Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS (2406.15136v1)

Abstract: 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.

• The paper uses JWST/NIRISS transmission spectroscopy over two transits to differentiate between a mini-Neptune atmosphere and a water world scenario on LHS 1140 b.
• It detects 5.8σ stellar contamination from unocculted faculae and rules out H₂-rich compositions at >10σ significance.
• The study integrates spectral retrieval and global climate modeling, underscoring the need for further multi-wavelength observations to refine atmospheric characterization.

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

The paper of exoplanetary atmospheres has become an integral part of understanding planetary systems, and with the advent of instruments like the James Webb Space Telescope (JWST), our ability to probe these distant worlds has been substantially enhanced. The paper focuses on the transmission spectroscopy of LHS 1140 b, a temperate exoplanet residing in the habitable zone of its star, utilizing the Near Infrared Imager and Slitless Spectrograph (NIRISS) on JWST.

Key Findings and Methodology

LHS 1140 b presents itself as an intriguing subject due to its location in the radius valley which separates gas-rich mini-Neptunes from rocky super-Earths. The planet's potential to host an atmosphere that could support liquid water makes its atmospheric characterization critical. The authors employed transmission spectroscopy using NIRISS on JWST to discern between a potential mini-Neptune atmosphere with a thin hydrogen envelope and a water world scenario with significant water content by mass.

The data were collected over two transits, with an unexpected simultaneous capture of LHS 1140 c's transit in one instance. Primary results from the spectroscopy indicate the presence of unocculted faculae causing stellar contamination in the transmission spectrum, which was detected with high confidence (5.8σ). This observation, covering about 20% of the stellar surface, poses challenges in discerning planetary atmospheric signals from stellar activity effects.

Further analysis utilized spectral retrieval methods, indicating a possible N₂-rich atmosphere rather than a hydrogen-rich one, given that H₂-rich compositions at varied solar metallicities were ruled out at an exceptionally high significance (>10σ). This prompts the hypothesis that LHS 1140 b could either lack an atmosphere or have one with significant molecular weight, complicating the initial mini-Neptune categorization. Additionally, Global Climate Models (GCM) predict that any significant water content likely results in cloud formation below the transit photosphere, somewhat limiting its spectral visibility.

Implications and Future Research Directions

The tentative detection of an N₂-rich atmosphere, though not definitive, suggests that LHS 1140 b might lean towards a planetary archetype distinct from hydrogen-dominated mini-Neptunes, potentially resembling a secondary atmosphere planet with a substantial rocky or icy component. This finding raises important questions about atmospheric evolution and retention, especially for temperate exoplanets orbiting M-type stars.

Practically, the differentiation between a mini-Neptune and a water world has significant bearings on our understanding of planetary formation and atmospheric retention theories. Additionally, from the perspective of habitability, confirming the nature of LHS 1140 b's atmosphere would be a stepping stone in assessing its potential to host life.

To firmly establish the atmospheric composition of LHS 1140 b, additional observations are necessary. The paper suggests that future efforts should include a combination of eclipse and broader wavelength transmission spectroscopy, ideally with repeated JWST observations. This will help disentangle the complex interactions of light with both stellar and planetary constituents. Furthermore, full advantage must be taken of JWST's capabilities, integrating multi-wavelength data to achieve comprehensive atmospheric characterization.

In conclusion, the observations of LHS 1140 b contribute significantly to the ongoing discourse on the diversity of exoplanetary atmospheres. This paper underlines the intricate relationship between atmospheric signals and stellar activity, advocating for innovative modeling approaches and collaborative datasets to achieve clarity in future exoplanetary atmospheric studies.