Carbon-bearing Molecules in a Possible Hycean Atmosphere

Nikku Madhusudhan; Subhajit Sarkar; Savvas Constantinou; Måns Holmberg; Anjali A. A. Piette; Julianne I. Moses

doi:10.3847/2041-8213/acf577

Carbon-bearing Molecules in a Possible Hycean Atmosphere

Nikku Madhusudhan
, Subhajit Sarkar
, Savvas Constantinou
, Måns Holmberg
, Anjali A. A. Piette
, Julianne I. Moses

Physics and Astronomy

Research output: Contribution to journal › Letter › peer-review

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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.

Original language	English
Article number	L13
Number of pages	16
Journal	The Astrophysical Journal Letters
Volume	956
Issue number	1
DOIs	https://doi.org/10.3847/2041-8213/acf577
Publication status	Published - 9 Oct 2023

Bibliographical note

Acknowledgments:
This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope as part of Cycle 1 GO Program 2722 (PI: N. Madhusudhan). We thank NASA, ESA, CSA, STScI, everyone whose efforts have contributed to the JWST, and the exoplanet science community for the thriving current state of the field. This work is supported by research grants to N.M. from the the UK Research and Innovation (UKRI) Frontier Grant (EP/X025179/1), MERAC Foundation, Switzerland, and the UK Science and Technology Facilities Council (STFC) Center for Doctoral Training (CDT) in Data Intensive Science at the University of Cambridge (STFC grant No. ST/P006787/1). N.M. and M.H. acknowledge support from STFC and the MERAC Foundation toward the doctoral studies of M.H. N.M. thanks Tony Roman, Elena Manjavacas, Nestor Espinoza, and Sara Kendrew at STScI for their help with planning our JWST observations. J.M. acknowledges support from JWST-GO-02722, which was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. We thank the anonymous referees for their valuable comments on the manuscript. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the NASA Astrophysics Data System and the Python packages NUMPY, SCIPY, and MATPLOTLIB.

This work was performed using resources provided by the Cambridge Service for Data Driven Discovery operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/P020259/1), and DiRAC funding from STFC (www.dirac.ac.uk).

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Cite this

@article{78b27f700037486793d7961f5c163942,

title = "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 \textasciitilde{}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.",

author = "Nikku Madhusudhan and Subhajit Sarkar and Savvas Constantinou and M{\aa}ns Holmberg and Piette, \{Anjali A. A.\} and Moses, \{Julianne I.\}",

note = "Acknowledgments: This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope as part of Cycle 1 GO Program 2722 (PI: N. Madhusudhan). We thank NASA, ESA, CSA, STScI, everyone whose efforts have contributed to the JWST, and the exoplanet science community for the thriving current state of the field. This work is supported by research grants to N.M. from the the UK Research and Innovation (UKRI) Frontier Grant (EP/X025179/1), MERAC Foundation, Switzerland, and the UK Science and Technology Facilities Council (STFC) Center for Doctoral Training (CDT) in Data Intensive Science at the University of Cambridge (STFC grant No. ST/P006787/1). N.M. and M.H. acknowledge support from STFC and the MERAC Foundation toward the doctoral studies of M.H. N.M. thanks Tony Roman, Elena Manjavacas, Nestor Espinoza, and Sara Kendrew at STScI for their help with planning our JWST observations. J.M. acknowledges support from JWST-GO-02722, which was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. We thank the anonymous referees for their valuable comments on the manuscript. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the NASA Astrophysics Data System and the Python packages NUMPY, SCIPY, and MATPLOTLIB. This work was performed using resources provided by the Cambridge Service for Data Driven Discovery operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/P020259/1), and DiRAC funding from STFC (www.dirac.ac.uk).",

year = "2023",

month = oct,

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doi = "10.3847/2041-8213/acf577",

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TY - JOUR

T1 - Carbon-bearing Molecules in a Possible Hycean Atmosphere

AU - Madhusudhan, Nikku

AU - Sarkar, Subhajit

AU - Constantinou, Savvas

AU - Holmberg, Måns

AU - Piette, Anjali A. A.

AU - Moses, Julianne I.

N1 - Acknowledgments: This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope as part of Cycle 1 GO Program 2722 (PI: N. Madhusudhan). We thank NASA, ESA, CSA, STScI, everyone whose efforts have contributed to the JWST, and the exoplanet science community for the thriving current state of the field. This work is supported by research grants to N.M. from the the UK Research and Innovation (UKRI) Frontier Grant (EP/X025179/1), MERAC Foundation, Switzerland, and the UK Science and Technology Facilities Council (STFC) Center for Doctoral Training (CDT) in Data Intensive Science at the University of Cambridge (STFC grant No. ST/P006787/1). N.M. and M.H. acknowledge support from STFC and the MERAC Foundation toward the doctoral studies of M.H. N.M. thanks Tony Roman, Elena Manjavacas, Nestor Espinoza, and Sara Kendrew at STScI for their help with planning our JWST observations. J.M. acknowledges support from JWST-GO-02722, which was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. We thank the anonymous referees for their valuable comments on the manuscript. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the NASA Astrophysics Data System and the Python packages NUMPY, SCIPY, and MATPLOTLIB. This work was performed using resources provided by the Cambridge Service for Data Driven Discovery operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/P020259/1), and DiRAC funding from STFC (www.dirac.ac.uk).

PY - 2023/10/9

Y1 - 2023/10/9

N2 - 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.

AB - 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.

U2 - 10.3847/2041-8213/acf577

DO - 10.3847/2041-8213/acf577

M3 - Letter

SN - 2041-8205

VL - 956

JO - The Astrophysical Journal Letters

JF - The Astrophysical Journal Letters

IS - 1

M1 - L13

ER -

Carbon-bearing Molecules in a Possible Hycean Atmosphere

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