Astrobiologists often compare the search for life beyond our solar system to scanning a desert for pockets of water. Intriguingly, many of the most common exoplanets – known as sub-Neptunes – may contain large amounts of water. These steam worlds, sized between Earth and Neptune, are believed to harbor water-rich interiors. However, because they orbit much closer to their stars than Earth does to the Sun, their surfaces are far too hot to sustain liquid water. Instead, they are blanketed by deep steam atmospheres above layers of supercritical water, an exotic state that behaves as neither liquid nor gas.

Since steam worlds were first proposed two decades ago, researchers have sought to better understand their composition and origins. Scientists at the University of California, Santa Cruz have now developed a more precise model to simulate the internal structure and evolution of sub-Neptunes. Led by postdoctoral researcher Artem Aguichine, the team’s work incorporates laboratory data on water under extreme conditions, including phases such as supercritical fluid and superionic ice-states thought to exist in the deep interiors of Uranus, Neptune, and possibly sub-Neptunes.

The James Webb Space Telescope recently confirmed steam in several sub-Neptune atmospheres, and many more observations are expected. Accurate models are essential for linking these atmospheric measurements to what lies beneath. Unlike the smaller icy moons that earlier models focused on, sub-Neptunes are far more massive and lack solid ice crusts, making their internal behavior uniquely complex.

By tracing how water is distributed within these planets over billions of years, researchers hope to refine our understanding of planetary formation – and perhaps identify new environments where life could emerge. Upcoming missions like the European Space Agency’s PLATO telescope will further test and improve these models, guiding the next steps in the search for life beyond Earth.

For more details, read the original paper and article by UC Santa Cruz.