Sidereal Messenger: Brazilian helps explain “valley” between super-earths and mini-Neptunes

When carrying out the first comprehensive census of exoplanets, after more than 5,000 stars were discovered, astronomers were faced with a conundrum: although there are many worlds 1 to 4 times the size of Earth, they fall into two blocks, with a marked absence. of those that would have a diameter of something around 1.8 of the terrestrial one. This “valley” in the exoplanet distribution chart lacks explanation, and now a group led by a Brazilian researcher seems to have found a pretty convincing one.

Once it was found that the highest concentrations of exoplanets in this size range are around 1.4 Earth radii (the so-called super-Earths) and 2.4 Earth radii (the mini-Neptunes), the first explanatory hypothesis was that they were basically all similar in origin, born from rocky cores and varying only in their ability to preserve a primordial atmosphere rich in hydrogen and helium. Those that were initially large enough to retain this atmosphere ended up larger, like mini-Neptunes, and those that lost it, swept away by the radiation of their stars, joined the super-Earth group.

Now, André Izidoro’s team, from Rice University, in the USA, proposes an alternative. In an article published in the Astrophysical Journal Letters, the group suggests that the “valley” may not have to do with this photoevaporation of the atmosphere, but would arise from the natural process of formation of the systems. By running simulations of the process, the researchers suggest that as planets emerge from the disk of gas and dust, their interaction with the disk causes them to migrate inward and end up in orbits resonant with each other (where the period of one planet is a multiple of another). After the disk dissipates, the planets come out of this exact lock and the system goes through a phase of instability, with giant collisions (like the one that occurred here between Earth and a body the size of Mars to form the Moon).

The collisions would dissipate the primordial atmosphere of these worlds, and then what would separate the super-Earths from the mini-Neptunes would be the amount of water present in them: the driest would end up with a diameter around 1.4 of the terrestrial, and the most watery (more than 10% of the mass in water) would have a typical diameter of 2.4 times that of Earth.

The advantage of this explanation for the “valley” is that it emerges naturally from the formation process, which in turn accounts for several other features revealed by exoplanet censuses, such as the orbital patterns and approximate masses of the planets. However, it is important not to underestimate nature’s creative capacity; it is quite possible that the photoevaporation mechanism also plays a role in some systems. And the test of the nines, case by case, will be observation. With the deepening of the study of exoplanets and the identification of their composition, it will be easier to discriminate between one hypothesis and another.

This column is published on Mondays, in Folha Corrida.

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