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

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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.

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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).


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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

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.

Correspondence to Angelos Tsiaras or Ingo P. Waldmann.

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

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