Astronomers have discovered that two exoplanets orbiting a red dwarf star that are unique in their composition. These two exoplanets are unlike any other discovered outside the Solar System. These two planets are filled with water.

These water worlds located in a planetary system 218 light-years away in the constellation Lyra are unique since the liquid makes up a large volume of of their composition. Astronomers observed exoplanets Kepler-138c and Kepler-138d with Hubble and the retired Spitzer space telescopes to make the watery discovery.

The team, led by PhD student Caroline Piaulet of the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal, published a detailed study of a planetary system known as Kepler-138 in the journal Nature Astronomy.

The planets, which are about one and a half times the size of the Earth, were discovered along with its host star by Nasa’s Kepler Space Telescope. While the water wasn’t detected directly, researchers, by comparing the sizes and masses of the planets to models, conclude that a significant fraction of their volume — up to half of it — should be made of materials that are lighter than rock but heavier than hydrogen or helium.

“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,” Björn Benneke, who led the discovery said in a statement.

The researchers maintained that with volumes more than three times that of Earth and masses twice as big, planets c and d have much lower densities than Earth. This is surprising because most of the planets just slightly bigger than Earth that has been studied in detail so far all seemed to be rocky worlds like ours.

“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star. Instead of an icy surface, Kepler-138 c and d would harbor large water-vapor envelopes,” explained Piaulet.

Recently, another team at the University of Montreal found another planet, called TOI-1452 b, that could potentially be covered with a liquid-water ocean.


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