Astronomers are discovering more and more super-Earths, planets around other stars whose sizes and masses lie somewhere between those of Earth and Neptune. We would like constraints on their composition to investigate whether they are more similar to rocky Earth or gaseous Neptune. To do this we need numerical models of their interiors. Such models often exclude any thermal effects, a choice justified by noting that a heated rocky planet expands by only a small amount. But this is not necessarily true for planets with thick oceans or watery atmospheres. Water has a rich and interesting thermal behaviour: at high pressure and temperature it can be in any of several exotic plasma and ice phases. Planets with thick water layers, known as waterworlds, cannot therefore be accurately represented by models that treat them as cold spheres. But understanding how waterworlds vary in size and structure is important as we seek to interpret new observations of super-Earths.

I developed temperature-dependent structure models of waterworlds, treating both the interior structure and the atmosphere and including both internal and external heating. In doing so, I synthesized an improved equation of state for water to better capture how it behaves when heated or pressurised. Using these models, I show the following: heat can significantly affect a watery planet’s size and structure; these planets can have large and diffuse yet opaque atmospheres; and a planet can have a hot extended steam atmosphere even if only moderately heated from the inside. My models are simpler than those based on energy transfer codes, yet are fast to evaluate and still capture thermal behaviour trends appropriately. I therefore suggest that they would be ideally suited to use in statistical models of planetary systems. I also explore how a planet might change size if it migrates or exists in an elliptical orbit, consider the astrobiological implications of heating a watery planet, and present the results of applying these models to a recently discovered potential waterworld.