Two worlds orbiting a small star 218 light-years away appear to be of a different type than anything we have in our Solar System.
The exoplanets are called Kepler-138c and Kepler-138d. Both are around 1.5 times the radius of Earth, and both appear to be soggy worlds consisting of thick, steamy atmospheres and incredibly deep oceans, all wrapped around a rocky, metallic interior.
“We previously thought that planets that were slightly larger than Earth were big balls of metal and rock, like enlarged versions of Earth, and that’s why we call them super-Earths.” says 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 fraction of their total volume is likely to be made up of water. It is the first time we have observed that they can be confidently identified as worlds. aquatics, a type of planet that astronomers have long theorized to exist.”
A recent analysis of another world found that could be a world of water, but follow-up observations will be needed to confirm. According to the researchers, their work on kepler-138 two oceanic planets is less uncertain.
Finding out what planets outside our Solar System (or exoplanets) are made of usually requires quite a bit of detective work. They are very far away and very dim compared to the light from the stars they orbit; direct images are very difficult to obtain and therefore very rare, and do not show much detail.
The composition of a exoplanet it is usually deduced from its density, which is calculated using two measurements: one taken from the eclipse (or transit) of light from the star across the planet, and the other from the star’s radial velocity or ‘wobble’.
The amount of starlight that is blocked by the transit tells us the size of the exoplanet, for which we get a radius. The radial velocity is induced by the exoplanet’s gravitational tug, seen as a regular but very small expansion and contraction of the wavelength of light from the star as it is pulled on. The amplitude of this motion can tell us how much mass an exoplanet has.
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. Rocky worlds that are rich in metals 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.
The transit data shows that Kepler-138c and Kepler-138d have a radius of 1.51 times that of the Earth, and measurements of their respective tugs on Kepler-138 give us masses of 2.3 and 2.1 times the of the Earth, respectively. Those features, in turn, give us a density of around 3.6 grams per cubic centimeter for both worlds, somewhere between a rocky and a gaseous composition.
That’s pretty close to the Jovian ice moon. Europe, which has a density of 3.0 grams per cubic centimeter. It happens that it is covered by a liquid global ocean under an icy layer.
“Imagine larger versions of Europa or Enceladus, the water-rich moons that orbit Jupiter and Saturn but come much closer to their star.” says astrophysicist Caroline Piaulet from the University of Montreal, who led the research. “Instead of an icy surface, Kepler-138c and d would harbor large envelopes of water vapor.”
According to the team’s model, water would make up more than 50 percent of the exoplanets’ volume, extending to a depth of about 2,000 kilometers (1,243 miles). The Earth’s oceans, for context, have an average depth of 3.7 miles (2.3 miles).
But Kepler-138c and Kepler-138d are much closer to their star than Earth is. Although that star is a small, cool red dwarf, that close proximity would make the two exoplanets much, much hotter than our world. They have orbital periods of 13 and 23 daysrespectively.
This means that the oceans and atmospheres of these worlds are unlikely to look much like our ocean, the researchers say.
“The temperature in the atmospheres of Kepler-138c and Kepler-138d are probably above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets.” piaulet says.
“Only under that steamy atmosphere could there be liquid water at high pressure, or even water in another phase that occurs at high pressures, called supercritical fluid.”
Alien, in fact.
The research has been published in nature astronomy.
