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Hydrogen escaping from a pair of exoplanets smaller than Neptune

Nature volume 638, pages 636–639 (2025)Cite this article

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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.

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Fig. 1: Lyman-α transit lightcurves.
Fig. 2: Mass loss constraints.
Fig. 3: Atmospheric evolution of TOI-776 b and c.

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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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Acknowledgements

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.

Author information

Authors and Affiliations

  1. Eureka Scientific, Oakland, CA, USA

    R. O. Parke Loyd

  2. Imperial Astrophysics, Department of Physics, Imperial College London, London, UK

    Ethan Schreyer & James E. Owen

  3. Institute of Astronomy, University of Cambridge, Cambridge, UK

    James G. Rogers

  4. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

    Madelyn I. Broome & Ruth Murray-Clay

  5. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Evgenya L. Shkolnik

  6. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA

    David J. Wilson, Girish M. Duvvuri, Allison Youngblood, Kevin France & Isabella Longo

  7. University of Maryland Baltimore County, Baltimore, MD, USA

    Sarah Peacock

  8. NASA Goddard Space Flight Center, Greenbelt, MD, USA

    Sarah Peacock & Allison Youngblood

  9. Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC, USA

    Johanna Teske

  10. The Observatories of the Carnegie Institution for Science, Pasadena, CA, USA

    Johanna Teske

  11. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA

    Hilke E. Schlichting

  12. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, USA

    Girish M. Duvvuri

  13. Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO, USA

    Girish M. Duvvuri & Kevin France

  14. Hamburger Sternwarte, Hamburg, Germany

    P. Christian Schneider

  15. Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO, USA

    Kevin France

  16. Department of Astronomy, California Institute of Technology, Pasadena, CA, USA

    Steven Giacalone

  17. NASA Ames Research Center, Moffett Field, CA, USA

    Natasha E. Batalha

  18. United States Naval Observatory, Flagstaff Station, Flagstaff, AZ, USA

    Adam C. Schneider

  19. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    Travis Barman

  20. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    David R. Ardila

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  1. R. O. Parke Loyd
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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.

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Correspondence to R. O. Parke Loyd.

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Extended data figures and tables

Extended Data Fig. 1 Reconstructions of the EUV spectrum.

Reconstructions of the intrinsic EUV spectrum of TOI-776 as seen from Earth based on a PHOENIX stellar model (blue), DEM model (orange), and scaling relationship (green)49,50,51,55,56. Values in parentheses following the line labels are the luminosity integrated across the range 100–912 Å in units of 1028 erg s−1.

Extended Data Fig. 2 Demographic ___location of TOI-776 b & c.

Black points with error bars are planets TOI-776 b & c, shown in the context of a well-characterised sample of 1246 planets (grey points) from ref. 85 orbiting hosts with effective temperatures (Teff) ranging from 3500 to 6700 K. Shading indicates the relative density of points.

Extended Data Fig. 3 Transit spectra and lightcurves.

a, Spectra of the Lyman-α line in and out of transit. Unshaded regions indicate integration bands used to create lightcurves shown in Fig. 1 (‘transit’) and Extended Data Fig. 4 (‘reference’). The selection of these regions is explained in Methods: Data. Colours indicating epoch correspond between Fig. 1, this figure, and Extended Data Fig. 3. Line style represents pre-transit (solid) and transit (dashed). The large difference in flux density and line width between the 2022 Dec and earlier observations is an instrument resolution effect. Envelopes around the lines are 1σ uncertainties. b, Lightcurves integrated over the bands shown in a. Filled and open points correspond to solid and dashed lines in a. Lightcurves were fitted with an occulting disk transit to estimate signal significance (solid: best fit, dashed: 68% confidence interval, dotted: 95% confidence interval). Data and curves are normalised by the pre-transit flux from the best fit. c, Background count rates in the integrated band, due primarily to geocoronal airglow, normalised by the mean value for each observation epoch.

Extended Data Fig. 4 Emission line time series.

Lightcurves of integrated line fluxes from all epochs of G140L data. Lyman-α values are normalised to the mean from the first three visits, shown in parenthesis in the rightmost panel in units of 10−15 erg s−1 cm−2. Others are normalised to the global mean, also shown in parenthesis. Dashed grey lines indicate the optical transit of TOI-776 c.

Extended Data Fig. 5 Reference band time series.

a, Lightcurves of Lyman-α flux in the 100–250 km s−1 reference range (grey points) and in the −37–69 km s−1 range (coloured points, offset slightly in time for visual clarity) as a function of HST orbital phase from the 2021 June and 2022 June epochs. b, Lightcurves of Lyman-α flux in the 100–250 km s−1 reference range as a function of linear time. Filled points are from the first exposure and open points are from the second exposure of each epoch. Curves show polynomial fits of order 0, 1, and 2 with associated fit statistics shown in each panel, with emphasis on the zeroth-order fit (flat line) to indicate our choice not to detrend the data. Dashed grey lines denote optical ingress and egress.

Extended Data Fig. 6 Lyman-α profile fit.

a, G140M data for the ISM-absorbed Lyman-α line with 1σ uncertainties (black with shading) compared to the maximum-likelihood model for the ISM-absorbed line and 68% confidence region after passing through the instrument model (orange with shading). Estimated background levels, which are dominated by geocoronal airglow emission near the line, are plotted as well after applying the same flux scaling as applied to the signal (green). b, Maximum-likelihood model of the intrinsic Lyman-α line (blue), ISM absorption (grey), and absorbed line (orange) with 68% confidence regions (shading). The slight offset of the maximum-likelihood model from the 68% confidence region is due to a nearly flat posterior on the ISM column density over 1017–1018 cm−2.

Extended Data Table 1 Properties of the TOI-776 system
Full size table
Extended Data Table 2 Observing log
Full size table
Extended Data Table 3 Lyman-α line profile fit parameters
Full size table
Extended Data Table 4 Outflow model parameters
Full size table

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Loyd, R.O.P., Schreyer, E., Owen, J.E. et al. Hydrogen escaping from a pair of exoplanets smaller than Neptune. Nature 638, 636–639 (2025). https://doi.org/10.1038/s41586-024-08490-x

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