Scientists find new ‘waterworlds’ that look nothing like any planet in our solar system

Two planets are the first ever ‘water worlds’ to be confidently identified

Scientists have found two “water worlds” that are unlike anything seen in our solar system.

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

Previously, the researchers were thought to be far more standard planets – but they are actually far more pioneering than we realised, scientists say.

“We previously thought that planets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that’s why we called them super-Earths,” said Björn Benneke, one of the scientists on the new research 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 big fraction of their entire volume is likely composed of water. It is the first time we observe planets that can be confidently identified as water worlds, a type of planet that was theorized by astronomers to exist for a long time.”

Scientists have not directly detected the water, and that remains difficult at such a long distance. But investigation showed that up to half of the planet should be made up of something lighter than rock and heavier than hydrogen – and the most likely candidate for that material is water.

The two planets are in orbit the star Kepler-138, which is 218 light years and situated in the Lyra constellation. They have volumes three times that of Earth, and masses twice as big – but are much less dense than our own planet.

Planets around Kepler-138 were first discovered in 2014, when they were spotted using with the Kepler Space Telescope. They were found using the transit method, where scientists watch the for small dip that happens when the planets pass in front of their star.

The new research relied on further observations from Nasa’s Hubble and Spitzer space telescopes, which allows researchers to determine the composition of the planets. That extra data led scientists to the belief that the planets were in fact water worlds.

Scientists caution that the water worlds do not look like the classic planet we might imagine, with a surface primarily like the oceans on our Earth. The planets are instead likely to be so hot that the water will immediately turn to steam, creating a thick and dense atmosphere that may hide liquid water.

The planets are also outside of the habitable zone, since they are far too hot for liquid water and are likely inhospitable to life. But new research also found an extra 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 that take 38 days on Earth. But scientists are still unsure of its exact details, since it does not seem to pass in front of its host star.

The findings are described in a new paper, ‘Evidence for the volatile-rich composition of a 1.5-Earth-radius planet’, published in Nature Astronomy.