Two possible 'water world' exoplanets discovered by NASA's Hubble and Spitzer telescopes

Astronomers using NASA's Hubble and Spitzer telescopes have made a discovery which has brought them a micro-step closer to confirming that planets beyond our own might harbour Earth-like oceans.

A team of University of Montreal researchers discovered two exoplanets, Kepler-138 c and Kepler-138 d, that they believe are largely made up of water.

While water hasn't been directly detected on either exoplanet, by comparing their sizes and masses to models, astronomers concluded that a significant fraction of their volumes – up to half – should be made of materials lighter than rock but heavier than hydrogen or helium.

The most common of these candidate materials is water.

What is an exoplanet?

An exoplanet is any planet that orbits a star outside our solar system.

Considering Kepler-138 c and Kepler-138 d are 218 light-years away in the constellation Lyra, they are definitely in the exoplanet category.

Why is this discovery important?
While frozen water has been observed on multiple moons in our outer Solar System, scientists have so far been unsuccessful in finding planets with their own versions of Earth's lush oceans.

The discovery of these two possible "water worlds" could change that.

"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," explained Björn Benneke, professor of astrophysics at the University of Montreal.

An artist's illustration showing a cross-section of the Earth (left) and the exoplanet Kepler-138 d (right).(Supplied: Benoit Gougeon (University of Montreal))

"However, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a big fraction of their entire volume is likely composed of water.

"It is the best evidence yet for water worlds, a type of planet that was theorised by astronomers to exist for a long time."

Despite being three times the volume and twice the mass as Earth, Kepler-138 c and d have much lower densities than Earth.

Researchers were surprised at this discovery because most of the planets slightly bigger than Earth that have been studied in detail so far all seem to be rocky worlds like ours.

The closest comparison, say researchers, would be some of the icy moons in the outer Solar System that are also largely composed of water surrounding a rocky core.

"Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star," explained study lead Caroline Piaulet.

"Instead of an icy surface, they would harbour large water vapour envelopes."

Could humans survive on Kepler-138 c and d?
Unfortunately not. It would be all but impossible for humans to even reach these exoplanets due to their distance from us.

And even if were we were able to travel to them, their oceans might not be anything like the ones we're used to.

That's because the two possible water worlds are not located in the habitable zone, the area around a star where temperatures would allow liquid water on the surface of a rocky planet.

"The temperature in Kepler-138 d's atmosphere is likely above the boiling point of water, and we expect a thick dense atmosphere made of steam on this planet, Ms Piaulet said.

"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."

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