Astronomers discover that two distant exoplanets are mostly water

The scientists didn’t directly detect water but instead, they compared the sizes and masses of the planets to models and concluded that a significant part of their volume, up to half of it, would be made up of materials that are lighter than rock but heavier than hydrogen and helium. The most common of the candidate materials that fit this description is water.

“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. 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,” explained Björn Benneke, leader of the study published in the journal Nature Astronomy.

With volumes more than three times the Earth’s and masses twice as much, the two planets — Kepler-138c and Kepler-138d — have much lower densities than our planet. This came as a surprise to the scientists because most of the planets slightly bigger than Earth that have been studied until now have all turned out to be rocky worlds just like the pale blue dot.

But at the same time, researchers point out that these two planets may not have oceans like we have on Earth, right on the planet’s surface.

“The temperature in Kepler-138c’s and Kepler-138d’s atmospheres is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets. Only under that steam atmosphere there could potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid,” explained Caroline Piaulet, a member of the research team, in a press statement.