Worlds richer in water than Earth may be more common than believed

Water is an essential ingredient for life on Earth, and the water cycle helps keep our planet’s climate stable and benign. Thus, in the search for life in our galaxy, planets with liquid water on the surface are among the ideal candidates. A new study suggests that many of the planets known as super-Earths or mini-Neptunes can host large amounts of water, with compositions of up to 50% rock and 50% water. (By comparison, Earth is only 0.02% water.) But the water on those worlds is likely to lie beneath the crust, rather than flowing across the surface in the form of oceans or rivers.

Thanks to advances in observational instruments, the discovery of planets in other solar systems is increasing by leaps and bounds. And a larger number of well-characterized planets makes it possible to identify demographic patterns, just as looking at the population of an entire city can reveal trends that are difficult to detect at the individual level.

The recently published study analyzes all the planets detected in red dwarf stars (of spectral class M), a type of star less massive than the Sun and the most abundant in our galaxy, the Milky Way. “It was a surprise to discover evidence of so many aquatic worlds orbiting the most common type of star in the galaxy,” says Rafael Luque, co-author of the study and researcher at the Institute of Astrophysics of Andalusia (IAA) in Spain and the University of Chicago in USA. “It has huge consequences for the search for habitable planets.”

The discoveries of planets around M dwarfs are numerous, but they are indirect discoveries, made thanks to the study of the effects of the planets on their stars: either by analyzing the decrease in brightness that occurs when the planet passes in front of its star, or by studying the small gravitational pull that the planet exerts on it as it revolves around it.

“Each of the two different ways of discovering planets gives you complementary information. By capturing the decrease in brightness produced when a planet crosses in front of its star, we can determine the planet’s diameter, and by measuring the tiny gravitational attraction that a planet exerts on a star, we can calculate its mass”, points out Enric Pallé, a researcher at the Institute of Astrophysics of the Canary Islands and the University of La Laguna, in Spain, and co-author of the work.

By combining diameter and mass, the composition of the planet can be measured, and whether it is, for example, a gas giant planet like Jupiter or a small, dense, rocky planet like Earth. Studying a population of forty-three planets, a striking picture emerged: the low density of a large percentage of the planets suggests that these planets are probably half rock and half water.

Although the first idea that can arise when contemplating these proportions points to large oceans, these planets are so close to their suns that if there were water on the surface it would be in a supercritical gas phase, which would increase its radius. “But that is not what we see in the samples, which suggests that the water is not in the form of a surface ocean,” explains Rafael Luque, who carried out a large part of the study during his thesis at the Institute of Astrophysics of the Canary Islands.

Artistic recreation of an aquatic planet. (Image: Pilar Montañes)

The finding contradicts the widely held idea that these worlds are either dry and rocky or have extensive, thin atmospheres of hydrogen, helium, or both. Rather, these worlds fall neatly into two families: rocky or watery. This scenario reinforces one of the most widely accepted theories of planet formation, which suggests that rocky worlds form in the inner parts of their solar systems, while water worlds form in the outermost regions and then migrate inward with the sun. weather.

Although the evidence is convincing, the next step is to obtain irrefutable proof that these planets are water worlds, which is expected to be achieved with the James Webb Space Telescope (JWST), recently launched into space by NASA and successor to the Space Telescope Hubble.

The new study is titled “Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars.” And it has been published in the academic journal Science. (Source: Silbia López de Lacalle / IAA / CSIC)