Abstract
Abstract Results from the Kepler mission indicate that the occurrence rate of small planets (<3 R ⊕) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the 2.6 R ⊕ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star ( 1368 − 107 + 114 W m−2) as the Earth receives from the Sun (1361 W m−2), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study we observed nine transits of K2-18b using Hubble Space Telescope/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.
Highlights
The recent discovery of the transiting 8.63 ± 1.35 M⊕ exoplanet K2-18b in the habitable zone of a bright, nearby M3dwarf provides us with an opportunity to carry out a spectroscopic study of the atmosphere of a habitable-zone planet outside our solar system (Montet et al 2015; Benneke et al 2017; Cloutier et al 2017, 2019; Sarkis et al 2018)
We present the detection of water vapor and clouds in the atmosphere of the habitable-zone exoplanet K218b
Our retrieval modeling shows that the data are best matched by a hydrogen-dominated atmosphere with water vapor absorbing above clouds in the mid-atmosphere (Figure 4)
Summary
The recent discovery of the transiting 8.63 ± 1.35 M⊕ exoplanet K2-18b in the habitable zone of a bright, nearby M3dwarf provides us with an opportunity to carry out a spectroscopic study of the atmosphere of a habitable-zone planet outside our solar system (Montet et al 2015; Benneke et al 2017; Cloutier et al 2017, 2019; Sarkis et al 2018). The short orbital periods for habitable-zone planets around mid M dwarfs (30–70 days) enables the observation of repeated transits within a relatively short time frame. These planets have no analogs in the solar system, and aside from the recent atmospheric detection for GJ 3470b (Benneke et al 2019), most atmospheric studies resulted in nondetections due to the prevalence of high-altitude clouds (Knutson et al 2014b; Kreidberg et al 2014; Crossfield et al 2017).
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