The two worlds orbiting the smallest star 218 light years apart appear to be unlike any we have in the Solar System.
The exoplanets are named Kepler-138c and Kepler-138d. Both are about 1.5 times the radius of Earth, and both appear to be scurrilous worlds, with thick, dense atmospheres, and crazy deep oceans, all surrounded by rocky-metallic interiors.
“The planets were a little bigger than the Earth, big balls of metal and rock, like scale versions of the Earth, and that’s why we called them super-Earths,” says astronomer Björn Benneke of the University of Montreal.
“But we have shown that these two planets, Kepler-138c and d, are very different in nature: a large part of the entire volume is probably composed of water. First, we notice planets that can be confidently identified as water worlds, the type of planet that astrologers have long speculated to exist.
Recent analysis of another world has found that it may be water, but further observations will be needed to confirm. According to the researchers, their work on the two Kepler-138 Oceanic planets is less uncertain.
Working out what happens to planets outside the Solar System (or exoplanets) usually requires some detective work. The orbits of the stars are very distant and very dark compared to the light of the stars; Direct images are very difficult and subsequently very rare to obtain, and do not show much detail.
The composition of an exoplanet is usually inferred from its density, which is calculated using two measurements, one taken from the eclipsing (or transit) of the star’s light by the planet, and the other from the star’s radial velocity or ‘labo’.
The mass of the stars that block the passage shows the size of the exoplanet from which we receive the radiation. The radial velocity is induced by the gravitational pull of the exoplanet, which is carried around by the just but minute expansion and contraction of the bright star as it escapes. The magnitude of this motion can tell us how much mass the exoplanet has.
Once you have the size and mass of an object, you can calculate its density.
A gaseous world, such as Jupiter or even Neptune, will have a relatively low density. World rocks that are rich in metals will have higher densities. At 5.5 P. * cubic centimeter; Earth is the densest planet in our Solar System; Saturn is the least dense, 0.69 grams per cubic centimeter.
Transit data show that Kepler-138c and Kepler-138d have radii 1.51 times that of Earth, and taking measurements on Kepler-138 each gives us masses of 2.3 and 2.1 times that of Earth, respectively. And those data give us a density of about 3.6 grams per cubic centimeter of both worlds – somewhere between a rocky and a gaseous composition.
Europa is quite close to the Jovian moon of ice, which has a density of 3.0 P/cm3. It is connected to the global ocean under the icy crust covered with liquid.
“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” says astrophysicist Caroline Piaulet of the University of Montreal, who led the research. “Instead of an icy surface, Kepler-138c and d would receive envelopes of many water vapors.”
According to the team’s model, water would make up more than 50 percent of the exoplanet’s volume, up to a depth of about 2,000 kilometers (1,243 miles). The Earth’s ocean, for context, has an average height of 3.7 kilometers (2.3 miles).
But Kepler and Kepler are much closer to their stars than to Earth. Although the star is a small, cool red dwarf, its proximity makes the two exoplanets much hotter than our world. They have orbital periods of 13 and 23 days respectively.
This means that the oceans and atmospheres on these worlds are unlikely to look like what our ocean researchers say.
“The temperature in the atmospheres of Kepler-138c and Kepler-138d is probably above the boiling point of water, and we expect a dense, dense atmosphere of steam on these planets,” says Piaulet.
“Just below that vapor 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.”
The research was published in Nature Astronomy.
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