Astrophysicists have made the first measurements of the atmosphere of a 'super-Earth' planet known as GJ 1214b.
GJ 1214b is 2.7 times the size of Earth and 6.5 times as massive. Super-Earths are generally mostly solid, consisting of some combination of rock and ice.
Discovered a year ago, the planet was believed to have a thick, gaseous atmosphere - but nobody was sure. And while the new observations rule out a number of possibilities, they still reveal plenty more.
"This is the first super-Earth known to have an atmosphere," says Jacob Bean of the Harvard-Smithsonian Center for Astrophysics. "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."
There were three plausible atmospheric possibilities for GJ 1214b. The most intriguing was a thick blanket of steam vaporized by the nearby star. The second option was that the planet was a mini-Neptune with a rocky core surrounded by ices and a hydrogen/helium atmosphere. The third model was a big, rocky world with a soupy mix of gases - mainly hydrogen - recently emitted by volcanoes.
To study the planet's atmosphere, the team observed it when it crossed in front of its star, filtering the light through its atmosphere.
"This is the first super-Earth to have its atmosphere analyzed. We've reached a real milestone on the road toward characterizing these worlds," says Bean.
Harvard astronomer David Charbonneau, who led the team that discovered GJ 1214b, agrees. "In less than 10 years, we've gone from studying the atmospheres of alien worlds like Jupiter, to Neptunes, to super-Earths. Earth-sized worlds are next, although they'll be the most difficult," he says.
The spectrum of GJ 1214b proved to be featureless, which ruled out a cloud-free atmosphere composed primarily of hydrogen. If hydrogen is abundant, it must be cloaked by a thick blanket of clouds or haze. A dense, steamy atmosphere also fits the bill.
"A lot of people are putting this planet under a microscope," says Bean. "In the next year, we should have some solid answers about what it's truly like."