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For astrobiologists, the search for life beyond our solar system could be likened to where one would look in a vast desert—essentially, where there's water. And it turns out that one of the most common types of exoplanet observed in planetary systems beyond ours has a size and mass that indicate a water-rich interior. They are categorized as "sub-Neptunes" because their size and mass are between that of Earth and Neptune.

But because these types of exoplanets tend to be much closer to their host star than Earth is to the sun, sub-Neptunes are too hot to have liquid water on their surface and support life. Instead, they would have atmospheres made of steam, over layers of an exotic phase of water that behaves like neither gas nor liquid. Since the existence of these "steam worlds" were first predicted 20 years ago, interest in their exact makeup and evolution has grown.

Now, astrobiologists and astronomers at the University of California, Santa Cruz, have developed a more precise way to model these steam worlds to help better understand their composition, and ultimately, how they formed in the first place. "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," said Artem Aguichine, a postdoctoral researcher at UC Santa Cruz who led the development of the new model.

The work is explained in published on July 24 in The Astrophysical Journal and is co-authored by Professor Natalie Batalha, head of UC Santa Cruz's astrobiology initiative, along with Professor Jonathan Fortney, chair of the university's Astronomy and Astrophysics Department.

More than icy moons

For the first time in history, the James Webb Space Telescope (JWST) confirmed the presence of steam on a handful of sub-Neptunes. Astronomers expect JWST to observe dozens more, which is why such models are critical to connecting what we see from the exoplanet's surface to what is inside of them.

The models historically used to characterize sub-Neptunes were developed to study the icy moons in our solar system, such as Jupiter's moon Europa and Saturn's moon Enceladus. Aguichine says sophisticated models can help interpret what space telescopes like JWST reveal about sub-Neptunes.

Icy moons are small, condensed bodies with layered structures: icy crusts over liquid water oceans. Sub-Neptunes are much different. They are vastly more massive—10 to 100 times as much—and, again, they orbit much closer to their stars. So they don't have icy crusts and liquid oceans like Europa or Enceladus. Instead, they develop thick steam atmospheres and layers of "supercritical water."

This exotic, supercritical phase of water has been recreated and studied in laboratories on Earth, exhibiting behavior that is far more complex than simple liquid water or ice—thus, making it difficult to model accurately. Some models even suggest that, under extreme pressure and temperature conditions inside sub-Neptunes, water may even transform into "superionic ice," a phase in which water molecules reorganize so 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. So, to model sub-Neptunes, researchers need to understand how water behaves as pure steam, as a supercritical fluid, and in extreme states like superionic ice. This team's model accounts for the experimental data on the physics of water under extreme conditions and advances the theoretical modeling that's required.

"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," Batalha explained. "In the future, we may find that a subset of these water worlds represent new niches for life in the galaxy."

By modeling the distribution of water in these common exoplanets, scientists can trace how water—one of the universe's most abundant molecules—moves during the formation of planetary systems. Indeed, Aguichine said water has a range of fascinating properties:

• It is both a chemical acid and base, participating in chemical balance
• It is good at dissolving salts, sugars, and amino acids
• It creates hydrogen bonds—giving water a higher viscosity, a higher boiling point, a greater capacity to store heat, and more.

"Life can be understood as complexity," Aguichine said, "and water has a wide range of properties that enable this complexity."

Looking back and forward

He also stressed that their modeling focuses not on static snapshots of sub-Neptunes, but accounts for their evolution over millions and billions of years. Because planetary properties change significantly over time, modeling that evolution is essential for accurate predictions, he said.

The modeling will soon be put to the test by continued observations with JWST, and also with future missions such as the European Space Agency's upcoming launch of the PLAnetary Transit and Oscillation (PLATO) of stars telescope, a mission designed to find Earth-like planets in the habitable zone of their host star.

"PLATO will be able to tell us how accurate our models are, and in what direction we need to refine them," Aguichine said. "So really, our models are currently making these predictions for the telescopes, while helping shape the next steps in the search for life beyond Earth."