The abundant population of water exoplanets

The abundant population of water exoplanets


A study points to the existence of an abundant population of exoplanets of water and rock around M-type dwarf stars, the most common in our galaxy.

Water is basic to life. At least as we know it. For this reason, astronomers strive to find planets in which this element is present, better in liquid form. A study, led by researchers Rafael Luque, from the University of Chicago and the Institute of Astrophysics of Andalusia (IAA-CSIC), and Enric Pallé, from the IAC and the University of La Laguna (ULL), points out that these aquatic worlds could not be as rare as we think. In fact, they could be legion. The conclusions of their work have just been published in the journal Science.

This is not a new theory, but the first experimental proof that these planets are common “and, in fact, almost as abundant as the terrestrial planets”explains Luque in a statement. To prove this, the team took a detailed look at the radius and mass of 43 known small exoplanets around M-dwarf stars, which represent 80% of the stars in the Milky Way.

When the researchers analyzed the sample they found something unexpected: the densities of a large percentage of the planets suggested that they were too light relative to their size to be made up of rock alone. Therefore, the researchers believe that these planets should be made up of half rock and half water or another lighter molecule. “We found that it is the density of the planet and not the radius, as previously thought, that separates the dry planets from the wet ones”clarifies Luke.

water exoplanets

However, these planets are so close to their suns that any surface water would exist in a supercritical gas phase, enlarging their size. So scientists think that, in this type of population, the water would likely be embedded in rock or pockets below the surface, rather than flowing like oceans or rivers.

These conditions would be similar to those on Jupiter’s moon Europa, but very different from what occurs on our planet. “Earth is a dry planet even though almost all the water is on its surface, which gives it a very wet appearance. The water of the Earth is only 0.02% of its total mass, while in the aquatic worlds it is 50% of the mass of the planet»Palle points out.

With this finding, the existence of a new type of exoplanet is confirmed for the first time. “We 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 (water worlds or water worlds in English) and mini -Neptunes with extended atmospheres of hydrogen and/or helium»describes Pallé.

It contradicts the general idea

The finding contradicts the widely held idea that these worlds are either dry and rocky or have extensive, thin atmospheres of hydrogen and/or helium.

Instead, the study suggests that, unlike rocky planets, these water-rich worlds formed outside the so-called “snow line,” that is, at a distance where the temperature was low enough for lighter compounds such as water will solidify and form solid ice grains, subsequently migrating inward. «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»says Palle.

In the same way that looking at the population of an entire city can reveal trends that are difficult to see at the individual level, studying a population of planets has helped scientists identify hitherto unknown patterns. “Due to errors in mass and radius of our measurements, an individual planet can sometimes fit into different categories (terrestrial, water worlds, etc.). It is when we look at a population of planets, as we do here, that patterns of distinct composition can be resolved.”Luke explains.

aquatic worlds

According to the researchers, the next steps to be taken are to understand the internal structure of the water worlds, that is, where the water is stored, and whether these planets can host a small detectable supercritical water vapor atmosphere. “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.”Palle points out.

“It is also critical to understand whether our finding applies to populations of small planets around other types of stars.”highlights Luke. “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”underlines.

The new discoveries of planets around M dwarf stars made by the mission have been essential for this work. Transiting Exoplanet Survey Satellite (TESS) of NASA, as well as the mass determinations carried out by the CARMENES spectrograph, installed in the Calar Alto 3.5 m telescope, located in Almería (Spain).

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