Imagine a planet where the entire atmosphere is like a steamy sauna. That’s essentially what a “steam world” is like, although there’s more to these exoplanets and their sweltering environments that makes them special.
A recent study published in The Astrophysical Journal explores the bizarre nature of water on these exoplanets, categorized as “sub-Neptunes” because their size and mass are between that of Earth and Neptune.
Although the lack of liquid water on the surface of steam worlds makes it impossible for anything to live there, knowing how they’ve evolved could still be a critical step in the search for extraterrestrial life.
Sifting Through Steam Worlds
Decades after the existence of steam worlds was initially predicted, the James Webb Space Telescope (JWST) made the first confirmed sighting of one in 2024. Located nearly 100 light years away, the exoplanet (GJ 9827 d) was found to be twice the size of Earth, with an atmosphere made almost entirely of hot water vapor. Researchers believe that future JWST observations will discover a lot more steamy exoplanets like it.
Steam worlds like GJ 9827 d aren’t hospitable to the kind of life that would be found on Earth. This is because they tend to be much closer to their host star than Earth is to the Sun; liquid water is nowhere to be found on their surfaces, but water takes a few different forms elsewhere on the planets.
Below the steamy atmosphere lies a region of “supercritical water,” a perplexing phase of water that is neither gas nor liquid, but has properties of both states. Supercritical water doesn’t just exist on planets beyond our Solar System; it exists on Earth as well, spewing from hydrothermal vents deep in the ocean. It has also been recreated in laboratories, even being used in nuclear engineering, bioenergy, and waste treatment.
Read More: JWST May Have Found Strongest Evidence of Life on Exoplanet K2-18b
Phases of Water on Steam Worlds
Beyond its steamy and supercritical states, water on sub-Neptunes may transform into “superionic ice” due to extreme pressure and temperatures. In this phase, water molecules reorganize so hydrogen ions move freely through an oxygen lattice. As a result, superionic ice possesses both solid and liquid properties.
These unusual states of water inspired researchers to create an improved model of steam worlds, which have often been examined with models developed to study icy moons in our Solar System. Sub-Neptunes, though, are 10 to 100 times more massive and don’t have icy crusts.
The researchers wanted to understand how water on sub-Neptunes could behave as pure steam, as supercritical fluid, and in extreme states like superionic ice. Knowing how these exoplanets have evolved to contain these forms of water, they believe, could unlock the next steps in finding life on other water planets.
“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,” said co-author Natalie Batalha, a professor of astronomy and astrophysics at the University of California, Santa Cruz, in a statement.
“In the future, we may find that a subset of these water worlds represent new niches for life in the galaxy,” Batalha continued.
Full Steam Ahead to Search for Alien Life
Combining information from previous atmospheric and interior models, the researchers came up with a new model to map the evolution of a water-rich sub-Neptune. The model indicates that the radius of these types of planets are smaller than predicted by previous models, and that the water deep in their interior is colder than previously suggested.
It also confirmed that water in the interior can transition from a plasma state to superionic ice.
These properties — changing radii and a planet’s composition — are crucial for studying a sub-Neptune’s evolution over billions of years. And even though steam worlds themselves aren’t the answer to extraterrestrial life, modeling them could help scientists refine predictions for the existence of water-rich, Earth-like exoplanets.
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