In the past decade we’ve discovered thousands of planets around other stars, but we’re only able to observe a few of the larger ones directly. This means that while we can determine some of an exoplanet’s properties such as its size and mass, it’s far more difficult to determine other properties such as the composition of its atmosphere.
If a planet passes in front of its star from our vantage point, then it is theoretically possible to determine some of the compounds that make up its atmosphere. When a planet passes in front of its star, the atmosphere absorbs certain wavelengths of light, and those wavelengths depend upon the types of molecules in the atmosphere. So far we’ve been able to study the composition of gas giant atmospheres, but recently a team of astronomers observed the atmosphere of a “super-Earth” sized planet.
55 Cancri e has a mass about 8 times that of Earth, or about half that of Neptune. We don’t have a similar planet in our own solar system, so understanding this type of planet is a big goal of astronomy. This particular planet orbits it star at about 1/20th the distance of Mercury from the Sun. It’s star-facing side is estimated to have a temperature of about 2000°C, so there was some question as to whether it would have an atmosphere at all. Using observations from the Hubble’s Wide Field Camera 3, the team found not only that the planet has an atmosphere, but that the atmosphere contains significant amounts of hydrogen cyanide (HCN).
An atmosphere with significant HCN means that 55 Cancri e is likely a carbon planet, meaning that it would likely have an iron core like Earth, but instead of silicates its crust would consist of oxygen-carbon compounds. So it would likely have a crust of graphite, diamond and carbonate minerals. Carbon planets would also likely lack water, since carbon bonds so well with oxygen there would likely be little left to bond with hydrogen to form water. It was suspected that 55 Cancri e was a carbon planet because its parent star contains much more carbon than our Sun. Now it seems our suspicions were correct.
Paper: A. Tsiaras, et al. Detection of an atmosphere around the super-Earth 55 Cancri e. arXiv:1511.08901 [astro-ph.EP]