LHS 1140 b Is a Potentially Habitable Water World

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 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 favor 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.

Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS

“…The NIRISS observations are more likely consistent with the water-world scenarios presented by Cadieux et al (2024), with or without a secondary atmosphere, a conclusion also supported by recent transmission spectroscopy data obtained with JWST/NIRSpec (Damiano et al 2024). Of all nearby small, temperate planets, LHS 1140 b is the most likely to have retained a secondary atmosphere based on its low instellation and high surface gravity.…”

“…The consistent facula parameters from our LHS 1140 b and LHS 1140 c analyses confirm that these unocculted faculae lie outside the stellar surface sampled by the two planetary transit chords. We additionally note that Damiano et al (2024) reported a spot-crossing event during a different set of two transits of LHS 1140 b observed with NIRSpec in 2023 July. Their observations, conducted 124 days prior to our NIRISS data set (about one full rotation of LHS 1140), disfavor unocculted heterogeneities (their Table 4).…”

“…An airless Europa-like surface (A B = 0.6; Buratti & Veverka 1983) would require ∼19 visits to detect the secondary eclipse, i.e., observing every eclipse for nearly 5 yr. Detecting and discriminating the water-world cases (an Earth-like or CO 2 -rich atmosphere) is out of reach through eclipse photometry at 15 μm only and would probably require the combination of many filters (e.g., 15 + 21 μm) over many eclipses. Cadieux et al (2024) estimated that 12 transits (six times each of NIRSpec G235 and G395) would be required to detect the CO 2 features longward of 2.8 μm, an exposure time in line with the nine additional G395 visits proposed by Damiano et al (2024) to detect the CO 2 feature at 4.3 μm. While G395 .…”

“…These observations imply a large fractional coverage (>80%) of stellar active regions, which is somewhat surprising given that LHS 1140 is a relatively old (>5 Gyr; Cadieux et al 2024), slowly rotating star (P rot = 131 ± 5 days; Dittmann et al 2017), with low levels of flaring activity (Medina et al 2022). Recent observations of two transits of LHS 1140 b with JWST/NIRSpec (G235H/ G395H, 1.7-5.2 μm) ruled out an H 2 -rich atmosphere and favored a high mean molecular weight atmosphere (Damiano et al 2024). Further TLS characterization with JWST is required, especially in the context of seeking the detection of small atmospheric signals that could be severely affected/ biased by stellar activity.…”