NASA Identifies Telltale Signs of a Super Earth’s Atmosphere

2009121600001224 A team of astronomers, including two NASA Sagan Fellows, has made the first characterizations of a super-Earth's atmosphere, by using a ground-based
telescope. A super-Earth is a planet up to three times the size of Earth and weighing up to 10 times as much. The findings are a significant milestone toward eventually being able to probe the atmospheres of Earth-like planets for signs of life.

The team determined the planet, GJ 1214b, is either blanketed with a thin layer of water
steam or surrounded by a thick layer of high clouds. If the former, the planet itself would
have an icy composition. If the latter, the planet would be rocky or similar to the
composition of Neptune, though much smaller.

"This is the first super-Earth known to have an atmosphere," said Jacob Bean, a NASA
Sagan Fellow and astronomer at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Mass. "But even with these new measurements, we can't say yet what that
atmosphere is made of. This world is being very shy and veiling its true nature from us."

GJ 1214b, first discovered in December 2009, is 2.7 times the size of Earth and 6.5 times
as massive. Previous observations of the planet's size and mass demonstrated it has a low
density for its size, leading astronomers to conclude the planet is some kind of solid body
with an atmosphere.

The planet orbits close to its dim star, at a distance of 0.014 astronomical units. An
astronomical unit is the distance between Earth and the sun, approximately 93 million
miles. GJ 1214b circles too close to its star to be habitable by any life forms.

Bean and his team observed infrared light as the planet crossed in front of its star. During
such transits, the star's light filters through the atmosphere. Gases absorb the starlight at
particular wavelengths, leaving behind chemical fingerprints detectable from Earth. This
same type of technique has been used to study the atmospheres of distant "hot Jupiters,"
or Jupiter-like planets orbiting close to their stars, and found gases like hydrogen,
methane and sodium vapor.

In the case of the super-Earth, no chemical fingerprints were detected; however, this
doesn't mean there are no chemicals present. Instead, this information ruled out some
possibilities for GJ 1214b's atmosphere, and narrowed the scope to either an atmosphere
of water steam or high clouds. Astronomers believe it's more likely the atmosphere is too
thin around the planet to let enough light filter through and reveal chemical fingerprints.

"A steamy atmosphere would have to be very dense – about one-fifth water vapor by
volume — compared to our Earth, with an atmosphere that's four-fifths nitrogen and one-
fifth oxygen with only a touch of water vapor," Bean said. "During the next year, we
should have some solid answers about what this planet is truly like."

The team, which included Bean's co-authors — Eliza Miller-Ricci Kempton, a NASA Sagan
Fellow at the University of California in Santa Cruz, and Derek Homeier of the Institute for
Astrophysics in Gottingen, Germany — examined GJ 1214b using the ground-based Very
Large Telescope at Paranal Observatory in Chile.

"This is an important step forward, narrowing our understanding of the atmosphere of
this planet," said NASA Exoplanet Exploration Program Scientist Douglas Hudgins at NASA
Headquarters in Washington. "Bizarre worlds like this make exoplanet science one of the
most compelling areas in astrophysics today."

Casey Kazan via NASA/JPL


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