Uranus appears to have far more water frozen as ice in its interior than astronomers thought, potentially settling a long-running mystery about whether it formed differently to its closest neighbour, Neptune.

Ice giants like Uranus and Neptune have thick, gassy atmospheres. This makes it hard to know what is inside the planets’ interiors or how they formed. Scientists can, however, measure gases in their atmospheres, which they can then link to processes and elements deeper inside.

Carbon monoxide in a planet’s atmosphere is often associated with its deepest parts being rich in water or ice, but while neighbouring Neptune has displayed abundant carbon monoxide suggestive of an ice-rich centre, Uranus has been lacking, which has led some astronomers to argue it instead has a rocky interior. If true, this would mean that Neptune and Uranus formed in very different ways and aren’t as similar as they appear.

Now, Thibault Cavalié at the University of Bordeaux in France and his colleagues have found carbon monoxide in Uranus’s lower atmosphere for the first time, suggesting it is far richer in water than previously suspected.

“We find that Uranus is more on the ice-giant side than on the rock-giant side,” says Cavalié. “It tells us that this controversy is over now. We have to be careful when we say things like that, because things also depend on modelling, but that’s the feeling we have.”

Cavalié and his colleagues used the Atacama Large Millimeter/submillimeter Array telescope in Chile to observe Uranus three times between 2022 and 2024 and detected significant amounts of carbon monoxide in the planet’s lower atmosphere. They then used several models with different ratios of rock and ice to try to reproduce the amount of carbon monoxide they measured, finding that they could only reproduce it with the ice-rich models.

They also detected carbon monoxide in Uranus’s upper atmosphere, but this suggests it is from a different source than from inside Uranus, probably from a comet striking the planet several centuries ago, says Cavalié.

Finding carbon monoxide is an important step in understanding Uranus’s deep interior, but it isn’t clear where it might be coming from, says Vanesa Ramirez at Leiden University in the Netherlands. “Interpreting atmospheric abundances requires assumptions about chemistry, mixing and internal structure, all of which remain uncertain for Uranus.”

These assumptions and the wide range of models used to simulate Uranus’s interior means there are many different rock-to-ice ratios that are compatible with the available data, says Ramirez. “On its own, it does not settle the question of whether Uranus should be regarded primarily as an ice-rich or rock-rich giant.”