Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b

Bézard, Bruno; Charnay, Benjamin; Blain, Doriann

doi:10.1038/s41550-022-01678-z

Cited by 18 publications

(15 citation statements)

References 22 publications

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“…Originally, the observed transmission spectrum of the planet in the near-infrared (1.1-1.7 μm) with the Hubble Space Telescope (HST) WFC3 spectrograph suggested a H 2 -rich atmosphere with strong H 2 O absorption (Benneke et al 2019b;Tsiaras et al 2019;Madhusudhan et al 2020). However, other studies highlighted the degeneracy between H 2 O and CH 4 in the observed HST spectrum (Bézard et al 2022;Blain et al 2021) and potential contributions due to stellar heterogeneities (Barclay et al 2021), rendering the previous H 2 O inference inconclusive.…”

Section: Introductionmentioning

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: Introductionmentioning

confidence: 99%

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Madhusudhan,

Sarkar,

Constantinou

et al. 2023

ApJL

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“…While methane is known to display a similar 1.4 μm absorption feature as water (Bézard et al 2022), it remains disfavored in our free chemistry retrieval analysis. The spectral signatures of methane include a feature not only around 1.4 μm but also at 1.2 and 1.7 μm, as shown by SCARLET forward atmosphere models for a pure methane envelope and a solar composition atmosphere (Figure 6).…”

Section: Upper Limits On Other Moleculesmentioning

confidence: 99%

Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d

Roy,

Benneke,

Piaulet

et al. 2023

ApJL

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Recent work on the characterization of small exoplanets has allowed us to accumulate growing evidence that sub-Neptunes with radii greater than ∼2.5 R ⊕ often host H2/He-dominated atmospheres both from measurements of their low bulk densities and from direct detections of their low mean molecular mass atmospheres. However, the smaller sub-Neptunes in the 1.5–2.2 R ⊕ size regime are much less understood and often have bulk densities that can be explained either by the H2/He-rich scenario or by a volatile-dominated composition known as the “water world” scenario. Here we report the detection of water vapor in the transmission spectrum of the 1.96 ± 0.08 R ⊕ sub-Neptune GJ 9827 d obtained with the Hubble Space Telescope (HST). We observe 11 HST Wide Field Camera 3 transits of GJ 9827 d and find an absorption feature at 1.4 μm in its transit spectrum, which is best explained (at 3.39σ) by the presence of water in GJ 9827 d’s atmosphere. We further show that this feature cannot be caused by unocculted starspots during the transits by combining an analysis of the K2 photometry and transit light source effect retrievals. We reveal that the water absorption feature can be similarly well explained by a small amount of water vapor in a cloudy H2/He atmosphere or a water vapor envelope on GJ 9827 d. Given that recent studies have inferred an important mass-loss rate (>0.5 M ⊕ Gyr−1) for GJ 9827 d, making it unlikely to retain a H-dominated envelope, our findings highlight GJ 9827 d as a promising water world candidate that could host a volatile-dominated atmosphere. This water detection also makes GJ 9827 d the smallest exoplanet with an atmospheric molecular detection to date.

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“…The teal line shows the transmission spectrum of the same planet but with the haze opacity artificially removed, while the transparent colored line shows these data at a spectral resolution of R = 1000. enough to host considerable CH 4 in their atmospheres via thermochemical equilibrium considerations have not produced detectable features (Stevenson et al 2010;Benneke et al 2019;Fu et al 2022). What few observational searches for CH 4 that do appear in the literature (e.g., Swain et al 2008;Guilluy et al 2019;Giacobbe et al 2021;Bézard et al 2022) have been called into question by other works or have not been reproduced. This "missing methane problem" could have a number of solutions.…”

Section: Current Exoplanet Landscapementioning

confidence: 99%

Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

Bergin

Kempton

Hirschmann

et al. 2023

ApJL

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We explore terrestrial planet formation with a focus on the supply of solid-state organics as the main source of volatile carbon. For the water-poor Earth, the water ice line, or ice sublimation front, within the planet-forming disk has long been a key focal point. We posit that the soot line, the location where solid-state organics are irreversibly destroyed, is also a key location within the disk. The soot line is closer to the host star than the water snow line and overlaps with the location of the majority of detected exoplanets. In this work, we explore the ultimate atmospheric composition of a body that receives a major portion of its materials from the zone between the soot line and water ice line. We model a silicate-rich world with 0.1% and 1% carbon by mass with variable water content. We show that as a result of geochemical equilibrium, the mantle of these planets would be rich in reduced carbon but have relatively low water (hydrogen) content. Outgassing would naturally yield the ingredients for haze production when exposed to stellar UV photons in the upper atmosphere. Obscuring atmospheric hazes appear common in the exoplanetary inventory based on the presence of often featureless transmission spectra. Such hazes may be powered by the high volatile content of the underlying silicate-dominated mantle. Although this type of planet has no solar system counterpart, it should be common in the galaxy with potential impact on habitability.

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Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b

Cited by 18 publications

References 22 publications

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d

Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

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