Scientists have discovered two ‘aquatic worlds’ unlike anything seen in our solar system.

The planets are almost entirely made up largely of water, marking the first time such worlds have ever been confidently identified by scientists.

Previously, seekers were thought to be much more standard planets – but they’re actually much more pioneering than we thought, scientists say.

“We used to think that planets a little bigger than Earth were big balls of metal and rock, like enlarged versions of Earth, and that’s why we called them super-Earths,” said Björn Benneke. , one of the scientists in the study. new researcher and professor of astrophysics at the University of Montreal.

“However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature: a large part of their total volume is probably made up of water. This is the first time we have observed planets that can be positively identified as water worlds, a type of planet that has been theorized by astronomers to have existed for a long time.

Scientists haven’t detected water directly, and it’s still difficult at such a long distance. But the survey showed that up to half of the planet should be made of something lighter than rock and heavier than hydrogen – and the likeliest candidate for this material is water.

Both planets orbit the star Kepler-138, which is 218 light-years away and located in the constellation Lyra. They have three times the volumes of Earth and twice the masses, but are much less dense than our own planet.

The planets around Kepler-138 were first discovered in 2014, when they were spotted using the Kepler space telescope. They were found using the transit method, where scientists watch for the small dip that occurs as planets pass in front of their star.

The new research built on other observations from Nasa’s Hubble and Spitzer space telescopes, which allows researchers to determine the composition of planets. This additional data led scientists to believe that the planets were in fact water worlds.

Scientists warn that water worlds don’t look like the classic planet we might imagine, with a surface mostly like the oceans on our Earth. Rather, planets are likely to be so hot that water will immediately turn to vapor, creating a thick, dense atmosphere that can hide liquid water.

The planets are also outside the habitable zone, as they are far too hot for liquid water and are likely inhospitable to life. But new research has also found an additional planet in the system, known as Kepler-138e – which is in the habitable zone.

The planet is smaller and further from its star, with years taking 38 days on Earth. But scientists are still unsure of its exact details, as it does not appear to pass in front of its host star.

The results are described in a new paper, “Evidence for the volatile-rich composition of a 1.5-Earth-radius planet”, published in natural astronomy.