Thousands of planets between the size of Earth and Neptune orbit other stars, and astronomers cannot tell whether they host liquid-water oceans or seas of molten rock. This matters because the James Webb Space Telescope has already spotted chemical signs in one such world, K2-18b, that some scientists interpret as evidence of a liquid ocean beneath a hydrogen atmosphere—a so-called "Hycean" planet that could support life. But the same chemical signal, ammonia depletion, could equally come from a magma ocean at the surface. Without a way to tell the difference, claims about habitability remain unverifiable. The team will model how sulfur dissolves differently in magma oceans versus water oceans, and how those differences leave detectable fingerprints in a planet's atmosphere. Sulfur affects both the light passing through the atmosphere and the formation of hazes that change a planet's brightness. By running hundreds of simulations across different atmospheric masses and stellar heating rates, the researchers will produce synthetic spectra that can be compared directly to JWST observations. This is fundamental science with no immediate practical application. It will, however, settle a central debate in exoplanet science: whether the most common type of known planet can ever be habitable, or whether those worlds are invariably molten.
View original technical description
Planets intermediate in mass between Earth and Neptune, sub-Neptunes, represent the frontier in our search for life in the Universe. These worlds have no analogue in the solar system, yet occur frequently around other stars. Their size and occurrence rate make them primary targets for characterisation with current observational facilities. It has been hypothesised these sub-Neptunes could harbour life: with a layer of liquid water beneath their hydrogen-rich atmospheres and above deeper water ice layers. Planet's with such a structure have been termed 'Hyceans', hydrogen atmospheres with liquid water oceans. However, whether such planets exist is intensely debated. On the one hand, early James Webb Space Telescope (JWST) observations of the atmosphere of the archetypal Hycean K2-18b, specifically the ammonia depletion of its atmosphere, have been used to argue for it having a liquid water ocean. In contrast, climate modelling suggests sub-Neptunes like K2-18b receive too high an instellation flux to have water oceans, and our recent work shows that ammonia depletion in the atmosphere of K2-18b is equally consistent with a magma ocean at its surface. New tracers of the climate and interior structure of this important class of planet are required to test climate models and the Hycean habitability hypothesis. We will develop observable tracers of sub-Neptunes being in a Hycean, magma ocean, or Neptune-like regime, by modelling how the differential dissolution of sulfur in sub-Neptune interiors leaves detectable atmospheric fingerprints. Sulfur is a key tracer of planetary processes, with its atmospheric abundance and chemistry sensitive to the presence and depth of a magma ocean. Critically, atmospheric sulfur species strongly affect observations we can make today: emission/transmission spectroscopy (e.g., H2S absorption) and planetary bond albedos (through sulfur-haze formation). This opens the possibility of distinguishing between Hycean, magma ocean, and Neptune-like interiors on sub-Neptunes. We will develop diagnostics of sub-Neptune interior structure by extending self-consistent magma-ocean climate models to include sulfur for the first time: this is possible because of our team's significant experience of coupled climate-interior modelling, the thermochemistry of sulfur in magmas, and atmospheric sulfur chemistry. A large ensemble of simulations will be run across key parameters of atmospheric mass, composition and instellation, self-consistently with predicted sub-Neptune interior structures. From these atmospheres synthetic transmission and emission spectra will be generated. These predictions will be used to interpret present JWST observations of K2-18b, and inform future observing strategies to understand the habitability of this enigmatic class of planet.
Plain English summaries and category classifications on this site are generated by AI and may not perfectly reflect the original research.
Is something wrong? Let us know