Carbon-bearing Molecules in a Possible Hycean Atmosphere (2309.05566v2)

Abstract: The search for habitable environments and biomarkers in exoplanetary atmospheres is the holy grail of exoplanet science. The detection of atmospheric signatures of habitable Earth-like exoplanets is challenging due to their small planet-star size contrast and thin atmospheres with high mean molecular weight. Recently, a new class of habitable exoplanets, called Hycean worlds, has been proposed, defined as temperate ocean-covered worlds with H2-rich atmospheres. Their large sizes and extended atmospheres, compared to rocky planets of the same mass, make Hycean worlds significantly more accessible to atmospheric spectroscopy with the JWST. Here we report a transmission spectrum of the candidate Hycean world, K2-18 b, observed with the JWST NIRISS and NIRSpec instruments in the 0.9-5.2 $\mu$m range. The spectrum reveals strong detections of methane (CH4) and carbon dioxide (CO2) at 5$\sigma$ and 3$\sigma$ confidence, respectively, with high volume mixing ratios of ~1% each in a H2-rich atmosphere. The abundant CH4 and CO2 along with the non-detection of ammonia (NH3) are consistent with chemical predictions for an ocean under a temperate H2-rich atmosphere on K2-18 b. The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet. The detection of CH4 resolves the long-standing missing methane problem for temperate exoplanets and the degeneracy in the atmospheric composition of K2-18 b from previous observations. We discuss possible implications of the findings, open questions, and future observations to explore this new regime in the search for life elsewhere.

• The paper detected methane at a 5-sigma level and carbon dioxide at 3-sigma in K2-18 b's atmosphere using JWST's transmission spectroscopy.
• It employed advanced retrieval models to estimate ~1% volume mixing ratios while noting the non-detection of water and ammonia.
• The study highlights potential biosignatures, including dimethyl sulfide, suggesting that Hycean worlds could be significant in the search for extraterrestrial habitability.

Insights into Carbon-bearing Molecules in the Atmosphere of K2-18 b

The paper, led by Madhusudhan et al., provides a comprehensive analysis of the atmosphere of K2-18 b, a candidate Hycean world, leveraging the capabilities of the James Webb Space Telescope (JWST). Observations reveal the presence of carbon-bearing molecules, notably methane ($\text{CH}_4$) and carbon dioxide ($\text{CO}_2$), with a high degree of confidence, marking significant progress in the characterization of sub-Neptunes and highlighting the potential of such planets in the search for habitable environments beyond Earth.

Methodology

The authors utilize transmission spectroscopy data obtained from JWST's NIRISS and NIRSpec instruments over the 0.9 to 5.2 micrometer range. The analysis of the data was conducted using sophisticated retrieval models that allow for estimation of the atmospheric composition, incorporating a variety of potential constituent gases, surface conditions, and the possibility of clouds or hazes. The retrievals were designed to ascertain the volume mixing ratios of several molecules, ensuring the robustness of the spectral interpretations made.

Key Findings

1. Methane and Carbon Dioxide Detection: The standout results include the significant presence of both $\text{CH}_4$ and $\text{CO}_2$, with reported mixing ratios around 1%. Methane is detected at a 5-sigma confidence level, resolving longstanding questions about its presence in temperate exoplanetary atmospheres. CO$_2$ is detected at a 3-sigma level, further corroborating theorized atmospheric models for such exoplanets.
2. Lack of Water and Ammonia: The paper surprisingly records a nondetection of $\text{H}_2\text{O}$ and $\text{NH}_3$, which may influence theories regarding the atmospheric processes and surface conditions on K2-18 b. This result is especially notable given previous studies suggesting the presence of water vapor based on less comprehensive data.
3. Potential Biomarkers: The spectrum suggests the presence of dimethyl sulfide (DMS), albeit with lower significance compared to methane or carbon dioxide, pointing towards possible biological activity under hypothetical scenarios. However, future observations are essential for confirming this potential biosignature.

Implications

The confirmation of methane and carbon dioxide as abundant atmospheric constituents has profound implications for our understanding of sub-Neptunes like K2-18 b. The results align with theoretical models that propose significant carbon compounds in the atmospheres of water-rich planets with hydrogen-dominated envelopes. The nondetection of certain molecules such as $\text{NH}_3$ and the potential presence of biomarkers suggest complex atmospheric chemistry that may challenge current photochemical models, driving further theoretical research in this area.

Future Prospects

This investigation sets the stage for future observational efforts using JWST and other high-sensitivity telescopes to probe deeper into the atmospheric dynamics and composition of K2-18 b and other similar exoplanets. Key future objectives will be the verification of DMS and the search for additional biosignatures, which would significantly advance the field of astrobiology and our understanding of life's potential beyond Earth.

Conclusion

The research by Madhusudhan et al. presents a significant advancement in exoplanet atmospheric studies, proposing K2-18 b as a viable candidate for habitability and biological activity investigations within the category of Hycean worlds. These findings not only bolster current planetary models but also expand the horizon for the exoplanet community in the continuous search for life in the universe.