LHS 1140 b is a potentially habitable water world

Mario Damiano; Aaron Bello-Arufe; Jeehyun Yang; Renyu Hu

doi:10.3847/2041-8213/ad5204

paper

LHS 1140 b is a potentially habitable water world

arXiv:2403.13265 · doi:10.3847/2041-8213/ad5204

Abstract

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 favors 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.

arXiv comment20 pages, 10 figures, 9 tables, accepted for publication in ApJL

LHS 1140 b is a potentially habitable water world

Abstract

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