An international scientific consortium led by Spanish astronomers has discovered an unprecedented exoplanet around a small red dwarf star located 31 light years from the solar system. What is unique about this planet is that it has much more mass than theoretical models predict.
Although the star is only one-tenth the mass of the Sun, it has a giant planet: “Around these stars there should only be planets the size of Earth or super-Earths that are somewhat more massive,” says Christoph Mordasini, professor at the University. Bern and member of the National Center for Research Competence PlanetS. “GJ 3512b, however, is a giant planet with a mass about half that of Jupiter and therefore at least an order of magnitude more massive than the planets predicted by theoretical models for stars so small. ” clarifies the expert in the journal Science that publishes the study.
The mysterious planet was detected by a Spanish-German research consortium called CARMENES , which has set itself the goal of discovering planets around the smallest stars in the cosmos. For this purpose, the consortium built a new instrument, which was installed at the Calar Alto Observatory in Almería, at an altitude of 2,100 m (it is not for nothing that it is the largest astronomical observatory in Europe), co-led by the Andalusian Institute of Astrophysics. .
Observations with this infrared spectrograph showed that the small star was regularly moving to and from us, a phenomenon triggered by a companion that had to be particularly massive in this case. Because this discovery was so unexpected, the scientists contacted, among others, the Berne de Mordasini research group, one of the world’s leading experts on the theory of planet formation, to discuss plausible formation scenarios for the exoplanet. giant.
The explanation?
“Our model of planet formation and evolution predicts that around small stars a large number of small planets will form,” summarizes Mordasini, referring to the famous planetary system known as Trappist-1 . This star comparable to GJ 3512 has seven planets with masses roughly equal to or even less than the mass of Earth. In this case, the Berne model calculations agree well with the observation. But this is not the case with GJ 3512.
“Our model predicts that there should be no giant planets around such stars,” Mordasini argues. One possible explanation for the failure of the current theory could be the mechanism underlying the model, known as central accretion. Planets are formed by the gradual growth of small bodies into larger and larger masses, a circumstance that experts call a “bottom-up process.”
Perhaps the giant planet GJ 3512b was formed by a fundamentally different mechanism , a so-called gravitational collapse. “A part of the gas disk in which the planets are formed collapses directly under its own gravitational force,” Mordasini explains: “A top-down process.” But even this explanation raises problems . “Why hasn’t the planet continued to grow and migrate closer to the star in this case? One would expect both if the gas disk had enough mass to become unstable under its gravity,” says the expert.
“The planet GJ 3512b is therefore an important discovery that should improve our understanding of how planets form around such stars,” Mordasini concludes.
The GJ 3512 type of solar system is likely rare: The team’s analyzes suggest that only 3% of stars with masses less than three-tenths of our Sun will host gas giant planets. In most cases, the star’s retinue of companions will be smaller, rockier orbs.
“For the first time, we have precisely characterized an exoplanet that cannot be explained by the central accretion formation model”, comments Juan Carlos Morales, from the Institute of Space Sciences (ICE-CSIC). “This exoplanet shows that the gravitational instability model can play a key role in the formation of giant planets.”
Researchers continue to monitor this system to learn more about a potential second world – possibly a planet similar to Neptune, with a longer orbital period – and perhaps even more planets, Morales says. In addition, they are examining another 300 red dwarfs to search for more exoplanets.
Will it finally be a system equivalent to our solar system but on a small scale?
Referencia: J. C. Morales et al. A giant exoplanet orbiting a very-low-mass star challenges planet formation models. Science, 2019 DOI: 10.1126/science.aax3198
