Many more planets in distant solar systems have large amounts of water than previously thought - as much as half water and half rock, surprise new findings suggest.

Thanks to better telescope instruments, scientists are finding signs of more and more planets in distant solar systems. A global team of scientists decided to take a population-level look at a group of planets that are seen around a type of star called an M-dwarf. These stars are the most common stars see around us in the galaxy, and scientists have catalogued dozens of planets around them so far.

These analyses had been done for individual planets, but much more rarely for the entire known population of such planets in the Milky Way galaxy. As the scientists looked at the numbers - 43 planets in all - they saw a surprising picture emerging.

The densities of a large percentage of the planets suggested that they were too light for their size to be made up of pure rock. Instead, these planets are probably something like half rock and half water.

Rafael Luque, researcher at the University of Chicago, USA, and first author, led the studies with co-author Enric Pallé of the Institute of Astrophysics of the Canary Islands and the University of La Laguna, Tenerife, Spain.

Luque said: “It was a surprise to see evidence for so many water worlds orbiting the most common type of star in the galaxy. It has enormous consequences for the search for habitable planets.”

The water could exist mixed into the rock or in pockets below the surface. Those conditions would be similar to Jupiter’s moon Europa, which is thought to have liquid water underground.

University of Chicago scientist Jacob Bean, who is carrying out further analysis, said: “I was shocked when I saw this analysis—I and a lot of people in the field assumed these were all dry, rocky planets.”

Though the evidence is compelling, Bean said he and the other scientists would still like to see "smoking gun proof” that one of these planets is a water world. That is something the scientists are hoping to do with JWST, NASA’s newly launched space telescope, which is the successor to Hubble.

The research paper was published in the journal Science in September 2022.