Hydrogen escaping from a pair of exoplanets smaller than Neptune

Creators: Loyd, R. O. Parke (Corresponding)¹; Schreyer, Ethan²; Owen, James E.²; Rogers, James G.³; Broome, Madelyn I.⁴; Shkolnik, Evgenya L.⁵; Murray-Clay, Ruth⁴; Wilson, David J.⁶; Peacock, Sarah^{7, 8}; Teske, Johanna^{9, 10}; Schlichting, Hilke E.¹¹; Duvvuri, Girish M.^{6, 12, 13}; Youngblood, Allison^{6, 8}; Schneider, P. Christian¹⁴; France, Kevin^{6, 13}; Giacalone, Steven¹⁵; Batalha, Natasha E.¹⁶; Schneider, Adam C.¹⁷; Longo, Isabella⁶; Barman, Travis¹⁸; Ardila, David R.¹⁹

1. Eureka Scientific
2. Imperial College London
3. University of Cambridge
4. University of California, Santa Cruz
5. Arizona State University
6. Laboratory for Atmospheric and Space Physics
7. University of Maryland, Baltimore County
8. Goddard Space Flight Center
9. Carnegie Institution for Science
10. Carnegie Observatories
11. University of California, Los Angeles
12. Vanderbilt University
13. University of Colorado Boulder
14. Universität Hamburg
15. California Institute of Technology
16. Ames Research Center
17. United States Naval Observatory
18. University of Arizona
19. Jet Propulsion Lab

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Abstract

Exoplanet surveys have shown a class of abundant exoplanets smaller than Neptune on close, <100-day orbits. 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. 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.

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Acknowledgement

Contributions by R.O.P.L were supported by NASA through programme HST-GO-16456. Additional support for R.O.P.L., M.I.B. and R.M.-C. was provided through programme HST-GO-16731. These programmes are administered through grants from the Space Telescope Science Institute, which is operated by the Associations of Universities for Research in Astronomy, under NASA contract NAS 5-26555. E.S. and J.E.O. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 853022, PEVAP). J.E.O. is supported by a Royal Society University Research Fellowship. Contributions by S.P. were supported by NASA under award number 80GSFC24M0006. R.M.-C. and E.S. acknowledge support from NASA XRP grant 80NSSC23K0282. This research is based on observations made with the NASA/ESA HST, obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA contract NAS 526555. These observations are associated with programmes 16456 and 16701. We thank R. Burn et al. for sharing detailed results of their formation–evolution model.

Contributions

R.O.P.L. identified targets and planned the observations of HST programme 16456, processed all HST data, reconstructed the Lyman-α profile, estimated the signal significance, wrote scripts to provide fit statistics for the outflow model, created figures and wrote the paper. E.S. conducted outflow modelling, interpreted the origin of the transit signals and drafted the ‘Outflow model’ section. J.E.O. conducted RHD modelling for the paper and the proposal for 16456, provided input to the paper and advised E.S.; J.G.R. modelled evolutionary tracks for the planets, drafted the ‘Evolutionary tracks’ section and provided Fig. 3. M.B. conducted RHD modelling of the planets and drafted the ‘RHD simulations’ section. E.L.S. originated the idea of targeting high radial velocity systems and drafted a first draft of the proposal for programme 16456. E.L.S. and J.T. suggested the observations may have implications for water content. R.O.P.L., J.E.O., R.M.-C., H.E.S., E.S. and J.G.R. jointly interpreted implications for water content and planetary formation and evolution. D.J.W. extracted initial STIS G140L spectra. D.J.W., A.Y., K.F. and J.T. assisted with data analysis and signal verification. S.P. generated the PHOENIX-based XUV reconstruction and drafted the ‘Lyman-α and XUV’ section. H.E.S. advised J.G.R.; G.M.D. reconstructed an XUV spectrum and drafted the ‘Lyman-α and XUV’ section. A.Y. identified targets and planned the observations of HST programme 16701. P.C.S. analysed X-ray data for the DEM XUV reconstruction. S.G. validated TOI-776 b and TOI-776 c as bonafide planets. I.L. measured ultraviolet line fluxes for input to XUV reconstructions. R.O.P.L., J.E.O., R.M.-C., A.C.S., T.B., S.P., S.G. and D.R.A. are members of the proposing team for programme 16456. A.Y., K.F., P.C.S., G.M.D. and D.J.W. are members of the proposing team for programme 16701. J.E.O., E.L.S., P.C.S., J.T., H.E.S., K.F., N.E.B., D.J.W., A.C.S., S.P., M.B., T.B. and D.R.A. provided feedback for the project and the paper.

Data Availability

The datasets analysed during this study are publicly available in the Mikulski Archive for Space Telescopes at https://doi.org/10.17909/a8jb-3759.

Code Availability

The Space Telescope Environment for Python and stistools package used to process the STIS data are publicly available at https://stistools.readthedocs.io/en/latest. Code for data reduction and analysis and for generating all figures and values except those relating to the evolutionary tracks is available on Zenodo at https://doi.org/10.5281/zenodo.13976674 (ref. ⁸⁴). The outflow model code is publicly available at https://github.com/eschreyer/LyA_code. The code used to generate evolutionary tracks is available at https://github.com/jo276/EvapMass, along with the efficiency interpolator as part of the main code package, and chains from the most recent run are present in the Zenodo archive. A beta release of the RHD code is available at https://github.com/mibroome/wind-ae/.

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Resource type: Journal Article
Publisher: Nature Publishing Group
Published in: Nature, 638, 636-639, ISSN: 0028-0836.
Languages: English