A distant star found ocean planets, almost half consisting of water

(ORDO NEWS) — New observations have shown that planets can rotate around a distant red dwarf, up to half of the volume of which is water.

She covers these worlds with a deep and hot ocean, which turns into a steam-filled atmosphere.

There are several classes of planets in the solar system, including rocky, Earth-like, gas and ice giants like Saturn and Neptune.

But planets that we have no analogues are also discovering near other stars: hot Jupiters, mini-Neptunes, massive super-Earths and ocean planets completely covered with water.

It is assumed that there are worlds containing water in huge quantities, where it is densely mixed with solid rocks in the form of ice.

New observations allow us to talk about a different kind of planets, which consist of almost half the water, while covered with many kilometers of liquid ocean and surrounded by a steam-filled hot atmosphere.

The work is devoted to the Kepler-138 star system , a red dwarf with dimensions and mass about half the size of the Sun, located 220 light years away.

A few years ago, Kepler-138 was studied by transit and Doppler methods.

The first relies on the amount of star light that a potential planet obscures when passing through its background, the second traces the weak vibrations of the star caused by the attraction of massive bodies rotating around it.

The combination of these approaches allows not only to detect planets, but also to estimate their size and mass, and then calculate the density.

Based on such data, it was shown that there are three planets in the system that are very close to the star.

The inner Kepler-138b resembles Mars, while the two outer ones (Kepler-138c and Kepler-138d) are super-Earths, dense rocky exoplanets several times more massive than ours.

However, the orbits of the planets are very closely spaced and can affect each other’s movements, which can distort estimates of their masses and sizes. Therefore, scientists decided to look at the system again.

The new observations were made using the Hubble and Spitzer space telescopes, and they did not agree with the previous conclusions.

They were able to be explained by assuming that there is a fourth planet in the system, Kepler-138e, which is farthest from the rest and does not pass against the background of the star, remaining invisible.

It is quite small, but it introduces distortions in the observations of the red dwarf, and hence in the data on the basis of which the characteristics of exoplanets were determined.

A distant star found ocean planets almost half consisting of water 2
Hypothetical internal structure of Kepler-138d in comparison with the Earth

Once again evaluating their size and mass, scientists came to the conclusion that the density of Kepler-138c and Kepler-138d is intermediate between the density of rocky planets (like Earth or Mars) and rarefied gas giants (like Saturn or Jupiter).

One could assume that they are solid bodies surrounded by a thick hydrogen-helium atmosphere.

However, the planets are very close to the red dwarf, and such an atmosphere would have been blown away by the stellar wind long ago.

Therefore, the authors of the new work associated the density of Kepler-138c and Kepler-138d with the presence of large amounts of water.

Apparently, it accounts for more than ten percent by mass and about half the volume of these exoplanets.

According to the authors of the work, their closest analogue can be called Jupiter’s moon Europa, covered with an ocean hundreds of kilometers deep, which is bound by a thick ice crust.

At the same time, both exoplanets are in much warmer conditions than Europe. Even the distant Kepler-138d is eight times closer to the star than the Earth is to the Sun.

And although the brightness of the red dwarf is quite small, its radiation is enough for the water on the surface of both planets to remain liquid and even boil.

Instead of an icy shell, they are covered by a hot ocean, which gradually turns into a steam-filled atmosphere.

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