Two worlds orbiting a nearby star could be more than half water

It appears that two worlds orbit a small star 218 light-years away of a type unlike anything we have in our solar system.

The outer planets are named Kepler-138c and Kepler-138d. Both are about 1.5 times the radius of Earth, and both appear to be moist worlds made up of thick, steamy atmospheres and insanely deep oceans, all wrapped around their rocky, metallic interior.

“We previously thought that planets that were a little larger than Earth were big balls of metal and rock, like spiraling versions of Earth, which is why we called them superplanets,” says astronomer Björn Benneke of the University of Montreal.

However, we have now shown that these two planets, Kepler-138c and d, are very different in nature: it is likely that much of their entire volume is made up of water. It is the first time we have observed planets that can be confidently identified as water worlds, a type of the planets astronomers assumed the existence of it for a long time.

A recent analysis of another world found that it could be an aquatic world, but follow-up observations would be needed to confirm. According to the researchers, their work on the two surrounding Kepler-138 planets is uncertain.

The identification of planets outside our solar system (or exoplanets) usually requires a great deal of investigative work. It is so far away, so dim, compared to the light of the stars it orbits; Live images are very hard to come by, and therefore very rare, and don’t 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 “wobble”.

The amount of starlight the transit blocks tells us the size of an exoplanet, from which we get the radius. Radial velocity is stimulated by the gravitational pull of an exoplanet, and is seen as a uniform but very small expansion and contraction in the wavelength of the star’s light as it is pulled back. The amplitude of this motion can tell us the mass of an exoplanet.

Once you have the volume and mass of an object, you can calculate its density.

A gaseous world, such as Jupiter or even Neptune, would have a relatively low density. Rocky, mineral-rich worlds would have a higher density. at 5.5 grams per cubic centimeter, Earth is the densest planet in our solar system; Saturn is less dense, at 0.69 grams per cubic centimeter.

Sectional diagram comparing Kepler-138d to Earth. (Benoit Goujon, University of Montreal)

Transit data shows that Kepler-138c and Kepler-138d have radii 1.51 times that of Earth, and their tug measurements on Kepler-138 give us masses that are 2.3 and 2.1 times that of Earth, respectively. These properties, in turn, give us a density of about 3.6 grams per cubic centimeter for both worlds – somewhere between rocky and gaseous composition.

This is very close to the Jovian icy moon Europa, which has a density of 3.0 grams per cubic centimeter. It happens to be covered in a liquid global ocean under an ice crust.

“Imagine larger versions of Europa or Enceladus, the water-rich moons that orbit Jupiter and Saturn but come very close 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 will harbor large envelopes of water vapor.”

According to the team’s modeling, water will make up more than 50 percent of the exoplanet’s volume, extending to a depth of about 2,000 kilometers (1,243 miles). Earth’s oceans, for example, 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 and cool red dwarf, such close proximity would make the two outer planets much hotter than our own. It has tropical periods of 13 and 23 days, respectively.

The researchers say this means that the oceans and atmospheres on these worlds are unlikely to look like ours.

“It is likely that the temperature in the atmospheres of Kepler-138c and Kepler-138d is above the boiling point of water, and we would expect a thick, dense atmosphere made of steam on these planets,” Piaolet says.

“Only below this vaporous atmosphere can there be liquid water under high pressure, or even water in another phase occurring at high pressures, called a supercritical fluid.”

Really alien.

Research published in natural astronomy.

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