Astronomers used the Hubble and Spitzer space telescopes when they discovered the evidence of exoplanets that could be "water worlds." The team from the University of Montreal found the exoplanets Kepler-138c and Kepler-138d, which are largely made up of water.
Although they did not directly detect water on either exoplanet, astronomers compared their sizes and masses to models and concluded that up to half of its volume is water, instead of the usual lighter than rock but heavier than hydrogen or helium materials.
Two Water Worlds Found Outside the Solar System
A detailed study of the planetary system Kepler-138, titled "Evidence for the Volatile-rich Composition of a 1.5-Earth-radius Planet," is published in the journal Nature Astronomy.
Researchers observed exoplanets Kepler-138c and Kepler-138d from 218 million light-years away and found that the planets are approximately one and a half times the size of Earth but mostly water, SciTech Daily reported. These planets are companions to Kepler-138b, which is closer to its star, and were discovered previously by NASA's Kepler Space Telescope.
Scientists previously thought that planets a bit larger than Earth were big balls of metal and rock, which is why they call them super-Earths. But the two water worlds show that they have different natures and a big fraction of them is likely composed of water. This is the first time they observed planets that they can confidently identify as water worlds, which were only theorized before.
Despite having volumes three times more than Earth and masses twice as big, the two exoplanets have lower densities than Earth. It is a surprising discovery as most of the planets bigger than Earth were rocky worlds.
The team said that the closest comparison to the two planets would be the icy moons in the outer Solar System, which have rocky cores. They are like larger versions of the Europa or Enceladus but are water-rich objects that orbit closer to the Sun.
They added that the two exoplanets may not have oceans like here on Earth given the boiling temperatures of its atmosphere. Only below the steam on these exoplanets could there be liquid water at high pressure or in another phase at high pressure known as a supercritical fluid.
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Detecting the Fourth Planet in Kepler-138
In 2014, scientists used NASA's Kepler Space Telescope to detect three planets orbiting the red dwarf star Kepler-138. According to Science Daily, they measured the dip in starlight as the planets momentarily passed by their star in transit.
They named the planets Kepler-138b, Kepler-138c, and Kepler-138d, which were discovered between 2014 and 2018. While the space telescope observations only showed transits by the three exoplanets, astronomers who used observations from Hubble and Spitzer space telescopes detected the presence of a fourth planet classified as Kepler-138e.
It was small and farther from the star and takes about 38 days to complete an orbit. Kepler-138e is in the habitable zone of its star, wherein it just receives the right amount of heat for the right temperature. It is neither too hot nor too cold to allow the presence of liquid water.
However, the exoplanet does not transit its host star so scientists were not able to measure its size. But scientists found out that Kepler-138c and Kepler-138d are twin planets with almost the same size and mass, while Kepler-138b is a small Mars-like planet.
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