Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b

Abstract

In the past decade, observations from space and the ground have found water to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous extrasolar planets1,2,3,4,5. Being the main molecular carrier of oxygen, water is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of water is of great importance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected6. Super-Earths—planets lighter than ten Earth masses—around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b—a planet of eight Earth masses in the habitable zone of an M dwarf7—with high statistical confidence (Atmospheric Detectability Index5 = 5.0, ~3.6σ (refs. 8,9)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms5,9. Although the suitability of M dwarfs to host habitable worlds is still under discussion10,11,12,13, K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Analysis of the K2-18 b white and spectral light curves, plotted with an offset for clarity.
Fig. 2: Best-fit models for the three different scenarios tested.
Fig. 3: Posterior distributions for the three different scenarios tested.

Data availability

The data analysed in this work are available through the NASA MAST HST archive (https://archive.stsci.edu/) programmes 13665 and 14682. The molecular line lists used are available from the ExoMol website (www.exomol.com). The final and intermediate results (reduced data, extracted light curves, light curve fitting results and atmospheric fitting results) are available through the University College London Exoplanets website (https://www.ucl.ac.uk/exoplanets) and the Open Science Framework (OSF) website at https://doi.org/10.17605/OSF.IO/N7DQX.

Code availability

All the software used to produce the presented results are publicly available through the University College London Exoplanets GitHub website (https://github.com/ucl-exoplanets/). More specifically, the codes used were Tau-REx (https://github.com/ucl-exoplanets/TauREx_public), Iraclis (https://github.com/ucl-exoplanets/Iraclis) and PyLightcurve (https://github.com/ucl-exoplanets/pylightcurve).

Change history

  • 25 September 2019

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

References

  1. 1.

    Tinetti, G. et al. Infrared transmission spectra for extrasolar giant planets. Astrophys. J. Lett. 654, L99–L102 (2007).

    ADS  Google Scholar 

  2. 2.

    Grillmair, C. J. et al. Strong water absorption in the dayside emission spectrum of the planet HD189733b. Nature 456, 767–769 (2008).

    ADS  Google Scholar 

  3. 3.

    Fraine, J. et al. Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet. Nature 350, 64–67 (2015).

    Google Scholar 

  4. 4.

    Macintosh, B. et al. Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager. Science 456, 767–769 (2008).

    Google Scholar 

  5. 5.

    Tsiaras, A. et al. A population study of gaseous exoplanets. Astron. J. 155, 156 (2018).

    ADS  Google Scholar 

  6. 6.

    de Wit, J. et al. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nat. Astron. 2, 214–219 (2018).

    ADS  Google Scholar 

  7. 7.

    Montet, B. T. et al. Stellar and planetary properties of K2 Campaign 1 candidates and validation of 17 planets, including a planet receiving Earth-like insolation. Astrophys. J. 809, 25 (2015).

    ADS  Google Scholar 

  8. 8.

    Benneke, B. & Seager, S. How to distinguish between cloudy mini-Neptunes and water/volatile-dominated super-Earths. Astrophys. J. 778, 153 (2013).

    ADS  Google Scholar 

  9. 9.

    Waldmann, I. P. et al. Tau-REx I: a next generation retrieval code for exoplanetary atmospheres. Astrophys. J. 802, 107 (2015).

    ADS  Google Scholar 

  10. 10.

    Segura, A. et al. Biosignatures from Earth-like planets around M dwarfs. Astrobiology 5, 706–725 (2005).

    ADS  Google Scholar 

  11. 11.

    Wordsworth, R. D. et al. Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone. Astrophys. J. 733, L48 (2011).

    ADS  Google Scholar 

  12. 12.

    Leconte, J. et al. 3D climate modeling of close-in land planets: circulation patterns, climate moist bistability, and habitability. Astron. Astrophys. 554, A69 (2013).

    Google Scholar 

  13. 13.

    Turbet, M. et al. The habitability of Proxima Centauri B. II. Possible climates and observability. Astron. Astrophys. 596, A112 (2016).

    Google Scholar 

  14. 14.

    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).

    ADS  Google Scholar 

  15. 15.

    Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ1214b. Nature 505, 69–72 (2014).

    ADS  Google Scholar 

  16. 16.

    Knutson, H. A. et al. Hubble Space Telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Astrophys. J. 794, 155 (2014).

    ADS  Google Scholar 

  17. 17.

    Tsiaras, A. et al. Detection of an atmosphere around the super-Earth 55 Cancri e. Astrophys. J. 820, 99 (2016).

    ADS  Google Scholar 

  18. 18.

    Wakeford, H. R. et al. Disentangling the planet from the star in late-type M dwarfs: a case study of TRAPPIST-1g. Astron. J. 157, 11 (2019).

    ADS  Google Scholar 

  19. 19.

    Benneke, B. et al. Spitzer observations confirm and rescue the habitable-zone super-Earth K2-18b for future characterization. Astrophys. J. 834, 187 (2017).

    ADS  Google Scholar 

  20. 20.

    Kopparapu, R. K. A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around Kepler M-dwarfs. Astrophys. J. Lett. 767, L8 (2013).

    ADS  Google Scholar 

  21. 21.

    Valencia, D., Tan, V. Y. Y. & Zajac, Z. Habitability from tidally induced tectonics. Astrophys. J. 857, 106 (2018).

    ADS  Google Scholar 

  22. 22.

    Cloutier, R. et al. Characterization of the K2-18 multi-planetary system with HARPS. A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit. Astron. Astrophys. 608, A35 (2017).

    Google Scholar 

  23. 23.

    Valencia, D., Guillot, T., Parmentier, V. & Freedman, R. S. Bulk composition of GJ 1214b and other sub-Neptune exoplanets. Astrophys. J. 775, 10 (2013).

    ADS  Google Scholar 

  24. 24.

    Zeng, L., Sasselov, D. D. & Jacobsen, S. B. Mass–radius relation for rocky planets based on PREM. Astrophys. J. 819, 127 (2016).

    ADS  Google Scholar 

  25. 25.

    Tsiaras, A. et al. A new approach to analyzing HST spatial scans: the transmission spectrum of HD 209458 b. Astrophys. J. 832, 202 (2016).

    ADS  Google Scholar 

  26. 26.

    Eastman, J. et al. Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian dates. Publ. Astron. Soc. Pac. 122, 935 (2010).

    ADS  Google Scholar 

  27. 27.

    Waldmann, I. P. et al. Tau-REx II: retrieval of emission spectra. Astrophys. J. 813, 13 (2015).

    ADS  Google Scholar 

  28. 28.

    Tennyson, J. et al. The ExoMol database: molecular line lists for exoplanet and other hot atmospheres. J. Mol. Spec. 327, 73–94 (2016).

    ADS  Google Scholar 

  29. 29.

    Tinetti, G. et al. A chemical survey of exoplanets with ARIEL. Exp. Astron. 46, 135–209 (2018).

    ADS  Google Scholar 

  30. 30.

    Allard, F., Homeier, D. & Freytag, B. Models of very-low-mass stars, brown dwarfs and exoplanets. Philos. Trans. R. Soc. A 370, 2765–2777 (2012).

    ADS  Google Scholar 

  31. 31.

    Claret, A. A new non-linear limb-darkening law for LTE stellar atmosphere models. Calculations for −5.0 < = log[M/H] < = +1, 2000 K < = T eff < = 50000 K at several surface gravities. Astron. Astrophys. 363, 1081–1190 (2000).

    ADS  Google Scholar 

  32. 32.

    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).

    ADS  Google Scholar 

  33. 33.

    Evans, T. M. et al. Detection of H2O and evidence for TiO/VO in an ultra-hot exoplanet atmosphere. Astrophys. J. Lett. 822, L4 (2016).

    ADS  Google Scholar 

  34. 34.

    Line, M. R. et al. No thermal inversion and a solar water abundance for the hot Jupiter HD 209458b from HST/WFC3 spectroscopy. Astron. J. 152, 203 (2016).

    ADS  Google Scholar 

  35. 35.

    Wakeford, H. R. et al. HST PanCET program: a cloudy atmosphere for the promising JWST target WASP-101b. Astrophys. J. Lett. 835, L12 (2017).

    ADS  Google Scholar 

  36. 36.

    McCullough, P. & MacKenty, J. Considerations for Using Spatial Scans with WFC3 Instrument Science Report WFC3 2012-08 (STSI, 2012).

  37. 37.

    Sarkis, P. et al. The CARMENES search for exoplanets around M dwarfs: a low-mass planet in the temperate zone of the nearby K2-18. Astron. J. 155, 257 (2018).

    ADS  Google Scholar 

  38. 38.

    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).

    ADS  Google Scholar 

  39. 39.

    Skilling, J. Nested sampling for general Bayesian computation. Bayesian Anal. 1, 833–860 (2006).

    MathSciNet  MATH  Google Scholar 

  40. 40.

    Feroz, F., Hobson, M. P. & Bridges, M. MULTINEST: an efficient and robust Bayesian inference tool for cosmology and particle physics. Mon. Not. R. Astron. Soc. 398, 1601–1614 (2009).

    ADS  Google Scholar 

  41. 41.

    Barber, R. J., Tennyson, J., Harris, G. J. & Tolchenov, R. N. A high-accuracy computed water line list. Mon. Not. R. Astron. Soc. 368, 1087–1094 (2006).

    ADS  Google Scholar 

  42. 42.

    Rothman, L. S. et al. HITEMP, the high-temperature molecular spectroscopic database. J. Quant. Spec. Rad. Transf. 111, 2139–2150 (2010).

    ADS  Google Scholar 

  43. 43.

    Yurchenko, S. N. & Tennyson, J. ExoMol line lists—IV. the rotation–vibration spectrum of methane up to 1500 K. Mon. Not. R. Astron. Soc. 440, 1649–1661 (2014).

    ADS  Google Scholar 

  44. 44.

    Yurchenko, S. N., Barber, R. J. & Tennyson, J. A variationally computed line list for hot NH3. Mon. Not. R. Astron. Soc. 413, 1828–1834 (2011).

    ADS  Google Scholar 

  45. 45.

    Foreman-Mackey, D. corner.py: scatterplot matrices in Python. J. Open Source Softw. 24, 1 (2016).

    Google Scholar 

Download references

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreements 758892, ExoAI; 776403/ExoplANETS A) and under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement numbers 617119 (ExoLights) and 267219 (ExoMol). We further acknowledge funding by the Science and Technology Funding Council (STFC) grants ST/K502406/1 and ST/P000282/1. The data used here were obtained by the Hubble Space Telescope as part of the 13665 and 14682 GO proposals (PI: B. Benneke).

Author information

Affiliations

Authors

Contributions

A.T. performed the data analysis and developed the HST analysis software Iraclis; I.P.W. developed the atmospheric retrieval software Tau-REx; G.T. contributed to the interpretation of the results; J.T. and S.N.Y. coordinated the ExoMol project. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Angelos Tsiaras or Ingo P. Waldmann.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary tables and figures

Supplementary Tables 1–4 and Supplementary Figs. 1–21.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Tsiaras, A., Waldmann, I.P., Tinetti, G. et al. Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b. Nat Astron 3, 1086–1091 (2019). https://doi.org/10.1038/s41550-019-0878-9

Download citation

Further reading