Scientists find an explanation for oddball, water-rich exoplanets: They make their own water

As more and more exoplanets are discovered throughout the galaxy, scientists find some that defy explanation—at least for awhile. A new study, published in Nature, describes a process that might explain why a large portion of exoplanets have water on their surface, even when it doesn't make sense.

Water where it shouldn't be

A particular category of exoplanets that are between the size of Earth and Neptune, referred to as "sub-Neptunes," generally have a rocky core, which is surrounded by an envelope of either hydrogen or water. This makes sense if the planet forms farther away from its host star, in a region where water can precipitate as ice. However, some of these planets are found much closer to their host stars, where it should be too hot to hold water at the surface.

While some planets may accumulate a certain amount of water from incoming comets and asteroids, that doesn't work for these planets either. The amount of water that is typically found on their surfaces is too high for such explanations. Past experiments have also shown that hydrogen can reduce iron in silicates, producing water. However, they came to the conclusion that only small amounts of water would be produced at the kind of high pressures experienced at the surface of a sub-Neptune planet.

Water from an unexpected source

The presiding explanation for the existence of these planets has been that they formed somewhere past the snow-line, where they were capable of accumulating ice, and then migrated inward. But the new study might have a better explanation—the planets make water from their own rocky cores and hydrogen atmospheres.

The study authors write, "Here we report experimental evidence of reactions between warm, dense hydrogen fluid and silicate melt that release silicon from the magma to form alloys and hydrides at high pressures. We found that oxygen liberated from the silicate melt reacts with hydrogen, producing an appreciable amount of water up to a few tens of weight percent, which is much greater than previously predicted based on low-pressure ideal gas extrapolation."

In other words, the high pressures on the sub-Neptune planets, which can be up to 10,000 times the pressure of Earth's atmosphere, cause the silicate rock to be in a magma form. The oxygen is then free to react with the hydrogen in the atmosphere and create water—and apparently, in very high amounts. These reactions also explain the existence of both hydrogen-rich and water-rich exteriors, as they appear to exist on a spectrum. As the hydrogen in a hydrogen-rich planet gets used up to make water, the planet shifts to a more water-rich planet.

"The reaction we report here suggests that these planet types may be fundamentally related: hydrogen-rich sub-Neptunes could be the precursors of water-rich sub-Neptunes and super-Earths. If an excess, unreacted H₂ atmosphere can be retained, sub-Neptunes with an H₂-rich atmosphere covering an H₂O-rich layer above the core (that is, hycean worlds) may be quite common," the study authors explain.

A new understanding of exoplanet formation

This new view of water formation on planets has a lot of implications for scientists. It challenges the idea that water-rich planets must form far from their stars, suggesting water worlds could be more common and form in unexpected places. It also changes the way scientists view the potential for life on other planets by expanding the range of planets that might have water.

Future experiments might incorporate a wider range of planetary materials and conditions in experiments to capture more planetary diversity and determine if similar processes might take place on other types of planets. Observational studies can also refine how exoplanet atmospheric data are interpreted, especially regarding water detection.

A News and Views on the research was also published in Nature.

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