Carbon-bearing Molecules in a Possible Hycean Atmosphere

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 owing 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 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 μm range. The spectrum reveals strong detections of methane (CH4) and carbon dioxide (CO2) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ∼1% each in a H2-rich atmosphere. The abundant CH4 and CO2, along with the nondetection 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.

Extended Habitability of Exoplanets Due to Subglacial Water

“…The recent detection of evidence for an ocean on the hycean world K2-18b (Madhusudhan et al 2023) was actually anticipated by this work (see in particular the location of K2-18 b in Figure 1). While the detection provides strong empirical support to the much-argued question, of whether planets closely orbiting M stars could support an atmosphere and water in spite of the eruptions during the early evolutionary stages of the host, this work gives the theoretical framework of how this could function during quiescece.…”

New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World

“…A variant of this model is the Hycean world (Madhusudhan et al 2021), i.e., a water world surrounded by a thin H/He-rich layer, as was recently proposed for the temperate mini-Neptune K2-18 b (Madhusudhan et al 2023). This new result opens the possibility that LHS 1140 b may be a lower-mass version of such a Hycean planet in the middle of the radius valley (Fulton et al 2017).…”

Clouds and Clarity: Revisiting Atmospheric Feature Trends in Neptune-size Exoplanets

“…We also note that, while we used water vapor as the major opacity source in this analysis, methane absorption has significant overlap with water in near-IR bands, and may dominate compared to water vapor in certain exoplanetary atmospheres <600 K (Bézard et al 2022). In fact, this has been identified in K2-18 b, previously thought to have water vapor in its atmosphere from HST data (Benneke et al 2019a), but now shown instead to have methane from broad-wavelength JWST observations (Madhusudhan et al 2023). With this in mind, we also ran methane-only retrievals for K2-18 b and TOI-270 d to These results echo previous analyses: our model comparisons provide evidence for a cloud-based mechanism shaping this trend (predicted by Fu et al 2017) over a haze-driven trend (predicted by Kreidberg 2017 andYu et al 2021).…”

“…Despite the proliferation of HST WFC3 transmission spectra in recent years, significant trends relating the clarity of these atmospheres to planetary and system parameters still evade us. WFC3, while especially sensitive to water vapor and uniform cloud absorption within a narrow spectral range, cannot characterize the full panoply of absorbers present in exoplanet atmospheres (perhaps most significantly shown by K2-18 b's JWST methane detection in Madhusudhan et al 2023 over the HST water detection from Benneke et al 2019a), nor precisely constrain physical cloud models. As already shown from the first year of JWST analyses, exoplanetary atmospheres are complex, dynamic, and contain clear evidence of atmospheric physics beyond anything HST could provide (Ahrer et al 2023;Alderson et al 2023;Feinstein et al 2023;JWST Transiting Exoplanet Community Early Release Science Team et al 2023;Rustamkulov et al 2023;Tsai et al 2023).…”