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. “The temperature in Kepler-138d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on this planet. Researchers caution the planets may not have oceans like those on Earth directly at the planet’s surface. “Instead of an icy surface, they would harbor large water-vapor envelopes.” “Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” explained Piaulet. 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. 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. With volumes more than three times that of Earth and masses twice as big, planets c and d have much lower densities than Earth. It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time.” “However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature and that a big fraction of their entire volume is likely composed of 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,” explained Björn Benneke, study co-author and professor of astrophysics at the University of Montreal. The most common of these candidate materials is water. Water wasn’t directly detected at Kepler-138c and d, but by comparing the sizes and masses of the planets to models, astronomers 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 new study found evidence for a fourth planet, too. These two planets and a smaller planetary companion closer to the star, Kepler-138b, had been discovered previously by NASA’s Kepler Space Telescope. Piaulet and colleagues observed exoplanets Kepler-138c and Kepler-138d with NASA’s Hubble and the retired Spitzer space telescopes and discovered that the planets could be composed largely of water. The team, led by Caroline Piaulet of the Institute for Research on Exoplanets (iREx) at the University of Montreal, published a detailed study of this planetary system, known as Kepler-138, in the journal Nature Astronomy today. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are unlike any planets found in our solar system. A team led by of researchers at the University of Montreal has found evidence that two exoplanets orbiting a red dwarf star are “water worlds,” where water makes up a large fraction of the entire planet.
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