In a harmonious rhythm: deciphering the "unsolvable puzzle" of a system with six planets

An international team of astronomers discovered a system of six planets orbiting the star HD110067 in a unique harmonic resonance

Astronomers using the space satellites CHEOPS and TESS, and academic groups have discovered an important new system of six transiting planets orbiting a bright star in harmonic rhythm. This rare feature allowed the team to determine the planetary orbits that initially seemed like an unsolvable puzzle.

CHEOPS is a joint mission of the European Space Agency and Switzerland, and thanks to collaboration with scientists who worked with data from NASA's TESS satellite, the international team was able to discover the system of planets orbiting The nearby star HD110067.

A very special feature of this system is its chain of resonances: the planets surround their host star in perfect harmony. The findings were recently published in the journal Nature.
The planets in HD110067's system revolve around the star in a very precise dance. When the planet closest to the star completes three revolutions around it, the other completes exactly two at the same time. This is called a 3:2 resonance.

"Of the more than 5,000 exoplanets that have been discovered orbiting stars other than the Sun, resonances are not rare, nor are systems with several planets. But what is very rare is to find systems where the resonances span such a long chain of six planets," notes Dr. Hugh Osborne, CHEOPS researcher at the University of Bern, director of the CHEOPS observation program involved in the study, and co-author of the publication.

This is the case with HD110067 whose planets form what is known as a "chain of resonances" in successive pairs of resonances 3:2, 3:2, 3:2, 4:3 and 4:3, resulting in the closest planet completing six revolutions while the outermost planet One cup is complete.

Although multiple planets were first discovered thanks to their transits, the exact arrangement of the planets was not clear at first. But the precise gravitational dance allowed the team of scientists to solve the puzzle of HD110067. Prof. Adrien Lello from the University of Geneva, responsible for the analysis of the orbital resonances, and a co-author of the study, explains: "A transit occurs when a planet, from our point of view, passes in front of its host star, hiding a tiny part of the star's light,
and creates an apparent decrease in its brightness."

From the first observations made by the TESS satellite, it was possible to determine that the two inner planets
Called b and c have coffee cycles of 9 and 14 days, respectively. But it was impossible to conclude
Conclusions about the other four planets identified that two were seen transiting once in 2020 and once in
2022 with a big gap of two years in the data, and the other two only happened once in 2022.
The solution to the puzzle of these four extra planets has finally begun to emerge thanks to observations
Using the CHEOPS space telescope of the European Space Agency. TESS is designed to scan the entire sky bit by bit to find short-period exoplanets, while CHEOPS is a focused mission, focusing on one star at a time with great precision. "Our CHEOPS observations allowed us to find that planet d's cycle is 20.5 days. They also ruled out many possibilities for the remaining three outer planets, e, f and g," Osborne reveals.

The team members therefore concluded that HD110067's three inner planets dance in a precise 3:2, 3:2 resonance chain: with the innermost planet orbiting the star nine times, the second orbiting six times and the third four times.

The team then considered the possibility that the other three planets were also part of the chain of resonances. "This led to dozens of possibilities for their orbital period," explains Lelo, "but combining existing observational data from TESS and CHEOPS with our model of the gravitational interactions between the planets allowed us to rule out all but one solution: a 3:2 chain, 3:2, 3:2, 4:3, 4:3". The scientists could therefore predict that the three outer planets (e, f and g) have The coffee cycles of
31, 41 and 55 days.

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