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

Two worlds orbiting a small star 218 light-years away appear to be of a different kind than anything we have in our 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 soggy worlds made up of thick, humid atmospheres and impossibly deep oceans, all wrapped around a rocky, metallic interior.

“We used to think that planets a little bigger than Earth were big balls of metal and rock, like enlarged versions of Earth, and that’s why we called them super-Earths,” says the astronomer. Björn Benneke from the University of Montreal.

“However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature: a large part of their total volume is probably made up of water. This is the first time we have observed planets that can be identified with certainty as water worlds, a type of planet that has been theorized by astronomers to have existed for a long time.”

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

Figuring out what the planets outside our solar system (or exoplanets) are made of usually requires a bit of detective work. They are very distant and very faint compared to the light of the stars around which they orbit; direct images are very difficult to obtain and therefore very rare, 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 eclipse (or transit) of starlight through the planet and the other from the radial velocity or “wobble” of the star.

The amount of starlight blocked by the transit tells us the size of the exoplanet, from which we get a ray. The radial velocity is induced by the exoplanet’s gravitational tug, thought to be a steady but very small wavelength expansion and contraction of light from the star as it is pulled. The amplitude of this motion can tell us the mass of an exoplanet.

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

A gaseous world, like Jupiter or even Neptune, will have a relatively low density. Metal-rich rocky worlds will have higher densities. At 5.5 grams per cubic centimeter, Earth is the densest planet in our solar system; Saturn is the least dense, at 0.69 grams per cubic centimeter.

A cross-sectional diagram comparing Kepler-138d to Earth. (Benoit Gougeon, University of Montreal)

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

This is quite close to the Jovian ice moon Europa, which has a density of 3.0 grams per cubic centimeter. It happens to be covered in a liquid world ocean under a shell of ice.

“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but much closer to their star,” says astrophysicist Caroline Piaulet of the University of Montreal, who conducted the research. “Instead of an icy surface, Kepler-138c and d would harbor large envelopes of water vapor.”

According to the team’s modeling, water would make up more than 50% of the volume of exoplanets, extending 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, much hotter than our world. They have orbital periods of 13 and 23 days, respectively.

This means that the oceans and atmospheres of these worlds are unlikely to look much like our own ocean, the researchers say.

“The temperature in the atmospheres of Kepler-138c and Kepler-138d is likely above the boiling point of water, and we expect a thick, dense atmosphere made of vapor on these planets,” Piaulet said.

“Only beneath this vaporous atmosphere could there potentially be liquid water at high pressure, or even water in another phase that occurs at high pressure, called supercritical fluid.”

Alien, indeed.

The research has been published in natural astronomy.

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