Science News | Astronomers Find That Two Exoplanets May Be Mostly Water

Washington [US], December 16 (ANI): A team led by UdeM astronomers has found evidence that two exoplanets orbiting a red dwarf star are "water worlds," planets where water makes up a large fraction of the volume. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are unlike any planets found in our solar system.

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

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Piaulet, who is part of Bjorn Benneke's research team, observed exoplanets Kepler-138c and Kepler-138d with NASA's Hubble and the retired Spitzer space telescopes and discovered that the planets -- which are about one and a half times the size of the Earth -- could be composed largely of water. These planets and a planetary companion closer to the star, Kepler-138b, had been discovered previously by NASA's Kepler Space Telescope.

Water wasn't directly detected, but by comparing the sizes and masses of the planets to models, they 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 (which constitute the bulk of gas giant planets like Jupiter). The most common of these candidate materials is water.

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"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 Benneke. "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."

"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 Benneke. "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."

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 have been studied in detail so far all seemed to be rocky worlds like ours. The closest comparison to the two planets, 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 Piaulet. "Instead of an icy surface, Kepler-138 c and d would harbor large water-vapor envelopes."

Researchers caution the planets may not have oceans like those on Earth directly at 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," Piaulet said. (ANI)

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