There’s much less water on distant planets than beforehand thought

There is way much less water on the surfaces of distant planets outdoors our photo voltaic system than beforehand thought, in line with new analysis.

These exoplanets should not have thick layers of water, as was typically speculated, the researchers report.

An exoplanet orbiting a dwarf star 124 light-years from Earth made headlines around the globe in April 2025. Researchers on the University of Cambridge reported that planet K2-18b may very well be a marine world with a deep, world ocean teeming with life.

However, the brand new examine now reveals that so-called sub-Neptunes resembling K2-18b are extremely unlikely to be worlds dominated by water and that situations there are removed from conducive to life.

“Water on planets is much more limited than previously believed,” notes Caroline Dorn, professor of exoplanets at ETH Zurich.

The examine was performed underneath the management of ETH Zurich, in collaboration with researchers from the Max Planck Institute for Astronomy in Heidelberg and the University of California in Los Angeles.

K2-18b is bigger than Earth however smaller than Neptune, inserting it in a category of planets that don’t exist in our photo voltaic system. However, observations present that they’re widespread in outer house. Some of those sub-Neptunes had been in all probability fashioned far-off from their central star, past the so-called snow line the place water freezes into ice and later migrates inwards.

Until now, it had been assumed that a few of these planets had been capable of accumulate notably giant quantities of water throughout their formation and now harbor deep, world oceans beneath a hydrogen-rich environment. Experts refer to those as Hycean planets: a mix of “hydrogen” and “ocean”.

“Our calculations show that this scenario is not possible,” says Dorn. This is as a result of a elementary vulnerability of earlier research was that they ignored any chemical coupling between the environment and the inside of the planet.

“We have now factored in the interactions between the planet’s interior and its atmosphere,” explains Aaron Werlen, a researcher on Dorn’s workforce and lead creator of the examine in The Astrophysical Journal Letters.

The researchers assume that, in an early stage of their formation, the sub-Neptunes went via a part wherein they had been lined by a deep, scorching magma ocean. A shell of hydrogen gasoline ensured that this part was maintained for tens of millions of years.

“In our study, we investigated how the chemical interactions between magma oceans and atmospheres affect the water content of young sub-Neptune exoplanets,” says Werlen.

To do that, the researchers used an current mannequin that describes planetary evolution over a particular time period. They mixed this with a brand new mannequin that calculates the chemical processes that happen between the gasoline within the environment, and the metals and silicates within the magma.

The researchers calculated the chemical equilibrium state of 26 completely different parts for a complete of 248 mannequin planets. The pc simulations confirmed that the chemical processes destroy most H2O water molecules. Hydrogen (H) and oxygen (O) connect themselves to metallic compounds, and these largely disappear into the planet’s core.

Even although the accuracy of such calculations has some limitations, the researchers are satisfied by the outcomes.

“We focus on the major trends and can clearly see in the simulations that the planets have much less water than they originally accumulated,” explains Werlen. “The water that actually remains on the surface as H2O is limited to a few per cent at most.”

In an earlier publication, Dorn’s group was already capable of present how most of a planet’s water is hidden within the inside.

“In the current study, we analyzed how much water there is in total on these sub-Neptunes,” explains the researcher, “According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50% of the planet’s mass, as was previously thought. Hycean worlds with 10-90% water are therefore very unlikely.”

This makes the seek for extra-terrestrial life tougher than hoped for. Conditions conducive to life, with enough liquid water on the floor, are prone to exist solely on smaller planets, which is able to in all probability be observable solely with observatories even higher than the James Webb Space Telescope.

Dorn finds the position of our Earth notably thrilling in gentle of the brand new calculations which present that the majority distant planets have comparable water content material to our planet.

“The Earth may not be as extraordinary as we think. In our study, at least, it appears to be a typical planet,” she says.

The researchers had been additionally stunned by a seemingly paradoxical distinction: the planets with essentially the most water-rich atmospheres will not be those who have accrued essentially the most ice past the snow line, however quite planets that fashioned inside the snow line. On these planets, the water didn’t come from ice crystals, however was produced chemically when hydrogen within the planetary environment reacted with oxygen from the silicates within the magma ocean to type H2O molecules.

“These findings challenge the classic link between ice-rich formation and water-rich atmospheres. Instead, they highlight the dominant role of the equilibrium between magma ocean and atmosphere in shaping planetary composition,” concludes Werlen. This can have far-reaching implications for theories of planetary formation and the interpretation of exoplanetary atmospheres within the age of the James Webb Telescope.

Source: ETH Zurich