Illustration by David A. Aguilar (CfA)

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The planet GJ 1214b orbits its dim, red host star in an artist's conception.

Illustration by David A. Aguilar (CfA)

Earth-Size Planet Has Hot and Steamy Atmosphere?

May be "most unique exoplanet we've found so far," expert says.

The atmosphere of a "super Earth" has been analyzed for the first time—and it ain't pretty.

The extrasolar planet, or exoplanet, GJ 1214b was discovered last year orbiting a dim, red star about 40 light-years from Earth. The planet is about 2.7 times larger than Earth and about 6.5 times more massive.

Based on the planet's density, astronomers estimate that GJ 1214b would be about three-quarters water with a solid core and an atmosphere—not unlike Earth.

But it seems the similarities stop there. The planet is so close to its star that any water would be turned to vapor, and the atmosphere should be so thick that the pressure would be immense.

Now new measurements show that GJ 1214b's atmosphere is made of either dense, ultrahot steam or a noxious, cloudy haze of hydrogen.

"Either way, it would be unpleasant if you were there," said Greg Laughlin, an astronomer at the University of California, Santa Cruz, who was not involved in the study.

Starlight Carries Chemical Imprints

Astronomers were able to "sample" the planet's atmosphere by watching the world pass in front of its host star, as seen from Earth.

Using the European Southern Observatory's Very Large Telescope in Chile, the team captured the light filtered around the edges of the planet when the world was blocking the star.

"That light has imprinted on it the signature of chemicals in the atmosphere," said study leader Jacob Bean of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Similar measurements have revealed gases such as hydrogen and sodium in the atmospheres of so-called hot Jupiters, exoplanets that are more massive than our gas giants but that orbit very close to their host stars.

This is the first time, however, that astronomers have been able to analyze the atmosphere of an Earth-size planet.

Planet Has "Noxious Soup" or Steamy Blanket?

The data show that GJ 1214b's atmosphere isn't, as previously theorized, a puffy, cloud-free layer of mainly hydrogen gas. Instead the atmosphere appears to block a significant amount of light, the team reports in this week's issue of the journal Nature.

For such an atmosphere to be made mostly of hydrogen, it must be topped by a thick layer of clouds like the atmosphere of Venus.

If that's the scenario, "it would make the atmospheres around the worst oil refineries look absolutely pristine," UC Santa Cruz's Laughlin said. Light that does filter through the haze would trigger chemical reactions in the hydrogen gas, creating a "noxious soup of stuff" near the planet's surface.

However, based on the new data, it's also possible that GJ 1214b's atmosphere is a dense blanket of hot water vapor, aka steam.

In that case, "it would be the most unique exoplanet we've found so far," study leader Bean said. "We have nothing like that in our solar system."