told astronomers the planet was mostly made of rock. *Image: ESO/L. Calcada*
03hd-209458b-bsar
HD 209458b ---------- Gas giant HD 209458b was the first exoplanet ever to have its atmosphere probed, when Harvard astronomer [David Charbonneau](https://www.cfa.harvard.edu/~dcharbon/Site/Welcome.html) and colleagues trained the Hubble Space Telescope on the Jupiter-sized world in 2002. That first observation found signals of atomic sodium. [Later measurements](http://stag-komodo.wired.com/wiredscience/2009/10/life-ingredients-exoplanet/) with Hubble and the Spitzer Space Telescope found traces of methane, carbon dioxide and water -- some of the basic ingredients of life. *Image: NASA/JPL-Caltech/T. Pyle (SSC)*
04hd189733b-bsar
HD 189733b ---------- Another way to probe exoplanet atmospheres is by measuring the light emitted by a transiting planet just before it slips out of view behind its star. That's how a team led by Caltech astronomer [Carl Grillmair](http://web.ipac.caltech.edu/staff/carl/) determined that gas giant HD 189733b has [methane](http://stag-komodo.wired.com/science/space/news/2008/03/exoplanet) in its atmosphere in 2008. *Image: NASA, ESA, and G. Bacon (STScI)*
05fomalhaut-b
Fomalhaut b ----------- Because Earth-sized exoplanets are so small, the best bet for studying their atmospheres is erasing the starlight and taking the planets' pictures directly. The first exoplanet to have its portrait taken was [Fomalhaut b](http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html), captured by Hubble in 2008. *Image: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center), M. Fitzgerald (Lawrence Livermore National Laboratory), and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory)*
06hr-8799
HR 8799 ------- The first system of planets to have its portrait taken orbits the star [HR 8799](http://stag-komodo.wired.com/wiredscience/2008/11/first-direct-im/), captured by ground-based telescopes in Hawaii and California in 2008. *Image: NASA/JPL-Caltech/Palomar Observatory*
07new-worlds-observer
Finding Earth Twins ------------------- Future prospects for finding Earth twins could involve placing a giant, flower-shaped shield in front of a space telescope to block starlight, like the proposed [New Worlds Observer](http://newworlds.colorado.edu/downloads/NWO.htm). A similar star shield was proposed for the [James Webb Space Telescope](http://stag-komodo.wired.com/wiredscience/2010/11/james-webb-overruns/), the planned successor to Hubble that recently sank into a pit of budget troubles. "Right now things are kind of a disaster with James Webb," Seager said. "It's not clear if there will be money for this new thing, but we're hopeful that things will work out. Some people are even hopeful that because James Webb is delayed, there will be more time for technology to develop." *Image: NASA and Northrop Grumman*
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