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Lyman-α transits favor hydrogen-rich atmospheres and rocky cores for TOI-776 b and c

Presentation #625.22 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
Lyman-α transits favor hydrogen-rich atmospheres and rocky cores for TOI-776 b and c

The origin and composition of sub-Neptune exoplanets, like TOI-776 b & c, is a subject of much debate. These two planets, with radii ≈ 1.9 and 2.0 R⊕, ages of a 1-4 Gyr, and an M1 host star, sit at the lower size boundary of the sub-Neptune population, within or just above the exoplanet radius valley. New Hubble Space Telescope observations of their Lyman-α transits show evidence of absorption by escaping hydrogen, the first around planets of this size and age. Hydrogen escape rates compatible with the observations suggest that TOI-776 b & c do not have water rich compositions, as has been proposed for TOI-776 b and other small sub-Neptunes orbiting M dwarf stars. Instead, it is more likely that these planets have hydrogen-helium envelopes surrounding iron-silicate cores. Such envelopes are probably stable against atmospheric escape to ages of 10 Gyr, although our models allow for a > 10% chance that TOI-776 b will be fully stripped to become a super Earth with radius ≈ 1.4 R. This result is expected if the demographic division between super-Earths and sub-Neptunes orbiting M dwarfs is primarily a product of atmospheric loss over Gyr timescales rather than differences in composition at birth.

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