Two exoplanets found orbiting a red dwarf could be mostly made of water, according to astronomers at the Université de Montréal (UdeM) in Québec. The exoplanets are located 218 light-years away in the constellation Lyra, and are unlike any other exoplanet discovered.

The two exoplanets are called Kepler-138 c and Kepler-138 d. Water was not directly detected on these planets, but astronomers concluded up to half of their volumes should be made of materials lighter than rock and heavier than hydrogen or helium. The most common substance fitting this criteria is water.

“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” said Caroline Piaulet of the university’s Trottier Institute for Research on Exoplanets, in a statement. “Instead of an icy surface, they would harbor large water-vapour envelopes.”

The worlds are considered “twin” planets, because they have virtually the same size and mass. Compared to Earth, the planets have lower densities, masses twice as big, and volumes that are at least three times greater. This is a surprising characteristic, because most exoplanets slightly bigger than Earth are rocky worlds.

The planets may not be covered in oceans like those found on Earth though, as they are outside the system’s habitable zones. And according to Piaulet, the atmosphere on Kepler-138 d is likely above water’s boiling point.

“Only under that steam atmosphere [could there] potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid,” she said.

Bjorn Benneke, an astrophysics professor at the UdeM, said in a statement that the existence of water worlds has been theoretical among astronomers. But if the planets do turn out to be water worlds, he hopes their characteristics will give astronomers a better idea of how to find similar planets.

“It is the best evidence yet for water worlds,” said Benneke.