The Chemistry of Exoplanet Atmospheres

Since 1995, over 6000 planets outside the solar system—exoplanets—have been discovered. These planets are incredibly diverse; the stochastic process of planet formation produces planets at all masses and orbits imaginable around their host stars. These range from the more familiar—like Earth-sized temperate terrestrial planets and cold gas giants—to the more exotic—hot Jupiters, intermediate mass sub-Neptunes, planets orbiting white dwarfs, and many more. We are now starting to characterize planets both familiar and exotic in more detail, with telescopes on the ground and in space allowing us to observe their spectra, providing a window into the compositions of their atmospheres. What we find is a very high level of complexity. The hottest giant planets seem more like stars than planets, with their molecules dissociated into atoms, and strong atomic features dominating their spectra. Cooler gas giants divert from chemical equilibrium, as their atmospheres convect and advect material faster than the timescales for chemical reactions. Photons further alter the chemistry of these atmospheres, with photochemical reactions leading to opaque hazes that obscure the features of some planets. Meanwhile, the smaller planets, sub-Neptunes, are like nothing in our solar system, with masses intermediate between Earth and Neptune. Their thinner hydrogen atmospheres can be enriched in molecules like methane, carbon dioxide, water, and sulfur species. Observations are beginning to reveal that these atmospheres likely overly a magma ocean, causing the atmosphere and interior to be coupled. Astronomers are now collaborating with geochemists to understand how this geological coupling occurs at the high pressures and temperatures of these magma oceans. The future frontier of exoplanet atmospheric chemistry adds another source of molecules at the surface: those formed by life. Future telescopes aim to observe biochemistry in the atmospheres of rocky planets, with biosignatures that indicate the presence of substantive surface life.