Hydrogen escaping from a pair of exoplanets smaller than Neptune

Exoplanet surveys have shown a class of abundant exoplanets smaller than Neptune on close, <100-day orbits^{1, 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 both^{5, 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-Neptunes^{5, 6, 7–8,15,16}.