Chemical conditions on Hycean worlds

Madhusudhan, Nikku; Moses, Julianne I.; Rigby, Frances; Barrier, Edouard

doi:10.1039/d3fd00075c

Cited by 11 publications

(40 citation statements)

References 144 publications

(301 reference statements)

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“…The observed mass, radius, and equilibrium temperature of K2-18 b have been known to be consistent with a degenerate set of internal structures (Madhusudhan et al 2020(Madhusudhan et al , 2023. These include (a) a Hycean world with a thin H 2 -rich atmosphere over a waterrich interior, (b) a mini-Neptune with a deep H 2 -rich atmosphere, or (c) a predominantly rocky super-Earth interior with a deep H 2 -rich atmosphere.…”

Section: Summary and Discussionmentioning

confidence: 81%

“…The 10 mbar temperature estimates for the three retrieval cases are shown in Table 3. While the H 2 -rich atmosphere can result in a significant greenhouse effect and warm the ocean surface, clouds and/or hazes play a crucial role in cooling the atmosphere and decreasing the temperature gradient (Madhusudhan et al 2020(Madhusudhan et al , 2021(Madhusudhan et al , 2023). The possible presence of clouds can allow more temperate conditions at the ocean surface compared to those predicted by cloud-free models (see Innes et al 2023).…”

Section: Clouds/hazes and Photospheric Temperaturementioning

confidence: 99%

“…Our derived atmospheric composition, in combination with K2-18 b's irradiation level, is seemingly inconsistent with a deepatmosphere, mini-Neptune scenario. In that scenario any photochemically produced carbon and nitrogen species would be recycled in the deep atmosphere back to their thermodynamically stable forms, CH 4 and NH 3 , with transport then returning these molecules back to the observable upper regions of the atmosphere (Tsai et al 2021;Yu et al 2021;Madhusudhan et al 2023). Although CO 2 can have a substantial mixing ratio in a H 2 -rich atmosphere with a high metallicity and/or low C/O ratio (e.g., Moses et al 2013), we find that the simultaneous presence of ∼1% CO 2 and CH 4 in a thick H 2 -rich atmosphere requires a moderately high C/H metallicity, a very low C/O ratio, (∼0.02) and efficient vertical quenching.…”

Section: Is a Deep Atmosphere A Possibility?mentioning

confidence: 99%

“…We also note that the retrieved abundances represent average chemical abundances in the observable photosphere (∼0.1-100 mbar) at the day-night terminator region. While the molecules with robust detections, CH 4 and CO 2 , have been to be relatively uniform in this pressure range for K2-18 b in several cases (Hu et al 2021;Yu et al 2021;Madhusudhan et al 2023), more precise observations in the future may be able to constrain nonuniform chemical abundances in the atmosphere.…”

Section: Is a Deep Atmosphere A Possibility?mentioning

confidence: 99%

“…We note that our upper limit for H 2 O corresponds to the planet's stratosphere at pressures below ∼100 mbar. Water vapor may very well be abundant at deeper levels in the atmosphere, but condensation of H 2 O is expected in the upper troposphere of this temperate planet (Benneke et al 2019b;Madhusudhan et al 2023), resulting in a comparatively dry stratosphere, as on Earth.…”

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confidence: 99%

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Carbon-bearing Molecules in a Possible Hycean Atmosphere

Madhusudhan,

Sarkar,

Constantinou

et al. 2023

ApJL

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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.

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Section: Summary and Discussionmentioning

confidence: 81%

Section: Clouds/hazes and Photospheric Temperaturementioning

confidence: 99%

Section: Is a Deep Atmosphere A Possibility?mentioning

confidence: 99%

Section: Is a Deep Atmosphere A Possibility?mentioning

confidence: 99%

mentioning

confidence: 99%

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Carbon-bearing Molecules in a Possible Hycean Atmosphere

Madhusudhan,

Sarkar,

Constantinou

et al. 2023

ApJL

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Possible Hycean conditions in the sub-Neptune TOI-270 d

Holmberg,

Madhusudhan

2024

A&A

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The JWST has ushered in a new era in atmospheric characterisations of temperate low-mass exoplanets with recent detections of carbon-bearing molecules in the candidate Hycean world K2-18 b. We investigated JWST observations of the TOI-270 system, with two sub-Neptunes simultaneously transiting the nearby M dwarf during the visit. We report our atmospheric characterisation of the outer planet TOI-270 d, a candidate Hycean world, with JWST transmission spectroscopy using the NIRSpec G395H instrument in the 2.7–5.2 μm range, combined with previous observations obtained with the HST WFC3 spectrograph (1.1–1.6 μm). The spectrum reveals strong signatures of CH4 and CO2 at 3.8–4.9σ and 2.9–3.9σ confidence, respectively, and no evidence of NH3. The abundant CH4 and CO2, at ∼0.1–1% mixing ratios, and the non-detection of NH3 are similar to the findings reported for K2-18 b and consistent with predictions for a Hycean world with a planet-wide ocean under a H2-rich atmosphere. We also report evidence of CS2 at a 2.3–3.0σ confidence and a potential inference of H2O at 1.6–4.4σ, depending on the data analysis approach, and discuss possible interpretations of these results. The spectrum does not provide strong constraints on the presence of clouds or hazes in the observable atmosphere, nor any evidence for the effects of stellar heterogeneities, which is consistent with previous studies. For the smaller inner planet TOI-270 b, we find that the spectrum is inconsistent with a featureless spectrum at ∼3σ, showing some preference for an H2-rich atmosphere in a super-Earth. We discuss the implications of our findings and future prospects.

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On the ocean conditions of Hycean worlds

Rigby,

Madhusudhan

2024

Monthly Notices of the Royal Astronomical Society

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Recent studies have suggested the possibility of Hycean worlds, characterized by deep liquid water oceans beneath H2-rich atmospheres. These planets significantly widen the range of planetary properties over which habitable conditions could exist. We conduct internal structure modelling of Hycean worlds to investigate the range of interior compositions, ocean depths and atmospheric mass fractions possible. Our investigation explicitly considers habitable oceans, where the surface conditions are limited to those that can support potential life. The ocean depths depend on the surface gravity and temperature, confirming previous studies, and span 10s to ∼1000 km for Hycean conditions, reaching ocean base pressures up to ∼6 × 104 bar before transitioning to high-pressure ice. We explore in detail test cases of five Hycean candidates, placing constraints on their possible ocean depths and interior compositions based on their bulk properties. We report limits on their atmospheric mass fractions admissible for Hycean conditions, as well as those allowed for other possible interior compositions. For the Hycean conditions considered, across these candidates we find the admissible mass fractions of the H/He envelopes to be ≲10−3. At the other extreme, the maximum H/He mass fractions allowed for these planets can be up to ∼4–8 per cent, representing purely rocky interiors with no H2O layer. These results highlight the diverse conditions possible among these planets and demonstrate their potential to host habitable conditions under vastly different circumstances to the Earth. Upcoming JWST observations of candidate Hycean worlds will allow for improved constraints on the nature of their atmospheres and interiors.

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Chemical conditions on Hycean worlds

Abstract: We investigate possible chemical conditions on a canonical Hycean world, focusing on the present and primordial molecular composition of the atmosphere, and the inventory of bioessential elements for the origin and sustenance of life in the ocean.

Cited by 11 publications

References 144 publications

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Possible Hycean conditions in the sub-Neptune TOI-270 d

On the ocean conditions of Hycean worlds

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