Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b

Abstract

Ever since the discovery of the first exoplanet, astronomers have made steady progress towards finding and probing planets in the habitable zone of their host stars, where the conditions could be right for liquid water to form and life to sprawl. Results from the Kepler mission indicate that the occurrence rate of habitable-zone Earths and super-Earths may be as high as 5-20%. Despite this abundance, probing the conditions and atmospheric properties on any of these habitable-zone planets is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid water clouds in the atmosphere of the 8.6 M ⊕ habitable-zone planet K2-18b. With a 33 day orbit around a cool M3 dwarf, K2-18b receives virtually the same amount of total radiation from its host star (1441 ± 80 W/m 2 ) as the Earth receives from the Sun (1370 W/m 2 ), making it a good candidate to host liquid water clouds. In this study we observed eight transits using HST/WFC3 in order to achieve the necessary sensitivity to detect water vapor. While the thick gaseous envelope of K2-18b means that it is not a true Earth analogue, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.

The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b

“…The atmospheric constraints are shown in Figure 1 and Table 1. We confirm the high-confidence detection of H 2 O in a H 2 -rich atmosphere as reported by Benneke et al (2019) and Tsiaras et al (2019). Our abundance estimates are consistent to within 1σ between all four model configurations and with Benneke et al (2019).…”

“…We confirm the high-confidence detection of H 2 O in a H 2 -rich atmosphere as reported by Benneke et al (2019) and Tsiaras et al (2019). Our abundance estimates are consistent to within 1σ between all four model configurations and with Benneke et al (2019). The derived H 2 O volume mixing ratio ranges between 0.02-14.80%, with median values of 0.7-1.6% between the 4 model cases, as shown in Table 1.…”

“…We retrieve the atmospheric properties of K2-18b using its broadband transmission spectrum reported by Benneke et al (2019). The data include observations from the HST WFC3 G141 grism (1.1-1.7 µm), photometry in the Spitzer IRAC 3.6 µm and 4.5 µm bands, and optical photometry in the K2 band (0.4-1.0 µm).…”

“…Given its mass (M p = 8.63 ± 1.35 M ⊕ , Cloutier et al 2019) and radius (R p = 2.610 ± 0.087 R ⊕ , Benneke et al 2019), K2-18b has a bulk density (2.67 +0.52 −0.47 g/cm 3 , Benneke et al 2019). This density, between that of Earth and Neptune, may be thought to preclude a purely rocky or icy interior and require a hydrogen-rich outer envelope.…”

“…However, the extent of such an envelope and the conditions at the interface between the envelope and the underlying interior have not been explored. We note that the mass and radius of the planet have recently been revised (Benneke et al 2019), which may have impacted inferences made using previous values (Cloutier et al 2017;Tsiaras et al 2019).…”