New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World

Abstract

The two-planet transiting system LHS 1140 has been extensively observed since its discovery in 2017, notably with Spitzer, HST, TESS, and ESPRESSO, placing strong constraints on the parameters of the M4.5 host star and its small temperate exoplanets, LHS 1140 b and c. Here, we reanalyze the ESPRESSO observations of LHS 1140 with the novel line-by-line framework designed to fully exploit the radial velocity content of a stellar spectrum while being resilient to outlier measurements. The improved radial velocities, combined with updated stellar parameters, consolidate our knowledge of the mass of LHS 1140 b (5.60 ± 0.19 M ⊕) and LHS 1140 c (1.91 ± 0.06 M ⊕) with an unprecedented precision of 3%. Transits from Spitzer, HST, and TESS are jointly analyzed for the first time, allowing us to refine the planetary radii of b (1.730 ± 0.025 R ⊕) and c (1.272 ± 0.026 R ⊕). Stellar abundance measurements of refractory elements (Fe, Mg, and Si) obtained with NIRPS are used to constrain the internal structure of LHS 1140 b. This planet is unlikely to be a rocky super-Earth, as previously reported, but rather a mini-Neptune with a ∼0.1% H/He envelope by mass or a water world with a water-mass fraction between 9% and 19%, depending on the atmospheric composition and relative abundance of Fe and Mg. While the mini-Neptune case would not be habitable, a water-abundant LHS 1140 b potentially has habitable surface conditions according to 3D global climate models, suggesting liquid water at the substellar point for atmospheres with relatively low CO2 concentration, from Earth-like to a few bars.

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

“…Transmission spectroscopy with Hubble Space Telescope (HST)/WFC3 shows a tentative signal of H 2 O/CH 4 near 1.4 μm in a low mean molecular weight atmosphere (Edwards et al 2021;Biagini et al 2024), but a combination of unocculted spots and faculae offers an alternative explanation -a hypothesis also supported by ground-based measurements (Diamond-Lowe et al 2020). 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).…”

“…LHS 1140 (Dittmann et al 2017;Ment et al 2019;Lillo-Box et al 2020;Cadieux et al 2024) is a keystone system for habitability studies. This M4.5 dwarf (Dittmann et al 2017) hosts two small transiting planets: an outer planet LHS 1140 b (R p = 1.73 R ⊕ ), a rare object in the middle of the radius valley (Fulton et al 2017(Fulton et al , 2018Cloutier & Menou 2020) on a 24.7-day temperate orbit, and an inner planet LHS 1140 c, a warm super-Earth (R p = 1.27 R ⊕ ) orbiting every 3.78 days.…”

“…The two planets receive 0.43 and 5.3 times the irradiation of Earth, respectively, with LHS 1140 b comfortably situated within the water condensation zone (Turbet et al 2023). Cadieux et al (2024) recently presented a joint study of almost all transit and radial velocity observations to date, obtaining a precision of 3% on the masses and 2% on the radii of LHS 1140 b and c. Among the temperate rocky exoplanets, only those in the TRAPPIST-1 system are currently characterized to such precision. Their analysis of the composition of LHS 1140 b shows that the planet could be enveloped in hydrogen (∼0.1% H/He by mass), akin to a smaller version of the temperate mini-Neptune K2-18 b, which has a transmission spectrum characterized by prominent CH 4 and CO 2 absorption bands in the near-infrared (Madhusudhan et al 2023).…”

“…Alternatively, LHS 1140 b could be a water world with a surface layer of condensed water representing 9%-19% of the total mass. A 3D Global Climate Model (GCM) of the water-world scenario predicts liquid water at the substellar point for a large range of atmospheric compositions (Cadieux et al 2024). The GCM also predicts a distinctive transmission spectrum featuring relatively small (∼15 ppm) CO 2 features at ∼2.8 and ∼4.3 μm.…”

Strong Fractionation of Deuterium and Helium in Sub-Neptune Atmospheres along the Radius Valley

“…To model the temporal evolution of the incident EUV flux, F EUV , for planets around M stars, we constructed a power-law function as in Cherubim et al (2023), Cloutier et al (2024), andCadieux et al (2024). Our model is informed by semiempirical spectra generated by the HAZMAT team for populations of M1 stars over a range of ages from 10 Myr to 5 Gyr (Figure 1; Peacock et al 2020):…”