arXiv subjects

Gregory J. Cooke

Publications and source records attributed to Gregory J. Cooke.

5 recordsLinked to original sources

arXiv2026

A Systematic Search for Trace Molecules in the Atmosphere of Exoplanet K2-18 b

The first transmission spectrum of the habitable-zone sub-Neptune K2-18 b with JWST has opened a new avenue for atmospheric characterisation of temperate low-mass exoplanets. The observations led to inferences of methane and carbon dioxide, as well as of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), both potential biosignatures. In the present work we conduct a broad and agnostic search for other chemical species in the atmosphere of K2-18 b. Our exploration includes 661 molecules, spanning a wide range of trace gases, including biotic, abiotic, and anthropogenic gases on Earth. We investigate possible preference for any of these gases, compared to a model only including the previously-detected CH$_4$ and CO$_2$, using three metrics: (a) preference in the JWST mid-infrared (MIR) spectrum, (b) preference in the JWST near-infrared spectrum, for species preferred in MIR and (c) plausible sources of production. We find that only DMS consistently results in Bayes factors $\ln B \geq 2.0$ across the datasets considered independently, though in the near-infrared this depends on detector offsets, as previously reported. The threshold of $\ln B \geq 2.0$ is motivated by the conventional threshold of $\ln B \geq 2.5$ for moderate preference, allowing for an empirical uncertainty of 0.5. A few other gases also provide comparable fits to a subset of the data or only with some of the retrieval codes used, but with limited known plausible sources. Our study highlights the need for further observations to distinguish between possible trace gases in the atmosphere of K2-18 b and theoretical work to establish their plausible sources.

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arXiv2026

Ecological modelling of hycean worlds

New observations are opening the possibility of characterising habitable environments in exoplanetary systems, with the recent example of the candidate hycean world K2-18 b. This motivates an exploration of the possible ecological conditions on such planets to better interpret biosignatures as well as understand the nature of potential life. On Earth, the Lotka-Volterra equations have been used to model numerous coupled populations within ecosystems, from interactions between large vertebrates, to systems with multiple microbial species. In this work, we apply the Lotka-Volterra equations to the ecology of habitable exoplanets for the first time, focusing on hycean worlds. We simulate scenarios in a vertical water column with between 1-5 bacterial species that thrive in anoxic environments on Earth, i.e. similar to predicted hycean conditions. We find that a wide range of ecological diversity is possible for microbial populations under hycean conditions. We demonstrate that dominating phototrophic bacteria at the top of a water column out-compete deeper dwelling phototrophic bacteria, analogous to bacterial blooms on Earth. Incorporating microbial viruses (bacteriophages) within our models can cause ecosystem collapse depending on the time of their introduction, and such phage inclusion can be beneficial to ecological diversity. Finally, our work shows that bacterial populations inhabiting tidally locked exoplanets may be more stable due to constant illumination of the ocean, but can have lower peak population densities in such cases when compared to seasonal scenarios. Our work provides an initial step towards understanding the possible ecological diversity on habitable worlds beyond Earth.

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arXiv2026

The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

The Great Oxidation Event (GOE) was a $200$ Myr transition circa 2.4 billion years ago that converted the Earth's anoxic atmosphere to one where molecular oxygen (O$_2$) was abundant (volume mixing ratio $>10^{-4}$). This significant rise in O$_2$ is thought to have substantially throttled hydrogen (H) escape and the associated water (H$_2$O) loss. Atmospheric estimations from the GOE onward place O$_2$ concentrations ranging between 0.1% to 150% PAL, where PAL is the present atmospheric level of 21% by volume. In this study we use WACCM6, a three-dimensional Earth System Model to simulate Earth's atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O$_2$ post-GOE. We find that O$_2$ indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O$_2$ leads to decreased O$_3$ densities that reduce local tropical tropopause temperatures by up to 17 K, which increases H$_2$O freeze-drying and thus reduces the primary source of hydrogen in the considered scenarios. The maximum differences between all simulations in the total H mixing ratio at the homopause and the associated diffusion-limited escape rates are a factor of 3.2 and 4.7, respectively. The prescribed CH$_4$ mixing ratio (0.8 ppmv) sets a minimum diffusion escape rate of $\approx 2 \times 10^{10}$ mol H yr$^{-1}$, effectively a negligible rate when compared to pre-GOE estimates ($\sim10^{12}-10^{13}$ mol H yr$^{-1}$). Because the changes in our predicted escape rates are comparatively minor, our numerical predictions support geological evidence that the majority of Earth's hydrogen escape occurred prior to the GOE. Our work demonstrates that estimations of how the hydrogen escape rate evolved through Earth's history requires 3D chemistry-climate models which include a global treatment of water vapour microphysics.

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arXiv2025

Exploring the Sub-Neptune Frontier with JWST

Sub-Neptune planets, with sizes and masses between those of Earth and Neptune, dominate the exoplanet population. Sub-Neptunes are expected to be the most diverse family of the exoplanet population, potentially including rocky gas dwarfs, water worlds, and mini-Neptunes, with a wide range of atmospheric, surface and interior conditions. With no analogue in the solar system, these planets open fundamental questions in planetary processes, origins, and habitability, and present new avenues in the search for life elsewhere. Atmospheric observations with the James Webb Space Telescope (JWST) are enabling unprecedented characterization of sub-Neptunes, starting with the first detections of carbon-bearing molecules in the habitable zone sub-Neptune K2-18 b. We survey the present landscape of JWST observations and atmospheric inferences of sub-Neptunes, which in turn provide key insights into their atmospheric processes, internal structures, surface conditions, formation pathways and potential habitability. The atmospheric abundance constraints reveal evidence of chemical disequilibria, and insights into the planetary mass-metallicity relation in the sub-Neptune regime. Similarly, for sub-Neptunes with H$_2$O-rich interiors, increasing atmospheric H$_2$O abundances with the equilibrium temperature may indicate the existence of a critical temperature for transition from H$_2$ dominated atmospheres with tropospheric cold traps to those with steamy atmospheres. The chemical abundances also provide initial evidence for diverse planet types, from potentially habitable hycean worlds to steam worlds with super critical water layers. These planet types serve as benchmarks for an emerging taxonomy of volatile-rich sub-Neptunes as a function of their equilibrium temperature and atmospheric extent, heralding a new era of chemical classification of low-mass exoplanets with JWST.

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arXiv2024

Considerations for Photochemical Modeling of Possible Hycean Worlds

JWST is revolutionising the study of temperate sub-Neptunes, starting with the first detection of carbon-bearing molecules in the habitable-zone sub-Neptune K2-18 b. The retrieved abundances of CH$_4$ and CO$_2$ and non-detection of NH$_3$ and CO in K2-18 b are consistent with prior predictions of photochemical models for a Hycean world with a habitable ocean. However, recent photochemical modeling raised the prospect that the observed abundances may be explained by a mini-Neptune scenario instead. In this study, we explore these scenarios using independent photochemical modeling with K2-18 b as a case study. We find the previous results to be sensitive to a range of model assumptions, such as the photochemical cross sections, incident stellar spectrum, surface pressure, UV albedo, and metallicity, significantly affecting the resulting abundances. We explore a wide model space to investigate scenarios that are compatible with the retrieved molecular abundances for K2-18 b. Our analysis shows that the previously favoured mini-Neptune scenario is not compatible with most of the retrieved abundances, while the Hycean scenarios, both inhabited and uninhabited, provide better agreement. An uninhabited Hycean scenario explains most of the abundance constraints, except CH$_4$ which is generally underabundant but dependent on the model assumptions. The inhabited Hycean scenario is compatible with all the abundances if the observed CH$_4$ is assumed to be predominantly biogenic. Our results underscore the importance of systematic photochemical modeling and accurate interpretation of chemical abundance constraints for candidate Hycean worlds.

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