Although formally there is only one planetary group called the “solar system”, science has already found more than 4,100 stars with planets orbiting around it. An international collaboration with significant Spanish participation has discovered a planetary system that adds to the list, but this one is unique in its kind. This is the star HD110067, which is home to six exoplanets that rotate around it in a synchronized dance, in a phenomenon known as orbital resonance. It is not common for systems to retain this characteristic, which indicates that the system has not undergone major changes throughout its billion-year history. Researchers consider it key to better understanding and explaining planetary formation processes. The discovery It was published today in the magazine Nature.
About 100 light years away, in the northern constellation of Coma Berenices, is HD110067, a star 20% smaller and cooler than the Sun. The first suspicions that this star hosted a planetary system came in 2020. The hunter of NASA’s TESS planets, recorded a decrease in brightness (similar to an eclipse), which indicated the existence of at least two planets passing in front of the star. Two years later, TESS observed the same star again, but presented data inconsistent with the first interpretation. This contradiction aroused the interest of Spanish astrophysicist Rafael Luque, from the astrophysics department at the University of Chicago, and his colleagues. “That’s when we decided to use CHEOPS. “We went to look for signs among all the potential periods that those planets could have,” says Luque.
With the help of ESA’s CHEOPS space telescope, they identified a third exoplanet. They realized they had found the key to unlocking the entire system because it was now clear that those three planets were in orbital resonance. “CHEOPS provided us with a configuration that allowed us to predict all the others,” says the astrophysicist. By combining data from both telescopes into A work they call “detective”, and with mathematical models of gravitational interactions, the existence of three outer planets was predicted.
Later observations confirmed that they were precisely where this rhythm, the resonance chain, predicted. The outermost planet takes 20,519 days to orbit, almost 1.5 times the orbital period of the next planet, which takes 13,673 days. This in turn is almost exactly 1.5 times the orbital period of the inner planet, at 9,114 days. That is, when the planet closest to the star makes three complete revolutions around it, the second makes exactly two during the same time. This is called 3:2 resonance. The six planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3 and 4:3, ESA explains, resulting in the closest planet completing six orbits in time in which the outermost planet performs a. The authors do not rule out the existence of more planets dancing coordinated in this system.
The exoplanets hosted by HD110067 belong to the group of so-called sub-Neptunes, that is, planets smaller than Neptune (four times the diameter of the Earth). When it comes to imagining what they are like, Ignasi Ribas, an astrophysicist at the CSIC, who also participated in the study and who has compiled years of work searching for exoplanets, clarifies that they are not similar to Earth: “They are very hot planets that can reach a temperature of 200 degrees.” The habitable zone of the system has not yet been deciphered either, that is, how many planets are in the temperate zone that would allow life. Future observations could also determine whether the planets have rocky or water-rich interior structures.
HD110067 is also the brightest known system with four or more planets. Some worlds that probably have atmospheres with a high presence of hydrogen and that are good candidates for carrying out new analyzes to determine the chemical composition and other properties of their atmospheres. Since it is a special system that invites you to study it further, what is needed for this? An approach with adequate equipment for observation and a good candidate for this is the space telescope James Webb, the most powerful and that has barely been in operation for a year. Rafael Luque, main author of the study, reveals that his working group aims to work with this instrument in the future.
A key discovery for the future
Planetary systems tend to form in resonance, but this is a rhythm that can be easily disturbed. “Clashes between planets, mergers or breakups, the birth of giant planets like Jupiter or the close passage of another star can alter the orbital balance,” explains Ribas. Among the thousands of multiplanetary systems, 99% are not in resonance, but could have been at one time. The HD110067 system is part of that scarce 1%, with a special value for science, as it can provide information to astronomers about the formation and subsequent evolution of the planetary system. Both Ribas and Luque compare the discovery to that of a fossil. “It shows us the configuration of a planetary system that has survived intact since its formation,” says Luque.
Astrophysicist Eva Villaver applauds the new study, in which she did not participate. “Each system that is discovered with unique characteristics contributes data to our understanding of training processes. Science studies these exoplanets in case they give us clues about what makes our solar system unique and why, although sub-Neptune planets are so frequent, we do not have them,” explains the researcher, who works as director of the Office Space and Society of the Spanish Space Agency. Thus, by studying exoplanets and distant solar systems, astronomers hope to solve the great mysteries that still surround the solar system and provide more information to the never-ending question of whether there is life elsewhere in our galaxy.
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