Exoplanets & Exomoons

LHS 1140 b Is A Potentially Habitable Water World

By Keith Cowing
Status Report
March 21, 2024
Filed under , , , , , , ,
LHS 1140 b Is A Potentially Habitable Water World
Transmission spectrum of LHS 1140 b. The two transit observations are color coded, i.e., blue for the G235H grating and green for the G395H grating. The models shown are randomly selected solutions from the posterior distributions of Scenario 4 (Table 4, corresponding to N2-H2O atmospheres with deep clouds). We have applied the best-fit offsets to the data. The offset between the two visits is ∼ 50 ppm, while the offset between the two detectors within each of the two visits is negligible (see Figure 4 and Table 5). — astro-ph.EP

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 H2-rich atmosphere or substantial water by mass.

Here we present a transmission spectrum of LHS 1140 b between 1.7 and 5.2 μm, 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 H2-rich atmospheres with varied size and metallicity, leaving a water world as the remaining scenario to explain the planet’s low density. Specifically, a H2-rich atmosphere would result in prominent spectral features of CH4 or CO2 on this planet, but they are not seen in the transmission spectrum.

Instead, the data favors a high-mean-molecular-weight atmosphere (possibly N2-dominated with H2O and CO2) with a modest confidence. Forming the planet by accreting C- and N-bearing ices could naturally give rise to a CO2– or N2-dominated atmosphere, and if the planet evolves to or has the climate-stabilizing mechanism to maintain a moderate-size CO2/N2-dominated atmosphere, the planet could have liquid-water oceans.

Our models suggest CO2 absorption features with an expected signal of 20 ppm at 4.2 μm. 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.

Self-consistent atmospheric models of LHS 1140 b compared to the JWST data. Top panel shows the massive H2-rich atmosphere models with varied metallicities as well as the small H2-dominated atmosphere with CO2 model (“hycean world”). χ 2 =1405, 506, and 289 for the 1×, 10×, and 100× solar metallicity models, respectively, and χ 2 =473 for the hycean model. These models are ruled out by the data with a p-value < 0.00001. Bottom panel shows the N2– and CO2-dominated atmosphere models. χ 2 =118 for both models. — astro-ph.EP

Potential processes that partition C and N species between the atmosphere, the ocean, and the HP ices in a cold water world like LHS 1140 b. Summarized based on Levi et al. (2017); Ramirez & Levi (2018); Levi & Cohen (2019); Marounina & Rogers (2020); Vazan et al. (2022); Kovaˇcevi´c et al. (2022). — astro-ph.EP

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

Comments: 18 pages, 10 figures, 5 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2403.13265 [astro-ph.EP] (or arXiv:2403.13265v1 [astro-ph.EP] for this version)
Submission history
From: Mario Damiano
[v1] Wed, 20 Mar 2024 03:02:16 UTC (18,440 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻