Water Vapor On The Habitable-Zone Exoplanet K2-18b

Transmission spectrum of K2-18b computed from our global spectroscopic and broadband transit light curve analysis (black points), and a random sampling of the model transmission spectra in the retrieval MCMC chain (blue curves). The main feature of the transmission spectrum is the prominent increase in transit depth with the 1.4 µm vibrational bands of water vapor covered by the HST/WFC3 data. The K2 data point is plotted at visible wavelengths and the Spitzer IRAC measurements are indicated at 3.6 µm and 4.5 µm.

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/m2) as the Earth receives from the Sun (1370 W/m2), 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 present-day telescopes.

Björn Benneke, Ian Wong, Caroline Piaulet, Heather A. Knutson, Ian J.M. Crossfield, Joshua Lothringer, Caroline V. Morley, Peter Gao, Thomas P. Greene, Courtney Dressing, Diana Dragomir, Andrew W. Howard, Peter R. McCullough, Eliza M.-R. Kempton Jonathan J. Fortney, Jonathan Fraine
(Submitted on 10 Sep 2019)

Comments: 10 pages, 6 figures, Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:1909.04642 [astro-ph.EP] (or arXiv:1909.04642v1 [astro-ph.EP] for this version)
Submission history
From: Björn Benneke [view email]
[v1] Tue, 10 Sep 2019 17:34:58 UTC (6,358 KB)
https://arxiv.org/abs/1909.04642
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