Hydrogen escaping from a pair of exoplanets smaller than Neptune

R O Parke Loyd; Ethan Schreyer; James E Owen; James G Rogers; Madelyn I Broome; Evgenya L Shkolnik; Ruth Murray-Clay; David J Wilson; Sarah Peacock; Johanna Teske; Hilke E Schlichting; Girish M Duvvuri; Allison Youngblood; P Christian Schneider; Kevin France; Steven Giacalone; Natasha E Batalha; Adam C Schneider; Isabella Longo; Travis Barman; David R Ardila

doi:10.1038/s41586-024-08490-x

Hydrogen escaping from a pair of exoplanets smaller than Neptune

Nature. 2025 Feb;638(8051):636-639. doi: 10.1038/s41586-024-08490-x. Epub 2025 Feb 12.

Authors

R O Parke Loyd¹, Ethan Schreyer², James E Owen², James G Rogers³, Madelyn I Broome⁴, Evgenya L Shkolnik⁵, Ruth Murray-Clay⁴, David J Wilson⁶, Sarah Peacock^{7

8}, Johanna Teske^{9

10}, Hilke E Schlichting¹¹, Girish M Duvvuri^{6

12

13}, Allison Youngblood^{6

8}, P Christian Schneider¹⁴, Kevin France^{6

13

15}, Steven Giacalone¹⁶, Natasha E Batalha¹⁷, Adam C Schneider¹⁸, Isabella Longo⁶, Travis Barman¹⁹, David R Ardila²⁰

Affiliations

¹ Eureka Scientific, Oakland, CA, USA. astroparke@gmail.com.
² Imperial Astrophysics, Department of Physics, Imperial College London, London, UK.
³ Institute of Astronomy, University of Cambridge, Cambridge, UK.
⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA.
⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.
⁶ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA.
⁷ University of Maryland Baltimore County, Baltimore, MD, USA.
⁸ NASA Goddard Space Flight Center, Greenbelt, MD, USA.
⁹ Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC, USA.
¹⁰ The Observatories of the Carnegie Institution for Science, Pasadena, CA, USA.
¹¹ Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA.
¹² Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, USA.
¹³ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO, USA.
¹⁴ Hamburger Sternwarte, Hamburg, Germany.
¹⁵ Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO, USA.
¹⁶ Department of Astronomy, California Institute of Technology, Pasadena, CA, USA.
¹⁷ NASA Ames Research Center, Moffett Field, CA, USA.
¹⁸ United States Naval Observatory, Flagstaff Station, Flagstaff, AZ, USA.
¹⁹ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
²⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.

PMID: 39939756
DOI: 10.1038/s41586-024-08490-x

Abstract

Exoplanet surveys have shown a class of abundant exoplanets smaller than Neptune on close, <100-day orbits^1-4. 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^5-14. 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^5-8,15,16.