Water World Exoplanets: Climate Simulations

Water World Exoplanets: Climate Simulations

Water worlds represent an exciting frontier in astronomy, having long been theorised to bridge the gap between rocky planets and larger ice giants. By mass, water worlds are hypothesised to be composed of 10–90% water and other volatile ices, such as ammonia and methane, with the remainder being made up of silicates and iron. In contrast, the rocky planets of the solar system are extremely ‘dry’—for example, despite having a surface covered by oceans, water constitutes less than 0.1% of the Earth’s total mass. Meanwhile, the ice giants Uranus and Neptune have thick outer envelopes composed of hydrogen-rich gas, whereas water worlds would be devoid of hydrogen.

This project will focus on developing atmospheric models for ‘sub-Neptunes’ (planets intermediate in size between Earth and Neptune), and water worlds in particular. Most existing models for exoplanet atmospheres assume a hydrogen-dominated atmosphere, given the observational focus on gas giants to date. Extending such models to water worlds will bring new challenges, such as a gas-phase composition dominated by a species (water) that may also condense in cooler regions of the atmosphere. At sufficiently low temperatures, some water worlds may form planet-wide liquid oceans at their surface (‘ocean worlds’). Detailed modelling for the climate of such planets will be a vital step in assessing their potential to host life, while also paving the way for climate simulations of more Earth-like planets that will be targeted by future observatories.

Key objectives

• Develop atmospheric and climate models for sub-Neptune exoplanets, with a particular focus on water worlds.

• Adapt and extend next-generation climate simulation tools to handle water-rich atmospheres and unfamiliar chemical regimes.

• Investigate atmospheric circulation, cloud formation, and heat transport in water world atmospheres.

• Interpret observations of water worlds by comparing atmospheric simulations with measured spectra.

• Assess the climatic conditions and potential habitability of water worlds and ocean worlds.