About a dozen of the roughly 500 currently known extra-solar planets are super-Earths: planets with masses between about two and ten earth-masses. For larger extra-solar planets, an estimate of their mass and radius provides a good handle on their composition. For example, most extra-solar planets are larger than Jupiter and, like Jupiter, are thought to be composed primarily of hydrogen and helium. Super-Earths, on the other hand, might have rocky cores and possibly an atmosphere, but models using only mass and radius cannot distinguish between this and other possibilities to determine the planet's composition.
Writing in this week's issue of Nature, SAO astronomer Jacob Bean and two colleagues report studying the atmosphere of a super-Earth using near-infrared techniques with a large ground-based telescope. The planet, GJ 1214b, has a mass of 5.69 earth-masses, and orbits very close to its star, passing through our line-of-sight to the star (a transit) every 1.58 days -- its "year". Each transit lasts about an hour, during which time the constituents in the planet's atmosphere can impose a characteristic signature on the starlight observed.
The astronomers analyzed the transit results, but found no signatures of any atomic or molecular species. This result suggests that the atmosphere is not composed of hydrogen - at least not unless there are very thick clouds blocking some of the view. Since, however, some molecules lack strong signatures at the wavelengths studied, the results are also consistent with the presence of one such species: hot water vapor (steam). The super-Earth around GJ 1214 orbits very close to the star, and hot atmospheric temperatures are expected. If there were steam in the atmosphere of this super-Earth, the astronomers note that the planet itself might plausibly be made of water ice, rather than rock. The tantalizing result illustrates the increasing sophistication of extra-solar planet research, and the possibility that future atmospheric studies can sort out the core compositions of this and other super-Earths. The results also emphasize the wide variety of planets that exist in the cosmos -- many more types than exist in our own solar system.