Exoplanets may have more water than we thought — but there’s a catch

For years, scientists have speculated about the nature of water on exoplanets, especially on super-Earths and sub-Neptunes — planets larger than Earth but smaller than Neptune. The prevailing view has been that these distant worlds could be covered by vast oceans, potentially making them prime candidates for hosting life. However, a groundbreaking study published in Nature Astronomy has turned this idea on its head.

The research reveals that instead of being concentrated at the surface, the majority of water on these planets might be hidden deep within their interiors, locked away in their cores and mantles.

Distribution of water

Water is an important component of exoplanets. It’s not just about habitability, it’s about the planets themselves. Whether at the surface or deep inside, water fundamentally influences planetary properties.

Here, on Earth, there’s a lot of water on the surface in rivers, lakes, and oceans. Other planets, that don’t have an atmosphere, may have trouble holding water on the surface. Take Mars, for example. We know it had water in the past, but when its atmosphere went poof, so did the surface water. But just because it doesn’t have surface water doesn’t mean it doesn’t have deeper water. A new study just showed that Mars may have a lot of water hidden deep in its crust — it’s just not in the form we’re familiar with.

Could the same be happening on other planets? Or rather, could planets have water hidden even deeper, in their mantles or cores?

The distribution of water within a planet is governed by a process known as water partitioning. This distribution is between different layers of the planet, such as the core, mantle, and surface. It is influenced by factors like pressure, temperature, and the chemical makeup of the planet’s interior.

Most exoplanets we know of are located close to their star. This means they’re hot — not ‘hot’ like a summer day, ‘hot’ as in oceans of molten lava. For this type of planet, the mantle and crust are not yet entirely formed and differentiated. Water also dissolves very well in this magma. But how is the water then distributed? Caroline Dorn, Professor for Exoplanets at ETH Zurich wanted to investigate.

Water in the core

Using advanced molecular dynamics simulations, the researchers investigated how water behaves under extreme pressures and temperatures in this type of scenario. The simulations revealed that water prefers to stay in the planet’s iron core rather than the silicate-rich mantle at these extreme pressures. This finding challenges the traditional view that water would primarily be present in the mantle or at the surface, as it is on Earth.

“The iron core takes time to develop. A large share of the iron is initially contained in the hot magma soup in the form of droplets.” The water sequestered in this soup combines with these iron droplets and sinks with them to the core. “The iron droplets behave like a lift that is conveyed downwards by the water,” explains Dorn.

“Most of the water on exoplanets is found deep in the interior and not on the surface,” Dorn adds.

This suggests that exoplanets are much more water-rich than previously thought, it’s just that the water is hidden deep inside the planet.

One of the most significant implications of this study is its impact on how scientists interpret the mass and radius data of exoplanets. Typically, the mass and radius of a planet are used to infer its composition. If a planet has a large radius for its mass, we assume it has a thick atmosphere or a large amount of surface water. But if the water is deep within the planet, it could lead to a massive underestimation of the amount of water present.

“This is one of the key results of our study,” says Dorn. “The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated into the core. Under certain circumstances, iron can absorb up to 70 times more water than silicates. However, owing to the enormous pressure at the core, the water no longer takes the form of H₂O molecules but is present in hydrogen and oxygen.”

Water worlds are probably not as common then

The study also has important implications for super-Earth water worlds — planets that are several times more massive than Earth and are considered to be covered by surface water.

The new study suggests that water worlds may not be as common. Planets that were water world candidates, and are likely to hold a lot of water, may have a lot of that water trapped within the core.

For instance, the study analyzed a group of planets that were previously thought to have up to 50% of their mass in water. The findings suggest that while these planets might still be water-rich, the water is likely distributed differently than previously assumed, with a significant portion locked within the interior rather than at the surface. This could mean that these planets have less surface water and, therefore, less potential for habitability than initially thought.

Ultimately, this study challenges many previous assumptions about exoplanets and water, but it needs further data to be validated. This is what the James Webb Space Telescope is working on. The telescope can track down the molecules in the atmosphere and for two years, it has been sending data from space to Earth. This data will go a long way toward confirming (or disproving) the findings of this study and helping us better understand the nature of water on exoplanets.

Journal Reference: Luo H, Dorn C, Deng J. The interior as the dominant water reservoir in super-Earths and sub-Neptunes. Nature Astronomy, 20 August 2024, doi: external page10.1038/s41550-024-02347-zcall_made

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