The planet, called GJ 1214b, 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 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, 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 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

Authors: Lisa Grossman

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