Narrow Loophole for H<sub>2</sub>-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs

Hu, Renyu; Gaillard, Fabrice; Kite, Edwin S.

doi:10.3847/2041-8213/acd0b4

Cited by 8 publications

(4 citation statements)

References 67 publications

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“…Any global ocean surface must then reside at pressures less than ∼10 bar. However, too shallow a H 2 atmosphere could be subject to escape over time (e.g., Kubyshkina et al 2018aKubyshkina et al , 2018bHu et al 2023), so there is a limited parameter range over which a habitable ocean could exist on K2-18 b without significant clouds/hazes. As mentioned previously, high-albedo tropospheric water clouds or scattering hazes can help alleviate the steam and supercritical water problem by reducing the stellar energy absorbed by the planet Madhusudhan et al 2021).…”

Section: A Potential Hycean Worldmentioning

confidence: 99%

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: A Potential Hycean Worldmentioning

confidence: 99%

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Madhusudhan,

Sarkar,

Constantinou

et al. 2023

ApJL

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“…Beyond this climate paradox, ∼1 bar of H 2 may also be susceptible to rapid escape driven by extreme-ultraviolet radiation (XUV; Hu et al 2023). Even if a 1 bar H 2 atmosphere could withstand modern XUV radiation, K2-18b likely experienced exceptionally high XUV fluxes during the host M star's pre-main sequence, potentially driving hundreds of bars of H 2 loss (Luger et al 2015), as so a remnant thin ∼1 bar atmosphere would be highly fortuitous.…”

Section: Inhabited Hycean Versus Mini-neptune: Evaluating Model Compl...mentioning

confidence: 99%

JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface

Wogan,

Batalha,

Zahnle

et al. 2024

ApJL

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The James Webb Space Telescope (JWST) recently measured the transmission spectrum of K2-18b, a habitable-zone sub-Neptune exoplanet, detecting CH4 and CO2 in its atmosphere. The discovery paper argued the data are best explained by a habitable “Hycean” world, consisting of a relatively thin H2-dominated atmosphere overlying a liquid water ocean. Here, we use photochemical and climate models to simulate K2-18b as both a Hycean planet and a gas-rich mini-Neptune with no defined surface. We find that a lifeless Hycean world is hard to reconcile with the JWST observations because photochemistry only supports <1 part-per-million CH4 in such an atmosphere while the data suggest about ∼1% of the gas is present. Sustaining percent-level CH4 on a Hycean K2-18b may require the presence of a methane-producing biosphere, similar to microbial life on Earth ∼3 billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with 100× solar metallicity should have 4% CH4 and nearly 0.1% CO2, which are compatible with the JWST data. The CH4 and CO2 are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model predicts H2O, NH3, and CO abundances broadly consistent with the nondetections. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H2 escape and potential supercriticality at depth, we favor the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data.

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“…Any global ocean surface must then reside at pressures less than ∼10 bar. However, too shallow a H 2 atmosphere could be subject to escape over time (e.g., Kubyshkina et al 2018a,b;Hu et al 2023), so there is a limited parameter range over which a habitable ocean could exist on K2-18 b without significant clouds/hazes. As mentioned previously, high-albedo tropospheric water clouds or scattering hazes can help alleviate the steam and supercritical water problem by reducing the stellar energy absorbed by the planet (Madhusudhan et al 2021;Piette & Madhusudhan 2020).…”

Section: A Potential Hycean Worldmentioning

confidence: 99%

Habitability and Biosignatures of Hycean Worlds

Madhusudhan

Piette

Constantinou

2021

ApJ

124

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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 H 2 -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 (CH 4 ) and carbon dioxide (CO 2 ) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ∼1% each in a H 2rich atmosphere. The abundant CH 4 and CO 2 along with the nondetection of ammonia (NH 3 ) are consistent with chemical predictions for an ocean under a temperate H 2 -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 CH 4 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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Narrow Loophole for H₂-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs

Cited by 8 publications

References 67 publications

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Carbon-bearing Molecules in a Possible Hycean Atmosphere

JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface

Habitability and Biosignatures of Hycean Worlds

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Narrow Loophole for H2-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs

Cited by 8 publications

References 67 publications

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Carbon-bearing Molecules in a Possible Hycean Atmosphere

JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface

Habitability and Biosignatures of Hycean Worlds

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Narrow Loophole for H₂-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs