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Arxiv: Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS Published: 6/21/2024 4:34:19 PM Updated: 6/21/2024 4:34:19 PM

Paper abstract: LHS 1140 b is the second-closest temperate transiting planet to the Earthwith an equilibrium temperature low enough to support surface liquid water. At1.730\pm0.025 R_\oplus, LHS 1140 b falls within the radius valleyseparating H_2-rich mini-Neptunes from rocky super-Earths. Recent mass andradius revisions indicate a bulk density significantly lower than expected foran Earth-like rocky interior, suggesting that LHS 1140 b could either be amini-Neptune with a small envelope of hydrogen (~0.1% by mass) or a waterworld (9--19% water by mass). Atmospheric characterization through transmissionspectroscopy can readily discern between these two scenarios. Here, we presenttwo JWST/NIRISS transit observations of LHS 1140 b, one of which captures aserendipitous transit of LHS 1140 c. The combined transmission spectrum of LHS1140 b shows a telltale spectral signature of unocculted faculae (5.8\sigma), covering ~20% of the visible stellar surface. Besides faculae,our spectral retrieval analysis reveals tentative evidence of residual spectralfeatures, best-fit by Rayleigh scattering from an N_2-dominated atmosphere(2.3 \sigma), irrespective of the consideration of atmospheric hazes. We alsoshow through Global Climate Models (GCM) that H_2-rich atmospheres of variouscompositions (100\times, 300\times, 1000\timessolar metallicity) areruled out to >10 \sigma. The GCM calculations predict that water cloudsform below the transit photosphere, limiting their impact on transmission data.Our observations suggest that LHS 1140 b is either airless or, more likely,surrounded by an atmosphere with a high mean molecular weight. Our tentativeevidence of an N_2-rich atmosphere provides strong motivation for futuretransmission spectroscopy observations of LHS 1140 b.