Institute for Advanced Study Astrophysics Seminar

ABSTRACT: The rapid growth in discovery of transiting planets has resulted not only in an abundance of known extrasolar planets, but planets for which interior composition may be inferred by density and atmospheric composition inferred by absorption spectroscopy. Transit searches that focus on the smallest main sequence stars, M dwarf stars, can discover the smallest planets, such as the planet GJ 1214b that has been inferred to be a water-world based on evidence for low density and lack of a Hydrogen/Helium atmosphere. Low mass stars, however, experience an extended pre-main sequence phase of increased luminosity powered by gravitational contraction and characterized by activity with substantial X-ray emission. The X-ray heating of the atmosphere of GJ 1214b during the first 0.5 Gyr of its history should have depleted it of a large fraction of its primordial Hydrogen, ruling out scenarios for a low density, Hydrogen or water rich composition. De-hydrogenation of ice-rich planet with near-solar relative composition of elements heavier than Helium would leave behind a rocky core surrounded by an envelope of nearly pure carbon dioxide. The low density and low atmospheric scale height of GJ 1214b can both be explained with a rocky core surrounded by a thick carbon dioxide envelope. This process can in principle also create a carbon dioxide ocean, which due to the abundance of M dwarf stars may be the most common liquid in the Universe.