For astrobiologists, the search for life beyond our Solar System could be likened to where one would look in a vast desert where there’s water.
The most intriguing targets are planets called sub-Neptunes, which get their name because they’re larger than Earth but smaller than Neptune.
What makes them fascinating is that their size and mass suggest they’re packed with water but not the kind of water we know.
These steam worlds orbit much closer to their host stars than Earth does to our Sun, making them far too hot for liquid oceans on their surfaces.
Instead, they’re shrouded in thick steam atmospheres that hover over layers of water in an exotic “supercritical” state. This strange phase of water, which scientists have recreated in Earth laboratories, behaves in ways far more complex than simple liquid or ice.
The James Webb Space Telescope has already detected steam on several sub-Neptunes, confirming what astronomers had theorised for decades.
Now, with dozens more observations expected, researchers need better tools to interpret what they’re seeing.
The challenge lies in the extreme nature of these worlds. Previous models were designed for studying icy moons like Europa and Enceladus in our Solar System; small, cold bodies with icy crusts over liquid oceans.
Sub-Neptunes are entirely different beasts, being 10 to 100 times more massive and subjected to crushing pressures and scorching temperatures that create water phases impossible to find on icy moons.
Under the most extreme conditions deep within these planets, water might even transform into “superionic ice,” a bizarre state where water molecules reorganise so that hydrogen ions move freely through an oxygen lattice. This phase has been produced in the lab and is thought to exist in the deep interiors of Uranus, Neptune, and potentially sub-Neptunes as well.
Led by postdoctoral researcher Artem Aguichine, the UC Santa Cruz team has created models that account for these exotic water phases and how they evolve over millions and billions of years.
“When we understand how the most commonly observed planets in the universe form, we can shift our focus to less common exoplanets that could actually be habitable,” – Artem Aguichine from UC Santa Cruz.
The research also serves as preparation for future missions. The European Space Agency’s upcoming PLATO telescope will search for Earth like planets in habitable zones, and these new models will help scientists interpret what they find.
As Aguichine put it, the models are making predictions for telescopes while helping shape humanity’s next steps in searching for life beyond Earth.
Understanding these steam worlds matters because they’re everywhere, among the most common planets we’ve discovered.
By deciphering how water behaves under such extreme conditions, we’re not just learning about distant worlds, but gaining insights into the fundamental processes that shape planetary systems throughout the universe.
Written by Mark Thompson/Universe Today.
