Helium-Enhanced Planets at the Edge of the Radius Gap
Abstract
Primordial hydrogen-helium envelopes surrounding sub-Neptune-sized planets are susceptible to mass loss driven by ionizing radiation from their host star. The effect of mass loss is imprinted on observed exoplanet populations in the form of a "photo-evaporation desert" and a "gap" at 1.6 Earth Radii in the planet radius distribution. To date, models of the mass-loss evolution of exoplanets have assumed that the planetary envelope composition stays constant over time. However, after an initial 0.1 Gyr phase of rapid hydrodynamic mass loss, sub-Neptunes may experience a subsequent phase of thermal escape modulated by diffusive separation between hydrogen and helium wherein they gradually become enhanced in helium and metals (relative to hydrogen) over billions of years. We predict that planets on the large radius edge of the "radius gap" in planet occurrence rates could be significantly enhanced in helium (or depleted in hydrogen) relative to solar composition. We have performed the first self-consistent calculations of the coupled thermal, mass-loss, and compositional evolution of hydrogen-helium envelopes surrounding sub-Neptune mass planets. Our simulations consistently produce planets with envelope helium mass fractions in excess of Y=0.5 (at planet ages of 5 Gyr) near the upper edge of the radius gap. Our results have important implications for the interpretation of atmospheric transmission and emission spectra of low-density sub-Neptune-size planets, which are prime targets for atmospheric characterization with HST and eventually JWST. Enhancement in helium relative to hydrogen will affect both the scale height and equilibrium chemical abundances in the atmosphere (e.g., CO relative to CH4). To date, most atmospheric retrieval analyses have fixed the ratio of hydrogen and helium to solar abundances; this assumption must now be relaxed. Our prediction further provides a new observational test for the extent to which the radius gap is caused by atmospheric mass loss versus an intrinsically bimodal outcome of planet formation.
- Publication:
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AAS/Division for Extreme Solar Systems Abstracts
- Pub Date:
- August 2019
- Bibcode:
- 2019ESS.....440006R