Researchers Find Evidence For Existence Of Water Worlds Planets

Researchers have found evidence for the existence of new types of planets they have called a “water worlds,” where water makes up a large fraction of the entire planet.

Researchers Find Evidence For Existence Of Water Worlds Planets

Researchers have found evidence for the existence of new types of planets they have called a “water worlds,” where water makes up a large fraction of the entire planet. These worlds, discovered in a planetary system 218 light-years away, are unlike any planets in our solar system. The team, led by Caroline Piaulet of the Institute for Research on Exoplanets (iREx) at the University of Montreal, published a detailed study of a planetary system known as Kepler-138 in the journal Nature Astronomy on December 15.

Piaulet, a member of Björn Benneke’s research team at the University of Montreal, used the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope to observe the exoplanets Kepler-138 c and Kepler-138 d. She discovered that the planets could be mostly made of water.

Water was not directly detected, but by comparing the planets’ sizes and masses to models, they conclude that a significant fraction of their volume, up to about half, should be made of materials lighter than rock but heavier than hydrogen or helium (which constitute the bulk of gas-giant planets like Jupiter). Water is the most common candidate material.

“Previously, we thought that planets a little larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, which is why we called them super-Earths,” Benneke explained. “However, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a large fraction of their entire volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that astronomers believe has existed for a long time”.

Planets c and d have much lower densities than Earth, despite having volumes more than three times that of Earth and masses twice as large. This is surprising because the majority of planets just slightly larger than Earth that have been studied in depth so far all appeared to be rocky worlds like ours. According to the researchers, the closest analogy would be some of the icy moons in the outer solar system, which are also largely composed of water surrounding a rocky core. “Think of larger versions of Europa or Enceladus, the water-rich moons that orbit Jupiter and Saturn, but brought much closer to their star,” Piaulet explained. “Rather than an icy surface, they would have large water-vapor envelopes.”

“The secure identification of an object with the density of the Solar System’s icy moons but significantly larger and more massive clearly demonstrates the great diversity of exoplanets,” said Grenoble Alpes University’s Jose-Manuel Almenara. “A variety of formation and evolution processes are expected to result in this.” Researchers warn that the planets may not have oceans like Earth’s directly on their surfaces. “The temperature in Kepler-138 d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on this planet. Only in that steam atmosphere could there be liquid water at high pressure or even water in another phase that occurs at high pressures, known as a supercritical fluid,” Piaulet explained.

The NASA/ESA/CSA James Webb Space Telescope will also allow for important future research. “Now that we have definitively identified the ‘water-world’ Kepler-138 d, the James Webb Space Telescope will be critical in revealing the atmospheric composition of such an exotic object,” said Austrian Academy of Sciences team member Daria Kubyshkina. “It will give us critical information enabling us to compare the composition of the icy moons of the solar system with that of their larger and heavier extrasolar counterparts.” Another team at the University of Montreal recently discovered a planet called TOI-1452b that may have a liquid-water ocean, but Webb will be needed to confirm this.

Astronomers announced the discovery of three planets orbiting Kepler-138, a red dwarf star in the constellation Lyra, using data from NASA’s Kepler Space Telescope in 2014. This was based on a measurable dip in starlight as each planet passed in front of the star for a brief moment. Benneke and his University of New Mexico colleague Diana Dragomir devised the plan to re-observe the planetary system with the Hubble and Spitzer space telescopes between 2014 and 2016 in order to catch more transits of Kepler-138 d, the system’s third planet, in order to study its atmosphere.

The secure identification of an object with the density of the solar system’s icy moons, but much larger and more massive, clearly demonstrates the great diversity of exoplanets, which is expected to be the result of a variety of formation and evolution processes. While earlier Kepler observations only revealed transits of three small planets around Kepler-138, Piaulet and her colleagues were surprised to discover that the Hubble and Spitzer observations required the presence of a fourth planet in the system, Kepler-138 e.

This newly discovered planet is small and far from its star, taking 38 days to complete an orbit. The planet is in its star’s habitable zone, a temperate region where it receives just enough heat from its cool star to be neither too hot nor too cold to support the presence of liquid water. The nature of this additional, newly discovered planet, on the other hand, remains unknown because it does not appear to transit its host star. The transit of the exoplanet would have allowed astronomers to calculate its size.

With Kepler-138 e in the mix, the masses of the previously known planets were recalculated using the transit timing-variation method, which involves tracking small variations in the precise moments of the planets’ transits in front of their star caused by the gravitational pull of other nearby planets.

Another surprise was discovered by the researchers: the two water worlds Kepler-138 c and d are “twin” planets, with nearly the same size and mass, despite previously being thought to be vastly different. Kepler-138 b, on the other hand, has been confirmed to be a small Mars-mass planet, one of the smallest exoplanets discovered to date. “As our instruments and techniques become more sensitive to finding and studying planets farther from their stars, we may find a lot more of these water worlds,” Benneke concluded.

Originally published at ESA