Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres1. Searches for helium, however, have hitherto been unsuccessful2. Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near-infrared transmission spectrum of the warm gas giant3 WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 ± 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 1010 to 3 × 1011 grams per second (0.1–4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Seager, S. & Sasselov, D. D. Theoretical transmission spectra during extrasolar giant planet transits. Astrophys. J. 537, 916–921 (2000).
Moutou, C., Coustenis, A., Schneider, J., Queloz, D. & Mayor, M. Searching for helium in the exosphere of HD 209458b. Astron. Astrophys. 405, 341–348 (2003).
Anderson, D. et al. The discoveries of WASP-91b, WASP-105b and WASP-107b: two warm Jupiters and a planet in the transition region between ice giants and gas giants. Astron. Astrophys. 604, A110 (2017).
Kreidberg, L. batman: Basic Transit Model cAlculatioN in Python. Publ. Astron. Soc. Pacif. 127, 1161 (2015).
Zhou, Y., Apai, D., Lew, B. W. P. & Schneider, G. A physical model-based correction for charge traps in the Hubble Space Telescope’s Wide Field Camera 3 near-IR detector and its applications to transiting exoplanets and brown dwarfs. Astron. J. 153, 243 (2017).
Fossati, L. et al. Metals in the exosphere of the highly irradiated planet WASP-12b. Astrophys. J. 714, L222–L227 (2010).
Dai, F. & Winn, J. N. The oblique orbit of WASP-107b from K2 photometry. Astron. J. 153, 205 (2017).
Castelli, F. & Kurucz, R. L. New grids of ATLAS9 model atmospheres. Preprint at https://arxiv.org/abs/astro-ph/0405087 (2004).
Kreidberg, L., Line, M., Thorngren, D., Morley, C. & Stevenson, S. Water, methane depletion, and high-altitude condensates in the atmosphere of the warm super-Neptune WASP-107b. Preprint at https://arxiv.org/abs/1709.08635 (2018).
Kuntschner, H., Bushouse, H., Kummel, M. & Walsh, J. R. WFC3 SMOV Proposal 11552: Calibration of the G102 Grism. Report No. WFC3–2009–18 (Space Telescope European Coordinating Facility, 2009); http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2009-18.pdf.
Christie, D., Arras, P. & Li, Z. Hα absorption in transiting exoplanet atmospheres. Astrophys. J. 772, 144 (2013).
Amundsen, D. et al. Accuracy tests of radiation schemes used in hot Jupiter global circulation models. Astron. Astrophys. 564, A59 (2014).
Tremblin, P. et al. Fingering convection and cloudless models for cool brown dwarf atmospheres. Astrophys. J. 804, L17 (2015).
Oklopčić, A. & Hirata, C. M. A new window into escaping exoplanet atmospheres: 10830 Å line of metastable helium. Astrophys. J. 855, L11 (2018).
Kulow, J. R., France, K., Linsky, J. & Loyd, R. O. P. Lyα transit spectroscopy and the neutral hydrogen tail of the hot Neptune GJ 436b. Astrophys. J. 786, 132 (2014).
Bourrier, V. Lecavelier des Etangs, A. Ehrenreich., D., Tanaka., Y. A. & Vidotto, A. A. An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b. Astron. Astrophys. 591, A121 (2016).
Lopez, E. D., Fortney, J. J. & Miller, N. How thermal evolution and mass-loss sculpt populations of super-Earths and sub-Neptunes: application to the Kepler-11 system and beyond. Astrophys. J. 761, 59 (2012).
Owen, J. & Wu, Y. Kepler planets: a tale of evaporation. Astrophys. J. 775, 105 (2013).
Jin, S. et al. Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation. Astrophys. J. 795, 65 (2014).
Chen, H. & Rogers, L. A. Evolutionary analysis of gaseous sub-Neptune-mass planets with MESA. Astrophys. J. 831, 180 (2016).
Fulton, B. et al. The California-Kepler survey. III. A gap in the radius distribution of small planets. Astron. J. 154, 109 (2017).
Lopez, E. & Fortney, J. J. Understanding the mass-radius relation for sub-Neptunes: radius as a proxy for composition. Astrophys. J. 792, 1 (2014).
Vidal-Madjar, A. et al. An extended upper atmosphere around the extrasolar planet HD209458b. Nature 422, 143–146 (2003).
Lecavelier des Etangs, A. et al. Evaporation of the planet HD 189733b observed in H i Lyman-α. Astron. Astrophys. 514, A72 (2010).
Jensen, A. G. et al. A detection of Hα in an exoplanetary exosphere. Astrophys. J. 751, 86 (2012).
Ryan, R. E. Jr et al. The Updated Calibration Pipeline for WFC3/UVIS: A Reference Guide to CALWF3 (Version 3.3). Report No. WFC3-2016-001 (Space Telescope Science Institute, 2016); http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2016-01.pdf.
Evans, T. M. et al. Detection of H2O and evidence for TiO/VO in an ultra-hot exoplanet atmosphere. Astrophys. J. 822, L4 (2016).
Claret, A. A new non-linear limb-darkening law for LTE stellar atmosphere models. Calculations for −5.0 < = log[M/H] < = +1, 2000 K <= Teff < = 50000 K at several surface gravities. Astron. Astrophys. 363, 1081–1190 (2000).
Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pacif. 125, 306–312 (2013).
Močnik, T., Hellier, C., Anderson, D. R., Clark, B. J. M. & Southworth, J. Starspots on WASP-107 and pulsations of WASP-118. Mon. Not. R. Astron. Soc. 469, 1622–1629 (2017).
Czesla, S., Klocová, T., Khalafinejad, S., Wolter, U. & Schmitt, J. H. M. M. The center-to-limb variation across the Fraunhofer lines of HD 189733. Sampling the stellar spectrum using a transiting planet. Astron. Astrophys. 582, A51 (2015).
Deming, D. et al. Infrared transmission spectroscopy of the exoplanets HD 209458b and XO-1b using the Wide Field Camera-3 on the Hubble Space Telescope. Astrophys. J. 774, 95 (2013).
Nutzman, P. & Charbonneau, D. Design considerations for a ground-based transit search for habitable planets orbiting M dwarfs. Publ. Astron. Soc. Pacif. 120, 317 (2008).
Irwin, J. M. et al. The MEarth-North and MEarth-South transit surveys: searching for habitable super-Earth exoplanets around nearby M-dwarfs. In Proc. 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (eds van Belle, G. & Harris, H. C.) 767–762 (2014); preprint at https://arxiv.org/abs/1409.0891.
Irwin, J. et al. The Monitor project: rotation of low-mass stars in the open cluster M34. Mon. Not. R. Astron. Soc. 370, 954 (2006).
Berta, Z. K. et al. Transit detection in the MEarth survey of nearby M dwarfs: bridging the clean-first, search-later divide. Astron. J. 144, 145 (2012).
Irwin, J. et al. On the angular momentum evolution of fully convective stars: rotation periods for field M-dwarfs from the MEarth transit survey. Astrophys. J. 727, 56 (2011).
Henry, G. W. Techniques for automated high-precision photometry of Sun-like stars. Publ. Astron. Soc. Pacif. 111, 845–860 (1999).
Eaton, J. A., Henry, G. W. & Fekel, F. C. in The Future of Small Telescopes in the New Millennium, Volume II – The Telescopes We Use (ed. Oswalt T. D.) 189–207 (Kluwer, Dordrecht, 2003).
Sing, D. K. et al. HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering. Mon. Not. R. Astron. Soc. 446, 2428–2443 (2015).
Aigrain, S., Pont, F. & Zucker, S. A simple method to estimate radial velocity variations due to stellar activity using photometry. Mon. Not. R. Astron. Soc. 419, 3147–3158 (2012).
Huitson, C. et al. An HST optical-to-near-IR transmission spectrum of the hot Jupiter WASP-19b: detection of atmospheric water and likely absence of TiO. Mon. Not. R. Astron. Soc. 434, 3252–3274 (2013).
Sing, D. K. et al. Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS. Mon. Not. R. Astron. Soc. 416, 1443–1455 (2011).
Tremblin, P. et al. Cloudless Atmospheres for L/T Dwarfs and Extrasolar Giant Planets. Astrophys. J. 817, L19 (2016).
Wakeford, H. R. et al. HAT-P-26b: A Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356, 628–631 (2017).
Evans, T. M. et al. An ultrahot gas-giant exoplanet with a stratosphere. Nature 548, 58–61 (2017).
Kramida, A. et al. NIST Atomic Spectra Database (version 5.5.1) https://physics.nist.gov/asd (2017).
Tennyson, J. et al. The ExoMol database: molecular line lists for exoplanet and other hot atmospheres. J. Mol. Spectrosc. 327, 73–94 (2016).
Gordon, I. E. et al. The HITRAN 2016 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 203, 3–69 (2017).
Rothman, L. S. et al. HITEMP, the high-temperature molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 111, 2139–2150 (2010).
Brammer, G., Pirzkal, N., McCullough, P. & MacKenty, J. Time-varying Excess Earth-glow Backgrounds in the WFC3/IR Channel. Report No. WFC3 2014-03 (Space Telescope Science Institute, 2014); http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2014-03.pdf.
Kreidberg, L. et al. A detection of water in the transmission spectrum of the hot Jupiter WASP-12b and implications for its atmospheric composition. Astrophys. J. 814, 66 (2015).
Wakeford, H. et al. The complete transmission spectrum of WASP-39b with a precise water constraint. Astron. J. 155, 29 (2017).
Andretta, V. & Jones, H. P. On the role of the solar corona and transition region in the excitation of the spectrum of neutral helium. Astrophys. J. 489, 375–394 (1997).
Vaughan, A. H. & Zirin, H. The helium line λ10830 Å in late-type stars. Astrophys. J. 152, 123–139 (1968).
Zirin, H. λ10830 He i observations of 455 stars. Astrophys. J. 260, 655–669 (1982).
Zarro, D. M. & Zirin, H. The dependence of He i λ10830 absorption strength upon X-ray emission in late-type stars. Astrophys. J. 304, 365–370 (1986).
Sanz-Forcada, J. & Dupree, A. K. Active cool stars and He i 10830 Å: the coronal connection. Astron. Astrophys. 488, 715–721 (2008).
Takeda, Y. & Takada-Hidai, M. Chromospheres in metal-poor stars evidenced from the He i 10830 Å line. Publ. Astron. Soc. Jpn. 63, S547–S554 (2011).
Andretta, V., Giampapa, M. S., Covino, E., Reiners, A. & Beeck, B. Estimates of active region area coverage through simultaneous measurements of the He i λλ 5876 and 10830 lines. Astrophys. J. 839, 97 (2017).
Isaacson, H. I. & Fischer, D. Chromospheric activity and jitter measurements for 2630 stars on the California planet search. Astrophys. J. 725, 875–885 (2010).
Andretta, V. & Giampapa, M. S. A method for estimating the fractional area coverage of active regions on dwarf F and G stars. Astrophys. J. 439, 405–416 (1995).
Deming, D. & Sheppard, K. Spectral resolution-linked bias in transit spectroscopy of extrasolar planets. Astrophys. J. 841, L3 (2017).
Li, H., You, J., Yu, X. & Du, Q. Spectral characteristics of solar flares in different chromospheric lines and their implications. Sol. Phys. 241, 301–315 (2007).
Rackham, B. et al. ACCESS I: an optical transmission spectrum of GJ 1214b reveals a heterogeneous stellar photosphere. Astrophys. J. 834, 151 (2017).
Rackham, B. V., Apai, D. & Giampapa, M. S. The transit light source effect: false spectral features and incorrect densities for M-dwarf transiting planets. Astrophys. J. 853, 122 (2018).
Husser, T.-O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, A6 (2013).
Beryugina, S. Starspots: a key to the stellar dynamo. Living Rev. Sol. Phys. 2, 8 (2005).
Afram, N. & Beryudina, S. Molecules as magnetic probes of starspots. Astron. Astrophys. 576, A34 (2015).
Gondoin, P. Contribution of Sun-like faculae to the light-curve modulation of young active dwarfs. Astron. Astrophys. 478, 883–887 (2008).
Parker, E. N. Dynamics of the interplanetary gas and magnetic fields. Astrophys. J. 128, 664–676 (1958).
France, K. et al. The MUSCLES treasury survey. I. Motivation and overview. Astrophys. J. 820, 89 (2016).
Youngblood, A. et al. The MUSCLES treasury survey. II. Intrinsic Lyα and extreme ultraviolet spectra of K and M dwarfs with exoplanets. Astrophys. J. 824, 101 (2016).
Loyd, R. O. P. et al. The MUSCLES treasury survey. III. X-ray to infrared spectra of 11 M and K stars hosting planets. . Astrophys. J. 824, 102 (2016).
Lecavelier des Etangs, A., Vidal-Madjar, A., McConnell, J. C. & Hebrard, G. Atmospheric escape from hot Jupiters. Astron. Astrophys. 418, L1–L4 (2004).
Salz, M., Czesla, S., Schneider, P. C. & Schmitt, J. H. M. M. Simulating the escaping atmospheres of hot gas planets in the solar neighborhood. Astron. Astrophys. 586, A75 (2016).
Ehrenreich, D. et al. A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature 522, 459–461 (2015).
We thank S. Seager, A. Dupree, V. Andretta, M. Giampapa and B. Drummond for discussions. This work is based on observations made with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. J.J.S. is supported by an STFC studentship. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 336792. Support for this work was provided by NASA grants under the HST-GO-14916 programme of the STScI. G.W.H. and M.H.W. acknowledge support from Tennessee State University and the State of Tennessee through its Centers of Excellence programme. The MEarth Team (J.I. and D.C.) gratefully acknowledges funding from the David and Lucille Packard Fellowship for Science and Engineering, the US National Science Foundation (NSF) and the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. This work was carried out in the framework of the National Centre for Competence in Research PlanetS supported by the Swiss National Science Foundation (SNSF). V.B., D.E., A.W. and S.U. acknowledge financial support from the SNSF. D.E. and V.B. acknowledge funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (project FOUR ACES; grant agreement No 724427). B.V.R acknowledges support from an NSF Graduate Research Fellowship (grant DGE-1746060) and the Earth in Other Solar Systems Team, NASA Nexus for Exoplanet System Science. IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF. J.M.G. acknowledges support from a Leverhulme Trust Research Project Grant.
Nature thanks D. Deming and S. Redfield for their contribution to the peer review of this work.
The authors declare no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 G102 white-light curve and broadband spectroscopic light curves covering the wavelength range 0.88–1.14 μm for WASP-107b.
a, Relative flux of the white-light curve with respect to systematics model results (blue points), with the best-fitting transit light curve plotted in black. b, White-light residuals and 1σ errors, after removing the combined transit and systematics components of the best-fitting model. c, Points are spectroscopic light curves divided by systematics model results, and black curves are best-fitting transit models, with vertical offsets applied for clarity. d, Best-fitting spectroscopic model residuals, with vertical offsets applied for clarity. Differently coloured points in c and d are used to highlight separate channels.
Extended Data Fig. 2 Narrowband (four-pixel-wide) spectroscopic light curves covering the wavelength range 1.06–1.12 μm.
a, Points are light curves divided by systematics model results, black curves are best-fitting transit models. b, Best-fitting model residuals, with vertical offsets applied for clarity. Differently coloured points correspond to different channels. The five non-overlapping channels used to measure the 10,833-Å line absorption are highlighted in blue.
We performed a Lomb–Scargle periodogram search and found a best-fitting period of 19.7 ± 0.9 days (dashed red line), with a relative amplitude of about 0.15%. Solid red lines show the results of the best-fitting sinusoidal model.
a, Nightly photometric observations of WASP-107, acquired with Tennessee State University’s C14 AIT at the Fairborn Observatory during the 2017 observing season. The number of observations (Nobs) was 120. ΔR is the relative flux in the Cousins R band. b, The frequency spectrum of the 2017 observations shows low-amplitude variability with a period (P) of 8.675 days (a frequency, f, of 0.115 cycles per day). c, The data phased to the 8.675-day period have a peak-to-peak amplitude of only 0.005 mag. HJD, heliocentric Julian Date; UCT, coordinated universal time; c/d, cycles per day; Tmin, best-fit ephemeris.
a, Measurements for 30 stars of different colour indices with 1σ errors from a previous work60. The two helium lines are expected to form in the same stellar-atmosphere regions and their equivalent widths are clearly correlated. Our 5,876-Å line measurement for WASP-107 (colour index B–V > 0.7), obtained from HARPS spectra, is plotted as a red line and the red-shaded region shows the 1σ error. b, Co-added spectra of WASP-107b around the 5,876-Å line of metastable helium from the HARPS radial-velocity campaign (blue line). Absorption lines are fitted with Gaussian profiles, and the best-fit results are shown as green, yellow and red lines, with the sum of the profiles shown in black. The best-fitting line profile of the 5,876-Å line is shaded in grey.
Extended Data Fig. 6 Effects of an inhomogeneous photosphere on the transmission spectrum of WASP-107b.
Lines show the stellar contamination produced by unocculted spots and faculae. Shaded regions indicate the 1σ uncertainty on the stellar contamination due to the uncertainty on spot- and faculae-covering fractions. a, The region around the 10,830-Å (air wavelength) helium triplet at the resolution of the PHOENIX spectra (resolving power, R = 500,000). (b) The full G102 wavelength range in 15-Å bins.
About this article
Cite this article
Spake, J.J., Sing, D.K., Evans, T.M. et al. Helium in the eroding atmosphere of an exoplanet. Nature 557, 68–70 (2018). https://doi.org/10.1038/s41586-018-0067-5
Astronomy & Astrophysics (2020)
Astronomy & Astrophysics (2020)
Research in Astronomy and Astrophysics (2020)
ExoTiC-ISM: A Python package for marginalised exoplanet transit parameters across a grid of systematic instrument models
Journal of Open Source Software (2020)
Why Is it So Cold in Here? Explaining the Cold Temperatures Retrieved from Transmission Spectra of Exoplanet Atmospheres
The Astrophysical Journal (2020)