LHS 1140 b Is a Potentially Habitable Water World

LHS 1140 b is a small planet orbiting in the habitable zone of its M4.5V dwarf host. Recent mass and radius constraints have indicated that it has either a thick H 2 -rich atmosphere or substantial water by mass. Here we present a transmission spectrum of LHS 1140 b between 1.7 and 5.2 μ m, obtained...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2024-06, Vol.968 (2), p.L22
Main Authors: Damiano, Mario, Bello-Arufe, Aaron, Yang, Jeehyun, Hu, Renyu
Format: Article
Language:English
Subjects:
Astronomy data analysis
Atmosphere
Atmospheric composition
Atmospheric models
Bayesian statistics
Carbon dioxide
Circumstellar habitable zone
Exoplanet atmospheric composition
Exoplanets
Habitable planets
Metallicity
Ocean models
Oceans
Planets
Transmission spectroscopy
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Summary:LHS 1140 b is a small planet orbiting in the habitable zone of its M4.5V dwarf host. Recent mass and radius constraints have indicated that it has either a thick H 2 -rich atmosphere or substantial water by mass. Here we present a transmission spectrum of LHS 1140 b between 1.7 and 5.2 μ m, obtained using the NIRSpec instrument on JWST. By combining spectral retrievals and self-consistent atmospheric models, we show that the transmission spectrum is inconsistent with H 2 -rich atmospheres with varied size and metallicity, leaving a water world as the remaining scenario to explain the planet’s low density. Specifically, a H 2 -rich atmosphere would result in prominent spectral features of CH 4 or CO 2 on this planet, but they are not seen in the transmission spectrum. Instead, the data favor a high mean molecular weight atmosphere (possibly N 2 dominated with H 2 O and CO 2 ) with a modest confidence. Forming the planet by accreting C- and N-bearing ices could naturally give rise to a CO 2 - or N 2 -dominated atmosphere, and if the planet evolves to or has the climate-stabilizing mechanism to maintain a moderate-size CO 2 /N 2 -dominated atmosphere, the planet could have liquid-water oceans. Our models suggest CO 2 absorption features with an expected signal of 20 ppm at 4.2 μ m. As the existence of an atmosphere on TRAPPIST-1 planets is uncertain, LHS 1140 b may well present the best current opportunity to detect and characterize a habitable world.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/ad5204