# Astronomers discover twin exoplanets that could be 'water worlds'

"It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time."
The low density of Kepler-138 c and Kepler-138 d – which are nearly identical in size – means that they must be composed largely of water.

Scientists at the University of Montreal have found two exoplanets that might be immersed in water.

Located in a planetary system 218 light-years away in the constellation Lyra, the 'water worlds' orbiting a red dwarf star are unlike any other planet ever discovered in the solar system. Dubbed Kepler-138 c and Kepler-138 d, the planets were observed with NASA's Hubble and the retired Spitzer space telescopes. The discovery was a follow-up of a study from 2014.

"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," Björn Benneke, study co-author and professor of astrophysics at the University of Montreal, explained in a statement.

"However, we have now shown that these two planets, Kepler-138 c, and d, are quite different and that a big fraction of their entire volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time," he said.

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

## The exoplanets do not have oceans like those on the Earth's surface

The exoplanets c and d have volumes more than thrice the Earth's and the masses twice as big. They also have much lower densities than Earth.

This is interesting because most planets slightly bigger than Earth seemed to be rocky worlds like our home planet. According to researchers, the closest comparison would be the icy moons in the outer solar system that are also largely composed of water surrounding a rocky core.

The researchers also state these planets may not have oceans like those on Earth directly at the planet's surface.

"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 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," Piaulet said.

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## The 'twin' water worlds have the same size and mass

Data also pointed to another planet, Kepler-138 e, which is small and farther from its star than the three others, taking 38 days to complete an orbit. However, the nature of this star is unknown.

With the finding of Kepler-138 e, the masses of the other two exoplanets were measured again via a transmit timing-variation method. The results were surprising.

Both the water worlds are "twin" planets, with virtually the same size and mass. Earlier, they were thought to be drastically different.

"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 of these water worlds," Benneke concluded.

Study Abstract:

The population of planets smaller than approximately 1.7 Earth radii (R⊕) is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial velocity mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate 1.51 ± 0.04 R⊕ planet Kepler-138 d (Teq,AB=0.3≈350KTeq,AB=0.3≈350K) combined with new radial velocity measurements of its host star obtained with the Keck/High Resolution Echelle Spectrometer. We find evidence for a volatile-rich ‘water world’ nature of Kepler-138 d, with a large fraction of its mass $M_{\rm{d}}$ contained in a thick volatile layer. This finding is independently supported by transit timing variations and radial velocity observations (Md=2.1+0.6−0.7M⊕Md=2.1−0.7+0.6M⊕), as well as the flat optical/infrared transmission spectrum. Quantitatively, we infer a composition of 11+3−4%11−4+3% volatiles by mass or ~51% by volume, with a 2,000-km-deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations (<0.003 M⊕) would have swiftly been lost on a ~10 Myr timescale. The bulk composition of Kepler-138 d therefore resembles those of the icy moons, rather than the terrestrial planets, in the Solar System. We conclude that not all super-Earths are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations. Finally, our photodynamical analysis also reveals that Kepler-138 c (with a Rc = 1.51 ± 0.04 R⊕ and a Mc=2.3+0.6−0.5M⊕Mc=2.3−0.5+0.6M⊕) is a slightly warmer twin of Kepler-138 d (that is, another water world in the same system) and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.