Scientists find water worlds 218 light years from Earth

By Mark Waghorn via SWNS

Two planets 218 light years from Earth are water worlds, according to new research.

They are located in a system in the constellation Lyra - and are unlike any found in our solar system.

Co-author Dr. Bjorn Benneke of the University of Montreal, Canada, said: "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.

"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."

The planets were discovered by NASA's Kepler Space Telescope. They are about one-and-a-half times the size of Earth - and may not have oceans directly at the surface.

Lead author Caroline Piaulet, a Ph.D. student at the same lab, said: "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."

Water wasn't directly detected. But computer models showed up to half their volume is made of materials lighter than rock and heavier than hydrogen or helium.

The latter constitute the bulk of gas giant planets like Jupiter. The most likely candidate is water.

It is believed the mysterious planets resembled the outer solar system's icy moons that are also largely composed of water surrounding a rocky core.

Piaulet said: "Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star.

"Instead of an icy surface, Kepler-138 c and d would harbor large water-vapor envelopes."

The planets are orbiting a red dwarf star named Kepler-138. The findings in the journal Nature Astronomy are based on dips in starlight as they pass by.

There are two other planets in the system known as Kepler-138d and the newly discovered Kepler-138 - which is small and farther from its star than the three others.

It 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 study also found the two water worlds Kepler-138c and d are “twin” planets with virtually the same size and mass.

The closer-in planet, Kepler-138b, on the other hand, is a small Mars-mass planet, one of the smallest exoplanets known to date.

Benneke added: "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-138 c and d."