- Steam worlds are exoplanets with hot atmospheres of water vapor. They are sub-Neptunes that orbit close to their stars.
- Researchers have developed a new model to study and understand these worlds better.
- Steam worlds are much larger and more massive than ocean moons. The steamy atmospheres lie above deep layers of an exotic phase of water unlike any water on Earth.
Exotic steam world exoplanets
You’ve heard about water worlds, exoplanets that could be completely covered in oceans or have deep layers of water beneath thick atmospheres. But what about steam world exoplanets? These are sub-Neptune planets that are too hot for surface oceans, but have steam-filled atmospheres. Now, a team of researchers led by the University of California, Santa Cruz (UC Santa Cruz), has developed a new model to better understand these intriguing worlds. The researchers said on August 25, 2025, that the steamy atmospheres would be above layers of an exotic phase of water. That water, called supercritical, is neither liquid nor gas.
A supercritical fluid is a substance at a temperature and pressure above its critical point. Distinct liquid and gas phases do not exist, and the fluid is below the pressure required to compress it into a solid.
These steam worlds are about 10-100 times more massive than the ocean moons in our solar system. Those moons – such as Europa and Enceladus among others – have global liquid water oceans beneath crusts of ice.
The researchers published their peer-reviewed findings in The Astrophysical Journal on July 24, 2025.
Too hot for liquid water
Steam worlds are unlike any planets in our solar system. They are what scientists call sub-Neptunes, so smaller than Neptune but larger than Earth. Most of the ones found so far tend to orbit their stars much closer than Earth orbits the sun. They are, therefore, a lot hotter.
In fact, they are too hot for liquid water to exist. However, they do have steamy atmospheres. So they are still water-rich, but not like Earth. Scientists first predicted their existence about 20 years ago.
New Model Aims To Demystify Steam Worlds Beyond Our Solar Systemastrobiology.com/2025/08/new-… #astrobiology #exoplanet
— Astrobiology (@astrobiology.bsky.social) 2025-08-26T21:35:15.131Z
Strange layers of water under steamy atmospheres
These unusual planets are also water-rich in another way also. But again, not like any place in our solar system. They have deep layers of water (and no solid surface) beneath their steamy atmospheres. That water, however, is not like the water we are familiar with. It is what scientists call supercritical water.
So, what is supercritical water? Scientists have recreated it in labs, but are still trying to understand it better. Basically, it is a phase of water that is more complex than usual liquid water or ice.
Are steam world exoplanets habitable?
Since they are extremely hot, these planets are unlikely to be habitable, at least for life as we know it. But understanding how they formed can help scientists find other planets that might be more habitable. Perhaps even a sub-set of steam worlds. Lead author Artyom (Artem) Aguichine at UC Santa Cruz said:
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.
And as co-author Natalie Batalha at UC Santa Cruz also noted:
The interiors of planets are natural ‘laboratories’ for studying conditions that are difficult to reproduce in a university laboratory on Earth. What we learn could have unforeseen applications we haven’t even considered. The water worlds are especially exotic in this sense. In the future, we may find that a subset of these water worlds represent new niches for life in the galaxy.
Water itself, in its various forms, is a complex substance. That might also help scientists in their search for extraterrestrial life. Aguichine said:
Life can be understood as complexity, and water has a wide range of properties that enables this complexity.
Icy ocean moons
The models used to study steam worlds are the same ones that scientists have used to study the icy ocean moons in our solar system. This includes ones like Europa, Enceladus and others. But there are, of course, key differences as well.
The ocean moons are much smaller, and cold on their surfaces. Their water layers are global oceans of regular liquid water beneath their icy crusts. But steam worlds are 10-100 times more massive and their “oceans” are layers of supercritical water, with no solid surface below.
In some models, the supercritical water – under the extreme pressures and temperatures inside sub-Neptunes – might even transform into something called superionic ice. This is an exotic form of ice where water molecules reorganize so that hydrogen ions can move freely through an oxygen lattice. It’s a cross between a solid and a liquid. Like supercritical water, it doesn’t exist naturally on Earth.
Bottom line: Researchers at UC Santa Cruz have developed a new model to study steam world exoplanets (sub-Neptunes) with hot steamy atmospheres and exotic layers of water.
Source: Evolution of Steam Worlds: Energetic Aspects
Via University of California, Santa Cruz
Read more: A steam world exoplanet with an earthlike atmosphere?
Read more: 2 alien water worlds with oceans 500 times deeper than Earth’s
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