Deuterium Escape on Photoevaporating Mini-Neptunes

Pin-Gao Gu^1*, Howard Chen^2,3

¹Institute of Astronomy & Astrophysics, Academia Sinica, Taipei, Taiwan
²Department of Aerospace, Physics, and Space Sciences, Florida Institute of Technology, Melbourne, Florida, USA
³Planetary Environments Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

* Presenter:Pin-Gao Gu, email:gu@asiaa.sinica.edu.tw

The deuterium-to-hydrogen (D/H) mass ratio provides crucial clues to planet formation and evolution. We investigate the evolution of the D/H ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in mini-Neptunes around solar-type stars. For the first time, the diffusion-limited approach in conjunction with energy-limited photoevaporation is considered in evaluating deuterium escape from evolving exoplanet H/He envelopes. We find that the planets with smaller initial gas envelopes and thus smaller sizes can lead to weaker atmospheric escape, which facilitates hydrogen-deuterium fractionation. Akin to the helium-enhanced envelopes of mini-Neptunes due to photoevaporating escape, the planets along the upper boundary of the radius valley are the best targets to detect high D/H ratios. The ratio can rise by a factor of ≲1.65 within 7.5 Gyr in our grid of evolutionary calculations. The D/H ratio is expected to be higher in thinner envelopes as long as the planets do not become bare rocky cores. This work was supported by the National Science and Technology Council in Taiwan through grants NSTC 111-2112-M-001-037 and 112-2112-M-001-035.

Keywords: astrophysics, planetary science