NASA: Two new exoplanets with “traces” of water found – What scientists discovered

A team led by researchers at the University of Montreal has found evidence that two exoplanets orbiting a “red dwarf” star are “water worlds” with water covering a very large portion of the planets. These worlds, located in a planetary system 218 light years from Earth, in the constellation Lyra and are unlike any other planet found in our solar system.

The team, led by Caroline Piaulet of the Trottier Institute for Exoplanet Research at the University of Montreal, published a detailed study of this planetary system, known as Kepler-138, in the journal Nature Astronomy.

Piaulet and her colleagues observed the exoplanets Kepler-138 c and Kepler-138 d with NASA’s Hubble and the retired Spitzer space telescopes and discovered that the planets may be largely composed of water. These two planets and a smaller one closer to the “red dwarf” star, Kepler-138 b, were previously discovered by NASA’s Kepler space telescope.

The new study also found evidence of a fourth planet.

Water was not directly detected in Kepler-138 c and d, but by comparing the sizes and masses of the planets with models they have, the astronomers conclude that a significant proportion of their volume – up to half – should consist of materials that are lighter than a rock and heavier than hydrogen or helium (which make up most of the gas giant planets like Jupiter). The most common of these candidate materials is the water.

“Previously we thought that planets that were slightly larger than Earth were big balls of metal and rock, and that’s why we called them super-Earths,” explained Björn Benneke, co-author of the study and professor of astrophysics at the University of Montreal. “However, we have now shown that these two planets, Kepler-138 c and d, are quite different and that a large percentage of their entire volume is likely water. It is the best evidence yet for water worlds, which have been thought by astronomers to exist for a long time.”

“Imagine types of moons like Europa or Enceladus, orbiting Jupiter and Saturn, but much closer to their starPiaulet explained. “Instead of a frozen surface, they would host large ‘envelopes’ of water vapor.”
The researchers warn that the planets may not have oceans like those on Earth directly on the planet’s surface. “The temperature in Kepler-138 d’s atmosphere is probably above the boiling point of water, and therefore a dense vapor atmosphere is expected on this planet. Only under this steam atmosphere could liquid water possibly exist at high pressure,” Piaulet said.

A new exoplanet in the system

The two possible water worlds, Kepler-138 c and d, are not in the habitable zone, the region around a star where temperatures would allow water to exist in liquid form on the surface of a planet. But the researchers found additional evidence for a new planet in the system, Kepler-138 e, in the habitable zone.

This newly discovered planet is small and farther from its star than the other three, taking 38 days to complete one orbit. The nature of this additional planet, however, remains an open question because it does not appear to be transiting its star. Observing the transit of the exoplanet would allow astronomers to determine its size.

The researchers had another surprise: they discovered that the two water worlds Kepler-138 c and d are “twin” planets, with essentially the same size and mass, whereas they were previously thought to be drastically different. The closest planet, Kepler-138 b, on the other hand, is confirmed to be a Mars-mass minor planet, one of smaller exoplanets which are known to this day.

“As our instruments and techniques become more precise at finding and studying planets farther from their stars, we may begin to find many more of these water worlds,” Benneke concluded.

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