Abstract
dc:description.abstractSub-Neptune planets have been shown to be common among the exoplanet popula- tion. The sub-Neptune regime, spanning ∼1-4 Earth radii, encompasses a diverse possible range of planetary conditions, with no analogue in our solar system. The bulk properties of these planets give rise to compositional degeneracies, with a wide range of compositions able to explain each mass and radius. In this thesis, I explore the diversity of interiors possible for temperate sub-Neptune exoplanets. To do this, I developed an internal structure model specialised for sub-Neptunes. This model relates the bulk properties of a planet to the possible interior compositions, incorporating equations of state to describe the behaviour of different planetary materials, including hydrogen/helium, water, silicates, and iron. I first use the model to conduct a theoretical exploration of the possible interiors and ocean depths of a new class of habitable sub-Neptune, known as hycean worlds. These planets are characterised by liquid water oceans at their surfaces beneath hydrogen- rich atmospheres. Hycean worlds have been the subject of recent investigations of habitability and the potential for biosignature detections, due to their being more conducive to atmospheric observations compared to Earth-like planets. I calculate the range of ocean depths possible for hycean worlds, dependent on the surface gravity and surface temperature. I then explore the range of possible interior compositions and ocean depths for five hycean candidates, placing constraints on the envelope and water mass fractions required for hycean conditions. Secondly, I apply the internal structure model to the scenario of a gas dwarf sub-Neptune. A gas dwarf is defined by the presence of thick hydrogen-rich envelope atop a rocky interior, with the possibility of a solid or magma ocean surface. The interior model is coupled to atmospheric observations and atmospheric models to evaluate the feasibility of a temperate sub-Neptune hosting such conditions. In this way, the inferences made about the atmospheric properties and composition from atmospheric observations can begin to ease the interior compositional degeneracies. The model framework is used to consider the plausibility of a gas dwarf scenario for the temperate sub-Neptune K2-18 b, which was recently observed with the James Webb Space Telescope (JWST). Lastly, I use the internal structure model to conduct a detailed exploration of the possible interior and surface conditions of TOI-270 d, a temperate sub-Neptune recently observed with JWST. This investigation, informed by the findings of the recent observations, spans possible scenarios of a gas dwarf, a hycean world, and a mini-Neptune, with a water-rich interior but no distinct surface. In the modelling of mini-Neptune scenarios I consider the potential for hydrogen and water to be mixed, which has been shown to occur across a wide range of pressures and temperatures relevant to sub-Neptunes. I conclude with a discussion of possible future directions for modelling the interiors of sub-Neptune exoplanets, and prospects for their characterisation. Potential directions include the incorporation of more complex treatments of planetary materials. I discuss the need for further experimental data to inform modelling efforts.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2025
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Rigby, Frances
- Advisor dc:contributor.advisor
-
- Madhusudhan, Nikku
Subjects
dc:subject × 1Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.123663
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/393275