Examined by researchers at the University of Montreal, the twin planets are located in the named system Kepler-138 at 218 light-years away, and they are unlike any other type of planet in our Solar System. Both, in fact, appear to be composed mostly of water.
Led by Caroline Piaulet and Björn Benneke, the team of researchers observed the world pair across the Hubble telescope and the Spitzer Space Telescope, now retired. The planets – named Kepler-138c and Kepler-138d – had previously been discovered by the Kepler telescope, along with their companion Kepler-138b.
The presence of water has not been observed directly but comparing the size and mass of planets to modelsthe scientists concluded that about half the volume it should be composed of materials that are lighter than rock, but heavier than hydrogen or helium. The most likely candidate for this type of material is water.
“Previously, we believed that exoplanets slightly larger than Earth were large balls of rock and metal, just like an enlarged version of Earth, which is why we called them super-Earths,” Benneke explained. ”However, we have shown that these two planets – Kepler-138c and – are fundamentally different: a large fraction of their volume is composed of water. It is the first time we have observed water worlds, although this type of planet has long been theorized by astronomers.”
Despite having a volume 3 times greater than that of the Earth, c and d planets are much less dense. It’s a surprising detail, since all the slightly larger-than-Earth planets observed so far appear to be rocky. The researchers explain that the closest example we have are some of the moons in our Solar System, such as Europe and Enceladus (just on Enceladus there could be an ocean with life forms).
The team also specified that these planets they might don’t have a ocean on the surface like the earthly one: “Kepler-138c and atmosphere temperatures are most likely above the boiling point of water and we expect the planets to have a thick thick atmosphere made of vapor,” said Caroline Piaulet. “Just underneath that vapor could there be water in a liquid state, or even water in a different state that happens under extreme pressures, called a supercritical fluid.”
