Astro Seminar by Kevin Heng: The Geoastronomy of Sub-Neptunes

One of the biggest surprises of exoplanet science is the discovery of the small exoplanet population between the sizes of Earth and Neptune. Super Earths appear to be scaled-up versions of rocky planets. However, it is less clear that sub-Neptunes are scaled-down versions of gas-giant planets, other than by analogy. The rocky cores and atmospheres of sub-Neptunes occupy comparable volumes (and thus have comparable extents in radius), but the cores dominate the mass budget if the atmospheres are hydrogen-dominated. Since the cores dominate the masses of sub-Neptunes, geochemical outgassing cannot be ignored. I will show that the temperature-pressure conditions at the surfaces of these cores exert a zeroth-order control on the atmospheric chemistry. Less massive (and smaller) sub-Neptunes have lower core surface pressures and temperatures, which leads to atmospheres with higher mean molecular weights. This naturally leads to a gradient in mean molecular weight across the radius valley of small exoplanets. The strength of this gradient is controlled by the oxygen fugacity of these cores and not by the elemental abundance of carbon. These results imply a need to shift one’s mental framework away from the planet formation-driven approach of prioritising the carbon-to-oxygen ratio and “metallicity” and towards the more geoscience-oriented approach of understanding the oxidation states of rocky mantles via the oxygen fugacity.  This research is part of the European Research Council (ERC) Synergy Grant awarded to Project Geoastronomy (PIs: Gaillard, Heng & Mozjsis).