Hydrogen escaping from a pair of exoplanets smaller than Neptune
File(s)
Accepted version
Author(s)
Type
Journal Article
Abstract
Exoplanet surveys have shown a class of abundant exoplanets smaller than Neptune on close, <100-day orbits1,2,3,4. These planets form two populations separated by a natural division at about 1.8 R⊕ termed the radius valley. It is uncertain whether these populations arose from separate dry versus water-rich formation channels, evolved apart because of long-term atmospheric loss or a combination of both5,6,7,8,9,10,11,12,13,14. Here we report observations of ongoing hydrogen loss from two sibling planets, TOI-776 b (1.85 ± 0.13 R⊕) and TOI-776 c (2.02 ± 0.14 R⊕), the sizes of which near the radius valley and mature (1–4 Gyr) age make them valuable for investigating the origins of the divided population of which they are a part. During the transits of these planets, absorption appeared against the Lyman-α emission of the host star, compatible with hydrogen escape at rates equivalent to 0.03–0.6% and 0.1–0.9% of the total mass per billion years of each planet, respectively. Observations of the outer planet, TOI-776 c, are incompatible with an outflow of dissociated steam, suggesting both it and its inner sibling formed in a dry environment. These observations support the strong role of hydrogen loss in the evolution of close-orbiting sub-Neptunes5,6,7,8,15,16.
Date Issued
2025-02-12
Date Acceptance
2025-12-04
Citation
Nature, 2025, 638, pp.636-639
ISSN
0028-0836
Publisher
Nature Research
Start Page
636
End Page
639
Journal / Book Title
Nature
Volume
638
Copyright Statement
© 2025, The Author(s), under exclusive licence to Springer Nature Limited. This is the author’s accepted manuscript made available under a CC-BY licence in accordance with Imperial’s Research Publications Open Access policy (www.imperial.ac.uk/oa-policy)
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Publication Status
Published