EFE.- Water is abundantly present on many small extrasolar planets but it does not flow in oceans and rivers as on Earth, but is probably embedded in rock or in pockets under the surface, according to a study conducted by the University of Chicago and the Spanish Institute of Astrophysics of the Canary Islands (IAC).

A study published this Thursday in the journal Science and which reveals that there is an abundant population of aquatic and rocky exoplanets around the M-type dwarf stars, which are the most frequent in the Milky Way.

For this study, led by researchers Rafael Luque, from the University of Chicago and the Institute of Astrophysics of Andalusia (IAA-CSIC), in southern Spain, and Enric Pallé, from the IAC and the University of La Laguna (ULL), in the Spanish Atlantic Archipelago, The radius and mass of 43 known small exoplanets around M dwarf stars, representing 80% of the stars in the Milky Way, have been analyzed in detail.

Rafael Luque explains in a press release that They found the first experimental evidence that water worlds exist as a population and are, in fact, almost as abundant as terrestrial planets.

According to the study, many more planets than previously thought could have large amounts of water, containing up to 50% of the total mass of the planet.

Enric Pallé told Efe that half of these planets are water, so the amount is huge and if it were in the form of an ocean it would be thousands of kilometers deep, but the data seems to indicate that the outermost layer is melted and the water is below and down to the core.

He added that at some point there will be liquid water and it is not known if there is life, but it is known that a large majority of planets are equal to Earth in composition, with which the probabilities of finding life greatly increase for this reason.

When the researchers analyzed the sample, they found something unexpected, as the densities of a large percentage of the planets suggested they were too light relative to their size to be made up of just rock.

Therefore, they believe that these planets should be composed of half rock and half water or some other lighter molecule, and They discovered that it is planet density and not radius, as previously thought, that separates dry planets from wet ones.commented Rafael Luque.

However, these planets are so close to their suns that any water on the surface would exist in a supercritical gas phase, increasing their size.

Therefore, scientists believe that in this type of population, water would likely be embedded in rock or in pockets below the surface, instead of flowing like oceans or rivers.

Read more: Astronomers discover exoplanets that could harbor human life

Water is abundantly present in the populations of exoplanets

These conditions would be similar to those of Europa, the moon of Jupiter, but very different from what happens on Earth, that it is a “dry” planet despite the fact that almost all the water is on its surface, which makes it look very wet. Earth’s water is only 0.02% of its total mass, whereas in the watery worlds it is 50% of the planet’s mass.”underlined Enric Pallé.

He adds that they have discovered that the small planets around these stars can be described by a discrete population of families: planets very similar to Earth, planets with 50% of their mass made up of water (aquatic worlds) and mini-Neptunes with extended hydrogen and/or helium atmospheres.

This discovery contradicts the widely held idea that these worlds are either dry and rocky or have a vast, thin atmosphere of hydrogen and/or helium.

And this suggests, on the contrary, that, unlike the rocky planets, These water-rich worlds formed outside the so-called “snow line”, i.e. at a distance where the temperature was low enough for lighter compounds such as water to solidify and form solid ice grains, then migrating inwards.

The distribution of sizes and densities of exoplanets is a direct consequence of the formation of planets at different distances from the star and not of the presence or absence of an atmosphere, commented Pallé.

The researchers explain that, in the same way that observing the population of an entire city can reveal trends that are difficult to see individually, the study of a population of planets has made it possible to identify previously unknown models.

“Because of the errors in mass and radius of our measurements, a planet, individually, can sometimes fit into different categories (terrestrial, “aquatic worlds”…) It is when we observe a population of planets, as we are doing here, when we can solve different compositional patterns,” Luque said.

According to the researchers, the next steps to take They will understand the internal structure of water worlds, i.e. where water is stored, and whether these planets can harbor a small detectable supercritical water vapor atmosphere.

Enric Pallé said that only planets around M stars in the habitable zone are accessible for atmospheric exploration by the James Webb Space Telescope (JWST) and future extremely large ground-based telescopes.

“It is also essential to understand whether our finding applies to populations of small planets around other types of stars,” said Luque, who pointed out that the precise masses of small planets around larger stars are more difficult to obtain, but could soon be provided by state-of-the-art ultrastable spectrographs.

For this job New discoveries of planets around M dwarf stars made by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, together with mass determinations made by the CARMENES spectrograph, installed at the 3.5m Calar Alto Telescope, have been essential in Almeria. (southern Spain).

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