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

• Published in: Astrophysical journal. Letters 2024-07, Vol.970 (1), p.L2
• 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-Ghorayeb, Caroline, Benneke, Björn, Cloutier, Ryan, Charnay, Benjamin, Cook, Neil J., Fournier-Tondreau, Marylou, Plotnykov, Mykhaylo, Valencia, Diana
• Format: Article
• Language: English
• Subjects: Astronomical models; Atmosphere; Bulk density; Circumstellar habitable zone; Climate models; Exoplanets; Extrasolar planets; Faculae; Global climate; Global climate models; Habitable planets; James Webb Space Telescope; M dwarf stars; Metallicity; Mini Neptunes; Ocean planets; Photosphere; Planetary atmospheres; Rayleigh scattering; Sciences of the Universe; Spectral signatures; Spectroscopy; Spectrum analysis; Stellar planets; Stellar surfaces; Super Earths; Transit; Transmission spectroscopy; Water
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad5afa

LHS 1140 b is the second-closest temperate transiting planet to Earth with an equilibrium temperature low enough to support surface liquid water. At 1.730 ± 0.025 R ⊕ , 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 be either a mini-Neptune with a small envelope of hydrogen (∼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 σ ), covering ∼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 a N 2 -dominated atmosphere (2.3 σ ), irrespective of the consideration of atmospheric hazes. We also show through Global Climate Models (GCMs) that H 2 -rich atmospheres of various compositions (100×, 300×, 1000× solar metallicity) are ruled out to >10 σ . 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 a N 2 -rich atmosphere provides strong motivation for future transmission spectroscopy observations of LHS 1140 b.