Renyu Hu (JPL) - Helium Atmosphere on Exoplanet GJ 436 b Formed by Irradiation Driven Escape

Many Neptune-sized exoplanets in orbits smaller than Mercury’s are thought to have experienced extensive atmospheric evolution. The consequence of this evolution has not been observed. Recently, the atmosphere of a warm Neptune-sized exoplanet GJ 436 b has been found to be poor in methane and rich in carbon monoxide, contrary to theoretical predictions for a hydrogen-dominated atmosphere, while the constraints on the bulk density of the planet require an extensive low-density gas envelope. The conundrum may be resolved if most of the original hydrogen on GJ 436 b were lost via irradiation-driven escape, and the remnant atmosphere is mainly helium with trace amounts of hydrogen, carbon and oxygen. Using a recently established photochemistry-thermochemistry kinetic-transport model to simulate the thick atmosphere on GJ 436 b, we find that a helium atmosphere with a hydrogen elemental abundance of 10-4 would lead to spectral features consistent with observations. Such atmospheres, although not found on any Solar System planets, may well be common on Neptune-sized exoplanets around stars more than 7 billion years old.