(ORDO NEWS) — Two worlds orbiting a tiny star 218 light-years away appear to be unlike anything else in our solar system.
The exoplanets are named Kepler-138c and Kepler-138d. Both are about 1.5 times the radius of the Earth, and both appear to be raw worlds made up of a thick vapor atmosphere and insanely deep oceans surrounded by a rock-metal subsurface.
“We used to think that planets that were slightly larger than the Earth were big balls of metal and stone, like enlarged copies of the Earth, so we called them super-Earths,” says astronomer Bjorn Bennecke from the University of Montreal.
“However, we have now shown that these two planets, Kepler-138c and d, are completely different in nature: most of their entire volume is probably composed of water.
This is the first time we have observed planets that can be confidently identified as water worlds, a type of planet that astronomers theorize has been around for a long time.”
Recent analysis of the other world suggested that it could be a water world, but follow-up observations would be needed to confirm. Their work on the two oceanic planets Kepler-138 is less questionable, the researchers say.
Figuring out what planets outside our solar system (or exoplanets) are made of usually requires a lot of detective work.
They are very far away and very dim compared to the light of the stars around which they revolve; direct images are very difficult to obtain and therefore very rare and do not show much detail.
The composition of an exoplanet is usually determined from its density, which is calculated using two measurements, one taken from the planet’s eclipse (or passage) of star light, and the other from the star’s radial velocity, or “wobble.”
The amount of starlight that is blocked by the passage tells us the size of the exoplanet, from which we get the radius.
The radial velocity is induced by the exoplanet’s gravitational pull, which is seen as a regular but very slight expansion and contraction of the wavelength of the star’s light as it pulls. The amplitude of this movement can tell us how much mass an exoplanet has.
Knowing the size and mass of an object, you can calculate its density.
A gaseous world like Jupiter or even Neptune would have a relatively low density. Rocky worlds rich in metals will have a higher density. At 5.5 grams per cubic centimeter, Earth is the densest planet in our solar system; Saturn has the lowest density: 0.69 grams per cubic centimeter.
Transit data show that Kepler-138c and Kepler-138d have radii 1.51 times Earth’s, and Kepler-138 measurements of their respective tugs give us masses 2.3 and 2.1 times Earth’s, respectively.
These characteristics, in turn, give us a density of about 3.6 grams per cubic centimeter for both worlds – something between a rocky and gaseous composition.
It’s pretty close to Jupiter’s icy moon Europa, which has a density of 3.0 grams per cubic centimeter. It so happened that it is covered with a liquid global ocean under an icy shell.
“Imagine larger versions of Europa or Enceladus, water-rich moons orbiting Jupiter and Saturn but much closer to their star,” says astrophysicist Caroline Piolet of the University of Montreal, who led the study. “Instead of an icy surface, Kepler-138c and d will contain large shells of water vapor.”
According to the team’s simulations, water will make up more than 50 percent of the volume of exoplanets, spreading downward. to a depth of about 2,000 kilometers (1,243 miles). Earth’s oceans, for context, have an average depth of 3.7 kilometers (2.3 miles).
But Kepler-138c and Kepler-138d are much closer to their star than Earth. Although this star is a small, cool red dwarf, this proximity would make the two exoplanets much hotter than our world. Their orbital periods are 13 and 23 days, respectively.
This means that the oceans and atmospheres on these worlds are unlikely to be very similar to ours, the researchers say.
“Temperatures in the atmospheres of Kepler-138c and Kepler-138d are likely above the boiling point of water, and we would expect these planets to have thick, dense vapor atmospheres,” Piole says.
“Only in this vapor atmosphere could there potentially be high pressure liquid water, or even water in another phase that occurs at high pressure, called a supercritical fluid.”
Indeed, an alien.
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