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The Deuterium-to-Hydrogen Ratio on Photoevaprating Sub-Neptunes

Presentation #625.10 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
The Deuterium-to-Hydrogen Ratio on Photoevaprating Sub-Neptunes

The deuterium-to-hydrogen (D/H) mass ratio has been one of the most studied isotopic ratios in planetary science, as this ratio provides crucial clues to planet formation, evolution, and habitability. Although potential observations of deuterium fractionation for exoplanets have been posited, there is no theoretical study yet to model H-D fractionation processes for exoplanets with significant H/He envelopes. Motivated by trendy studies of H-He fractionation on hot Jupiters and sub-Neptunes due to photoevaporation, we investigate the evolution of the D/H ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in sub-Neptunes around solar-type stars. Akin to the helium-enhanced envelopes of sub-Neptunes due to photoevaporating escape, we find that the planets along the upper boundary of the radius valley exhibit higher D/H ratios. The ratio can rise by a factor of about 1.65 within 7.5 Gyr in our grid of evolutionary calculations. The D/H ratio is expected to be even higher in thinner envelopes if the sub-Neptunes do not become bare rocky cores.

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