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

• Published in: arXiv.org 2024-06
• Main Authors: Cadieux, Charles, Doyon, René, MacDonald, Ryan J, Turbet, Martin, Artigau, Étienne, Lim, Olivia, Radica, Michael, Fauchez, Thomas J, Salhi, Salma, Dang, Lisa, Albert, Loïc, Coulombe, Louis-Philippe, Cowan, Nicolas B, Lafrenière, David, L'Heureux, Alexandrine, Piaulet, Caroline, Benneke, Björn, Cloutier, Ryan, Charnay, Benjamin, Cook, Neil J, Fournier-Tondreau, Marylou, Plotnykov, Mykhaylo, Valencia, Diana
• Format: Article
• Language: English
• Subjects: Atmosphere; Bulk density; Circumstellar habitable zone; Extrasolar planets; Faculae; Global climate models; James Webb Space Telescope; Metallicity; Photosphere; Rayleigh scattering; Spectral signatures; Spectroscopy; Spectrum analysis; Stellar surfaces; Transit; Water
• ISSN: 2331-8422

LHS 1140 b is the second-closest temperate transiting planet to the Earth with an equilibrium temperature low enough to support surface liquid water. At 1.730\(\pm\)0.025 R\(_\oplus\), LHS 1140 b falls within the radius valley separating H\(_2\)-rich mini-Neptunes from rocky super-Earths. Recent mass and radius revisions indicate a bulk density significantly lower than expected for an Earth-like rocky interior, suggesting that LHS 1140 b could either be a mini-Neptune with a small envelope of hydrogen (\(\sim\)0.1% by mass) or a water world (9--19% water by mass). Atmospheric characterization through transmission spectroscopy can readily discern between these two scenarios. Here, we present two JWST/NIRISS transit observations of LHS 1140 b, one of which captures a serendipitous transit of LHS 1140 c. The combined transmission spectrum of LHS 1140 b shows a telltale spectral signature of unocculted faculae (5.8 \(\sigma\)), covering \(\sim\)20% of the visible stellar surface. Besides faculae, our spectral retrieval analysis reveals tentative evidence of residual spectral features, best-fit by Rayleigh scattering from an N\(_2\)-dominated atmosphere (2.3 \(\sigma\)), irrespective of the consideration of atmospheric hazes. We also show through Global Climate Models (GCM) that H\(_2\)-rich atmospheres of various compositions (100\(\times\), 300\(\times\), 1000\(\times\)solar metallicity) are ruled out to \(>\)10 \(\sigma\). The GCM calculations predict that water clouds form 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 tentative evidence of an N\(_2\)-rich atmosphere provides strong motivation for future transmission spectroscopy observations of LHS 1140 b.