Using data from the NASA/ESA Hubble Space Telescope and NASA’s retired Spitzer space telescope, astronomers have discovered that two exoplanets around the red dwarf star Kepler-138 — Kepler-138c and Kepler-138d — could be composed largely of water.
Kepler-138 is an M-dwarf star approximately 218 light-years away in the constellation of Lyra.
Also known as KOI-304 or TIC 159376971, the star is about 57% the mass and 54% the radius of the Sun.
Kepler-138 is home to at least three exoplanets: Kepler-138b, Kepler-138c and Kepler-138d.
“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,” said Université de Montréal’s Professor Björn Benneke.
“However, we have now shown that these two planets, Kepler-138c and Kepler-138d, 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, Kepler-138c and Kepler-138d 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.
“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” said Université de Montréal Ph.D. student Caroline Piaulet.
“Instead of an icy surface, Kepler-138c and Kepler-138d would harbor large water-vapor envelopes.”
The astronomers 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,” 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.”
The new observations also suggest the presence of a fourth planet in the Kepler-138 system.
Named Kepler-138e, the planet is small and farther from its star than the three others, taking 38 days to complete an orbit.
The alien world is in the habitable zone of its star, a temperate region where a planet receives just the right amount of heat from its cool star to be neither too hot nor too cold to allow the presence of liquid water.
The nature of this additional planet, however, remains an open question because it does not seem to transit its host star.
“As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we might start finding a lot more water worlds like Kepler-138c and Kepler-138d,” Professor Benneke said.
The findings appear today in the journal Nature Astronomy.
Caroline Piaulet et al. Evidence for the volatile-rich composition of a 1.5-Earth-radius planet. Nat Astron, published online December 15, 2022; doi: 10.1038/s41550-022-01835-4
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