LHS 1140 b is a potentially habitable water world

TL;DR

This study uses JWST transmission spectroscopy to analyze LHS 1140 b, providing evidence that it is likely a water-rich world with a high-mean-molecular-weight atmosphere, making it a prime candidate for habitability.

Contribution

First detailed JWST transmission spectrum analysis of LHS 1140 b, constraining its atmosphere as likely water-rich rather than H2-dominated, and discussing implications for habitability.

Findings

Inconsistent with H2-rich atmospheres due to absence of CH4 and CO2 features.

Data favors a high-mean-molecular-weight atmosphere, possibly N2-dominated with water and CO2.

Potential for liquid water oceans if a stable CO2/N2 atmosphere is maintained.

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 $\mu$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 $\mu$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.