Supercritical steam? Hidden fourth star? Kepler telescope discovers extrasolar water world

After years of observation, exoplanets are full of life. Behind the data, a hidden world emerges

Astronomers discover bizarre binary exoplanet that could be a 'water world'

Two planets discovered by NASA's retired Kepler space telescope may be made of half water, according to new research.

The two exoplanets, called Kepler-138c and Kepler-138d, are located in the constellation Lyra and orbit a star 218 light-years from Earth. In 2014, data from the Kepler Space Telescope sparked scientists' interest. They decided to revisit the two planets using the Hubble Space Telescope and NASA's now-retired Spitzer Space Telescope, hoping to learn more about these distant worlds and their composition. Surprisingly, they are mostly water.

"We thought that these two planets, slightly larger than Earth, were giant balls of metal and rock — like a larger version of our own, which is why we call them 'super-Earths,'" Björn Bennik, a planetary astrophysicist at the University of Montreal in Canada and a co-author of the study, said in a statement.

"However, now we've discovered that Kepler-138C and D are very different: a large portion of their mass may be made up of water," he said. "This is the first time we've seen an exoplanet that can be confirmed as a 'water world' - a type of planet that astronomers have long hypothesized to exist."

An artist's conception of an exoplanet with an atmosphere containing water vapor. (Image credit: ESA/Hubble, M. Kohlmesser)

Astronomers still can't be sure if these planets are truly water-rich, as they still haven't directly detected the presence of water on any of these worlds. However, the researchers can calculate the density of each planet: They are about three times the size of Earth and only twice as massive, meaning they are much less dense than our planet.

(The fact that the two planets were so similar was surprising — scientists had expected them to be very different.) The lower densities suggest that both worlds are made of at least half some material heavier than the hydrogen and helium that make up the gas giants, but lighter than the rocky stuff that scientists had expected to find — water is one possibility.

The researchers think the two planets may be somewhat similar to the icy moons of the outer solar system, with an icy crust hiding an ocean of liquid water surrounding a rocky core. However, the two planets discovered by Kepler will be much hotter than these moons and contain very little ice.

"Imagine a larger version of Europa or Enceladus, the water-rich moons of Jupiter and Saturn, but much closer to their stars," Caroline Piollet, a doctoral student at the University of Montreal and lead author of the new study, said in a statement. "The temperatures in the atmospheres of Kepler-138c and Kepler-138d are likely above the boiling point of water, so we expect them to have thick atmospheres dominated by water vapor."

An artist's impression of the exoplanet Kepler-138D, compared to Earth. (Image credit: Benoît Goujon, University of Montreal)

(Beneath the vapor, scientists think there might be liquid water, or even liquid water that flows like a gas under high temperatures and pressures — a state called a supercritical liquid.)

The two bizarre worlds weren't the only surprises researchers found in the Hubble and Spitzer data. Initially, astronomers used Kepler's observations to find three planets, a finding published in 2014. Benneke was particularly interested in the outermost world, Kepler-138D, and highlighted observations from Hubble and Spitzer between 2014 and 2016.

The data point to the existence of a fourth planet, which will be called Kepler-138E. It will be a small world that orbits its star every 38 days, close enough to allow liquid water to exist on its surface, the scientist said. (Kepler-138B and C have orbital periods of 10 and 14 days, respectively.)

Meanwhile, additional observations suggest that the innermost planet, Kepler-138b, is about the size of Mars, which would make it one of the smallest members of the more than 5,000 exoplanets scientists have discovered and catalogued.

The researchers hope that Kepler-138 won't be the last planets to pose such puzzles. "As our instruments and techniques become more sensitive and capable of finding and studying planets farther from their stars, we may find more water worlds like Kepler-138C and D," Bennick said.

Related knowledge:

An exoplanet, or extrasolar planet, is a planet that is outside our solar system. The earliest documented evidence of the existence of exoplanets dates back to 1917, although they were not identified at the time. The first detected exoplanet was confirmed in 1992. Another planet, which was detected as early as 1988, was not confirmed until 2003. As of December 1, 2022, 5,284 exoplanets have been confirmed to exist in 3,899 planetary systems, of which 847 systems have more than one planet.

The Kepler Space Telescope is a retired space telescope of NASA, launched in 2009 to search for Earth-sized planets orbiting other stars. The spacecraft, named after astronomer Johannes Kepler, was launched into a heliocentric orbit synchronized with the Earth to carry out its mission, with William J. Borucki as its chief investigator. After nine and a half years of operation, the telescope's reaction system ran out of fuel. On October 30, 2018, NASA announced that it had been retired.

BY:Meghan Bartels

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