The Atmospheres of Super-Earths

A "terrestrial" planet is a planet that is earth-like in the sense that it is roughly ten thousand kilometers in diameter and has a density of rock: about five grams per cubic centimeter (about five times that of water). Mercury, Venus, Earth and Mars are the only terrestrial planets in our solar system, the Earth being the largest one. But the Earth is no longer the largest known terrestrial planet. Over the past few years, astronomers have detected eleven "super-Earths" -- planets with masses between one and ten earth-masses -- around other stars. Astronomers working to understand whether the Earth is in any way unique, and how it formed, are excited by the prospect of being able to study super-Earths to help sort out possible competing models for the Earth.

If super-Earths are to be habitable by life as we know it, they will need earth-like atmospheres, but determining if a distant super-Earth has an atmosphere at all, and if so how it might be categorized, is a daunting task. Harvard graduate student Eliza Miller-Ricci has undertaken a theoretical study of the possible atmospheric signatures of super-Earths as part of her recently completed Ph.D. thesis, part of which appears in a recent issue of the Astrophysical Journal. Together with CfA astronomer Dimitar Sasselov and a colleague, she argues that super-Earth atmospheres might all begin with a similar composition, but the differing planetary masses, compositions, and their stellar environments will eventually lead to three different, generic kinds of atmospheres: those like the present Earth or Venus that are rich in molecules other than hydrogen, those like the primitive Earth which was rich in hydrogen gas, and a third group that lies somewhere between these two extremes.

The scientists note that recent observations of transiting planets (ones whose orbits take them in front of their star as seen from Earth), have been able to detect atmospheric features, especially at infrared wavelengths. Although no transits of a super-Earth have yet been discovered, the authors address how to interpret such measurements when they can be, and are, made. Starting from some reasonable assumptions about their initial chemical composition based on known chemical abundances in star forming regions, they calculate what might happen to the mix as ultraviolet radiation, heating, escape into space, and other factors affect the gas. They conclude that infrared spectroscopy can discriminate between the two extreme cases in a relatively straightforward way, and calculate how those results should look affect the observations; they also suggest some possible future observing strategies. The results go a long way towards preparing astronomers for the amazing day when an Earth-like planet is discovered with an atmosphere that might support life.