Although there is formally only one group of planets called the “solar system,” science has already found more than 4,100 stars that have planets orbiting them. An international collaboration with significant Spanish participation has discovered a planetary system that adds to the list, but this one is unique of its kind. This is the star HD110067, which hosts six exoplanets that rotate around it in a synchronized dance, a phenomenon known as orbital resonance. It is not common for systems to maintain this property, suggesting that the system has not undergone major changes over its billion-year history. Researchers believe it is a key to better understanding and explaining the processes of planet formation. The finding was published today in the journal Nature.
About 100 light-years away, in the northern constellation Coma Berenices, is HD110067, a star 20% smaller and cooler than the Sun. The first suspicion that this star hosts a planetary system came in 2020. NASA’s TESS planet hunter, recorded a decrease in brightness (similar to a solar eclipse), indicating 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 that did not agree with the first interpretation. This contradiction piqued the interest of Spanish astrophysicist Rafael Luque of the Department of Astrophysics at the University of Chicago and his colleagues. “That’s when we decided on CHEOPS. “We looked for signs of all the possible periods that these planets could have,” says Luque.
With the help of ESA’s CHEOPS space telescope, they identified a third exoplanet. They realized that they had found the key to unlocking the entire system because it was now clear that these three planets were in orbital resonance. “CHEOPS gave us a configuration that allowed us to predict all others,” says the astrophysicist. By combining the data from both telescopes A work they call “Detective,” which used mathematical models of gravitational interactions to predict the existence of three outer planets.
Later observations confirmed that they were exactly where this rhythm, the resonance chain, had predicted. The outermost planet’s orbital period is 20,519 days, almost 1.5 times the orbital period of the nearest planet, which takes 13,673 days. At 9,114 days, this corresponds to almost exactly 1.5 times the orbital period of the inner planet. That is, if the planet closest to the star makes three complete revolutions around it, the second closest planet makes exactly two revolutions in the same time. This is called a 3:2 resonance. The six planets form a resonance chain in pairs of 3:2, 3:2, 3:2, 4:3 and 4:3, explains ESA, resulting in the nearest planet completing six orbits in the time in which the outermost planet performs one. The authors do not rule out the existence of other coordinately dancing planets in this system.
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The exoplanets hosted by HD110067 belong to the group of so-called sub-Neptunes, i.e. planets that are smaller than Neptune (four times the diameter of the Earth). When it comes to imagining what they look like, Ignasi Ribas, an astrophysicist at CSIC who was also involved in the study and has accumulated years of work searching for exoplanets, makes it clear that they are nothing like 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, i.e. how many planets there are in the temperate zone that would support life. Future observations could also reveal whether the planets have rocky or water-rich interior structures.
HD110067 is also the brightest known system with four or more planets. Some worlds are likely to have atmospheres with high levels of hydrogen and are good candidates for conducting new analyzes to determine the chemical composition and other properties of their atmospheres. Since this is a special system that invites further study, what is needed for this? One approach with sufficient observation equipment and a good candidate for this is the James Webb Space Telescope, the most powerful and only in operation for a year. Rafael Luque, lead author of the study, reveals that his working group would like to work with this instrument in the future.
A key discovery for the future
Planetary systems tend to form in resonance, but this rhythm can easily be disrupted. “Collisions between planets, mergers or break-ups, the birth of giant planets like Jupiter or the close flyby of another star can change the orbital balance,” explains Ribas. Of the thousands of multiplanetary systems, 99% are not in resonance, but may once have been. The HD110067 system is part of this scarce 1% and is of particular value to science because it can provide astronomers with information 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 remained intact since its formation,” says Luque.
Astrophysicist Eva Villaver praises the new study, in which she did not take part. “Each system discovered with unique properties contributes data to our understanding of training processes. “Science is studying these exoplanets in case they give us clues about what makes our solar system unique and why we don’t have them, even though sub-Neptune planets are so common,” explains the researcher, who serves as director of the Office Space and Society works for the Spanish Space Agency. Therefore, by studying exoplanets and distant solar systems, astronomers hope to solve the great mysteries that still surround the solar system and provide more information on the never-ending question of whether there is life elsewhere in our galaxy.
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