Artist's concept of a hycean world
Pablo Carlos Budassi / Wikimedia Commons

One of the most common types of planets out there doesn’t exist in our own solar system: “sub-Neptunes,” or planets slightly smaller than Neptune with a gaseous atmosphere. With no basis for comparison, we don’t really know what these planets are like. Some astronomers have even suggested that they might be water-rich, potentially hosting global oceans.

However, a new study has shed doubt on this water-rich scenario. A paper published in The Astrophysical Journal Letters argues that sub-Neptunes are dry, with no more than a few percent more water (by mass) than Earth. Unlike previous results, this new study accounts for interactions between the atmospheres and molten interiors as these planets are forming — interactions which ultimately destroy most of the planet’s water.

Primordial Interactions

Like the giant planets in our own solar system, sub-Neptunes should have an atmosphere composed primarily of hydrogen and helium. But they could also accumulate large amounts of water if they form beyond the snow line, where water condenses as ice.

Previous studies have suggested that some sub-Neptunes could even harbor vast oceans, maybe even making them potentially hospitable to life. These worlds are known as hycean worlds, a portmanteau of hydrogen and ocean, with 10-90% of their weight made up of water. However, those studies didn’t consider the interactions between the planet’s atmosphere and molten interior right after its formation.

“The energy from giant impacts during planet formation translates into large amounts of heat, much of which can be trapped by a primordial hydrogen-helium atmosphere,” says Aaron Werlen (ETH Zurich, Switzerland), a graduate student who led the study. “Numerous studies have shown that this naturally leads to long-lived magma oceans.”

The planets could therefore maintain surface temperatures of thousands of degrees for billions of years, as their hydrogen-helium atmosphere traps the leftover heat of formation.

“Interactions between the atmosphere and the magma oceans seem to be extremely important, and this study is making the right approach towards trying to understand those connections better,” says Yamila Miguel (Leiden University, The Netherlands), who wasn’t involved in the finding. The chemistry at play during this stage of a planet’s existence can have a significant impact on its composition and evolution.

The team simulated a sample of 248 planets, spanning a range of conditions expected for young sub-Neptunes. The researchers considered interactions between seven elements, including volatiles (elements that easily evaporate as gases, like hydrogen, carbon, or oxygen) and the deep interior. They accounted for 26 different phases of compounds made up of these elements.

They found that the reactions between the magma ocean and atmosphere would destroy most of the water, below 1.5% of the planet’s mass — even for planets that had initially accreted up to 30% water. For reference, Earth is only around 0.02-0.3% water, despite our large oceans. Under the high pressures and temperatures of a planets’ interior, chemical processes dissolve water into hydrogen and oxygen, which are then sequestered as metals.

Where’s the Water?

Hycean worlds have seen a spike in public interest due to hints of a potential biosignature in the atmosphere of K2-18b. However, with such a low percentage of water, sub-Neptunes are less likely to host large oceans, making it doubtful whether they could ever sustain life.

“The truth is that we don't know yet the nature of these planets, since we do not have any such kind in our solar system,” says Miguel. “We do not know if they are fully mixed or if they have layers.” Astronomers are just starting to determine what kinds of atmospheres and temperatures are required for oceans, she adds.

But there might be hope for a subset of these worlds.

In their simulations, the team found that sub-Neptunes forming inside the snow line had significant amounts of water in their atmospheres (between 10% and 90%), even though they had accumulated fewer volatiles than planets farther out. The results suggest that water mass fractions are governed more by primordial chemical reactions than by the accretion of ices themselves.

The overall amount of water isn't actually greater in the close-in planets, though. Worlds forming beyond the snow line have thicker hydrogen atmospheres, which dilutes the fraction of water in the atmosphere compared to planets that form closer-in. Nevertheless, the study offers the possibility that close-in planets might have water-rich atmospheres. For example, LHS 1140b, a super-Earth that astronomers think might host a nitrogen-rich atmosphere and surface ocean, could fall into this category.

The team hopes to look at more sub-Neptune and water world candidates to verify their models. “A key goal is to compare our predictions to James Webb Space Telescope spectra by computing atmospheric compositions directly,” Werlen says. “But much more work — both in modelling and in observations — is needed to achieve this.”

The paper “is one more study that is trying to take us closer to understanding the nature of these planets,” Miguel says. A better understanding of the chemical processes is necessary to reveal what’s happening in the deep interiors of these mysterious worlds.