Is our dream of finding ocean-covered exoplanets drying up?

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Sub-Neptune planets, often billed as possible "water worlds," may be more desert than deep sea, according to a new study.

For years, scientists thought these planets, which are larger than Earth but smaller than Neptune, could form far from their stars, sweeping up ice beyond the so-called "snow line." As the planets migrated inward, scientists have thought that ice might melt into oceans hidden beneath hydrogen skies. Such hypothetical worlds were dubbed "Hycean planets," a blend of "hydrogen" and "ocean."

"Our calculations show that this scenario is not possible," Caroline Dorn, an assistant professor of Physics at ETH Zürich in Switzerland who co-led the new study, said in a statement.

The results come just months after high-profile claims about K2-18b, an exoplanet about 124 light-years away, made global headlines as a likely ocean world "teeming with life." A team of scientists studying James Webb Space Telescope (JWST) observations had reported hints of a possible biomarker gas, dimethyl sulfide, on K2-18b — fueling speculation that the planet might be cloaked in a hydrogen-rich atmosphere above a vast global ocean. These are conditions that could potentially support life (as we know it).

But those claims were quickly met with pushback. Independent analyses of the same JWST data suggested the team's evidence for DMS was weak at best, while other experts cautioned that sub-Neptunes may not be ocean-bearing worlds at all, but rather volatile-rich planets wrapped in thick, hostile atmospheres.

In the new study, Dorn and her team modeled how sub-Neptunes evolve during their early lifetimes, when they are thought to be blanketed by hydrogen gas and covered for millions of years by molten rock. Unlike earlier studies, the researchers included chemical interactions between magma and the atmosphere, according to the statement.

Of the 248 model planets the team studied, "there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet's mass, as was previously thought," Dorn said in the statement. "Hycean worlds with 10-90 percent water are therefore very unlikely."

The team found that hydrogen and oxygen — the building blocks of H2O — tend to bind with metals and silicates in the interior, effectively sequestering water deep in the interior. Even planets that began with abundant ice ended up with less than 1.5% of their mass as water near the surface, the new study reports, far less than the tens of percent envisioned for Hycean planets.

"We focus on the major trends and can clearly see in the simulations that the planets have much less water than they originally accumulated," Aaron Werlen, a researcher on Dorn's team at ETH Zürich who co-led the new study, said in the same statement. "The water that actually remains on the surface as H2O is limited to a few per cent at most."

The researchers also found that the most water-rich atmospheres did not appear on planets formed far from their stars, where ice is plentiful, but rather on planets formed closer in. In these cases, water was generated chemically, as hydrogen in the atmosphere reacted with oxygen from the molten rock.

The implications are sobering for astrobiology. If Hycean planets do not exist, the most promising havens for liquid water, and potentially life, may lie on smaller, rocky worlds more akin to Earth.

Still, K2-18b remains a captivating target, scientists say. As a sub-Neptune, a type of planet missing from our own solar system but common across the galaxy, it could reveal fundamental insights into how planetary systems form and why ours turned out the way it did.

The new results also suggest that Earth may not be exceptional, with many distant worlds veiled in similarly modest traces of water.

"The Earth may not be as extraordinary as we think," Dorn said in the statement. "In our study, at least, it appears to be a typical planet."

The research was published on Sept. 18 in The Astrophysical Journal Letters.