arXiv subjects

Frances E. Rigby

Publications and source records attributed to Frances E. Rigby.

7 recordsLinked to original sources

arXiv2026

Sea Ice as an Origin of Life Location for Hycean and Ocean Worlds

Water worlds with potential deep oceans are at the forefront of currently observable exoplanetary habitability. The standard requirements for habitability are the presence of an energy source, a source of nutrients, and a solvent, all of which can be met in exoplanets with deep liquid water oceans. Such planets may be numerous, ranging from hycean worlds and ocean worlds to terrestrial-size planets with enhanced water content. However, abiogenesis requires additional conditions, namely a mechanism to accumulate nutrients to the high concentrations required for prebiotic chemistry, and an energy source to drive the chemistry. A deep ocean, and in some cases the high-pressure ice layers underneath, has traditionally been thought to prevent the required concentrations. Here we outline a location where nutrients could be concentrated in sufficiently high quantities to make abiogenesis possible -- at the interface between the open ocean and the surface ice shelf. Using a 3D General Circulation Model, we explore cases corresponding to several candidate habitable exoplanets which might have dayside sea ice. We discuss how the planet's climate influences the nature and behaviour of the sea ice. We investigate the ability of small-scale ice formation and melting processes to concentrate nutrients to prebiotically useful levels, and also explore the potential for meteoritic impactor fragments to provide a platform for prebiotic chemistry. We find that both of these mechanisms may plausibly provide concentrated chemical feedstock for prebiotic chemistry, which adds an important new perspective when evaluating the prospects of habitability in planets with deep oceans.

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arXiv2026

The Mid-Infrared Transmission Spectrum of the Temperate Sub-Neptune TOI-270 d

Observations of temperate sub-Neptunes with JWST have ushered in a new era for atmospheric characterization of small exoplanets. In particular, the MIRI instrument provides a unique opportunity to search for molecules that are not easily accessible in the near-infrared, as demonstrated by recent mid-infrared observations of K2-18 b. In this work, we present the first mid-infrared transmission spectrum of TOI-270 d observed using the JWST MIRI LRS (5-12 $μ$m) instrument. By leveraging archival MIRI LRS data, we establish a new empirical relation between the detector settling timescale and the flux, which helps accurately model the spectral light curves and improve the precision of the transmission spectrum. We find that there is notable evidence of molecular features in the MIRI transmission spectrum of TOI-270 d, favouring the presence of atmospheric absorption at $\ln B$ = 2.8-5.3 when comparing physically plausible atmospheric models with and without molecular line absorption. The data show excess absorption beyond what could be attributed to CH$_4$ and CO$_2$ detected previously, in line with recent near-infrared results. Through an agnostic search for 203 species, we identify several candidate trace molecules, most of which are complex molecules, evaluate their physical plausibility, and compare them against inferences from near-infrared observations. We also compare the MIRI spectrum of TOI-270 d to that of K2-18 b and find that random or systematic noise is unlikely to explain these observations. Future follow-up observations are necessary to definitively identify the additional absorbers beyond CH$_4$ and CO$_2$. These observations demonstrate the unique capability of JWST MIRI for atmospheric characterisation of temperate sub-Neptunes.

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arXiv2026

Transmission Spectrum of the Benchmark Temperate Exo-Neptune TOI-1231 b

The JWST is revolutionizing our understanding of the temperate sub-Neptune population through atmospheric spectroscopy. The nature of these planets remains debated, as their bulk properties are compatible with a range of interior scenarios, including mini-Neptunes, hycean worlds, and gas dwarfs, with different predicted atmospheric compositions. While theoretical studies have predicted compositional diagnostics for shallow- versus deep-atmosphere scenarios, there is a critical need for empirical constraints for a temperate planet that is a priori known to possess a deep H$_2$-rich atmosphere. The temperate exo-Neptune TOI-1231 b provides one such benchmark target. In this work, we present the JWST near-infrared (0.65--5.2 $μ$m) transmission spectrum of TOI-1231 b, observed with NIRISS single-object slitless spectroscopy and NIRSpec G395H, representing the first for a temperate exo-Neptune. The density of TOI-1231 b requires a thick H$_2$-rich atmosphere, making the planet a keystone reference case for testing mini-Neptune scenarios for sub-Neptunes. We report a strong detection of CH$_4$ ($\ln B = 54.5$-$69.6$) and moderate to strong evidence for CO$_2$ ($\ln B = 2.9$-$6.6$). We do not find significant evidence for any other prominent molecule, although we find high 95\% upper limits on the mixing ratios of NH$_3$ and CO, both of which are expected in deep H$_2$-rich atmospheres. We also do not find any significant evidence for sulfur-bearing species that have been inferred for some temperate sub-Neptunes. This composition is consistent with expectations for a temperate Neptune possessing a deep H$_2$-rich atmosphere with no distinct surface. We discuss the implications of our results for the characterization of temperate sub-Neptunes.

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arXiv2025

A JWST Transmission Spectrum of the Temperate Sub-Neptune TOI-732 c

In recent years, JWST has facilitated detections of carbon-bearing molecules in the atmospheres of temperate sub-Neptunes orbiting M dwarfs, ushering in a new era in the characterization of this intriguing planetary regime. We report the transmission spectrum of the temperate sub-Neptune TOI-732 c, observed with JWST NIRISS, NIRSpec G395H and MIRI LRS between 0.9-12 $μ$m. The observations provide evidence for methane (CH$_4$) in a H$_2$-rich atmosphere, at a volume mixing ratio of $\sim$1\%, and non-detection of NH$_3$ and HCN, along with nominal constraints on other prominent molecules H$_2$O, CO and CO$_2$, which are typically expected in H$_2$-rich atmospheres. We conduct a comprehensive survey of 250 chemical species and find moderate to strong evidence (up to $\ln B\sim 5.9$, $3.9σ$) for additional absorption due to one or more complex molecules including higher-order hydrocarbons and/or sulfur-bearing molecules. The spectral features are strongly degenerate among these molecules and with methane, which we find at $\ln B=3.2-8.8$ (up to $3.0-4.6$$σ$) significance. Two complex molecules are preferred with at least moderate evidence ($\ln B \gtrsim 2.5$) in both the near- and mid-infrared, while several others show such evidence in at least one of the two wavelength ranges. The preferred molecules are found in trace quantities on Earth, with no significant sources identified in other planetary atmospheres, requiring future work to assess their physical plausibility in this planet. Future observations are required to resolve the degeneracies and place more robust constraints on these species. We highlight the need for further theoretical and experimental work to robustly characterize the atmospheric and internal composition of TOI-732 c and similar sub-Neptunes.

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arXiv2025

The Surface and Interior Conditions of Temperate Sub-Neptune TOI-270 d

Sub-Neptune planets, with no analogue in our solar system, provide a wealth of information about exoplanet diversity, formation & evolution, and habitability. Their robust characterisation requires the coupling of physically informed atmosphere and interior models with precise atmospheric data to break compositional degeneracies. Recent JWST observations of the temperate sub-Neptune TOI-270 d revealed detections of CH$_4$ and CO$_2$ in its H$_2$-rich atmosphere, with tentative inferences of H$_2$O and CS$_2$ and a non-detection of NH$_3$. We conduct a theoretical exploration of the range of possible interiors for TOI-270 d based on the current observational constraints. We carry out internal structure modelling using a coupled atmosphere-interior model, including self-consistent atmospheric temperature structures informed by JWST observations. The bulk properties permit solutions spanning mini-Neptune, gas dwarf and hycean scenarios, with a wide range of possible surface conditions, which are strongly dependent on the atmospheric properties, including the presence of clouds/hazes. We explore the solutions allowing for surface water oceans on TOI-270 d, including under potentially habitable conditions. The atmospheric mass fractions permitting habitable surface conditions are found to be $\lesssim$$3.5\times10^{-5}$ and pressures $\lesssim$100 bar for the envelope temperature structures considered. We consider mini-Neptune interiors that are sufficiently warm for H$_2$O to be mixed with the H$_2$-rich envelope. Finally, we consider possible gas dwarf interiors, finding H$_2$-rich envelope mass fractions of $\sim$$1-5$ % are required to satisfy the bulk properties, with surface pressures $\sim10^4-10^5$ bar. Further theoretical and experimental studies in addition to future atmospheric observations will aid the characterisation of the possible interior and surface conditions on TOI-270 d.

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arXiv2024

Towards a self-consistent evaluation of gas dwarf scenarios for temperate sub-Neptunes

The recent JWST detections of carbon-bearing molecules in a habitable-zone sub-Neptune have opened a new era in the study of low-mass exoplanets. The sub-Neptune regime spans a wide diversity of planetary interiors and atmospheres not witnessed in the solar system, including mini-Neptunes, super-Earths, and water worlds. Recent works have investigated the possibility of gas dwarfs, with rocky interiors and thick H$_2$-rich atmospheres, to explain aspects of the sub-Neptune population, including the radius valley. Interactions between the H$_2$-rich envelope and a potential magma ocean may lead to observable atmospheric signatures. We report a coupled interior-atmosphere modelling framework for gas dwarfs to investigate the plausibility of magma oceans on such planets and their observable diagnostics. We find that the surface-atmosphere interactions and atmospheric composition are sensitive to a wide range of parameters, including the atmospheric and internal structure, mineral composition, volatile solubility and atmospheric chemistry. While magma oceans are typically associated with high-temperature rocky planets, we assess if such conditions may be admissible and observable for temperate sub-Neptunes. We find that a holistic modelling approach is required for this purpose and to avoid unphysical model solutions. We find using our model framework and considering the habitable-zone sub-Neptune K2-18 b as a case study that its observed atmospheric composition is incompatible with a magma ocean scenario. We identify key atmospheric molecular and elemental diagnostics, including the abundances of CO$_2$, CO, NH$_3$ and, potentially, S-bearing species. Our study also underscores the need for fundamental material properties for accurate modelling of such planets.

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arXiv2024

On the Ocean Conditions of Hycean Worlds

Recent studies have suggested the possibility of Hycean worlds, characterised by deep liquid water oceans beneath H$_2$-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 $\sim$1000 km for Hycean conditions, reaching ocean base pressures up to $\sim$6$\times$10$^4$ 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 $\lesssim$10$^{-3}$. At the other extreme, the maximum H/He mass fractions allowed for these planets can be up to $\sim$4-8$\%$, representing purely rocky interiors with no H$_2$O 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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