Strong Fractionation of Deuterium and Helium in Sub-Neptune Atmospheres along the Radius Valley
Abstract
We simulate atmospheric fractionation in escaping planetary atmospheres using IsoFATE, a new open-source numerical model. We expand the parameter space studied previously to planets with tenuous atmospheres that exhibit the greatest helium and deuterium enhancement. We simulate the effects of extreme-ultraviolet-driven photoevaporation and core-powered mass loss on deuterium–hydrogen and helium–hydrogen fractionation of sub-Neptune atmospheres around G, K, and M stars. Our simulations predict prominent populations of deuterium- and helium-enhanced planets along the upper edge of the radius valley with mean equilibrium temperatures of ≈370 K and as low as 150 K across stellar types. We find that fractionation is mechanism dependent, so constraining He/H and D/H abundances in sub-Neptune atmospheres offers a unique strategy to investigate the origin of the radius valley around low-mass stars. Fractionation is also strongly dependent on retained atmospheric mass, offering a proxy for planetary surface pressure as well as a way to distinguish between desiccated enveloped terrestrials and water worlds. Deuterium-enhanced planets tend to be helium dominated and CH4 depleted, providing a promising strategy to observe HDO in the 3.7 μm window. We present a list of promising targets for observational follow-up.
- Publication:
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The Astrophysical Journal
- Pub Date:
- June 2024
- DOI:
- 10.3847/1538-4357/ad3e77
- arXiv:
- arXiv:2402.10690
- Bibcode:
- 2024ApJ...967..139C
- Keywords:
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- Exoplanet astronomy;
- Exoplanet atmospheres;
- Exoplanet atmospheric composition;
- Exoplanet atmospheric evolution;
- Exoplanet formation;
- Exoplanet evolution;
- Exoplanets;
- Mini Neptunes;
- Super Earths;
- Isotopic abundances;
- 486;
- 487;
- 2021;
- 2308;
- 492;
- 491;
- 498;
- 1063;
- 1655;
- 867;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted by ApJ