CHEOPS finds unique planetary system

The CHEOPS space telescope has detected six planets orbiting the star TOI-178. Five of the planets are in a harmonic rhythm despite very different compositions – a novelty. CHEOPS is a joint mission by the European Space Agency (ESA) and Switzerland, under the aegis of the University of Bern in collaboration with the University of Geneva.

Musical notes that sound pleasant together can form a harmony. These notes are usually in a special relationship with each other: when expressed as frequencies, their ratios result in simple fractions,
such as four-thirds or three-halves. Similarly, a planetary system can also form a kind of harmony when planets, whose orbital period ratios form simple fractions, regularly attract each other with their gravity. When one planet takes three days to orbit its star and its neighbor takes two days, for example. Using the CHEOPS space telescope, scientists, led by astrophysicist Adrien Leleu of the Center for Space and Habitability of the University of Bern, the University of Geneva and the National Center of Competence in Research PlanetS, found such relationships between five of six planets orbiting the star TOI-178, located over 200 light years away from Earth. The results were published in the journal Astronomy and Astrophysics.

A missing piece in an unexpected puzzle

“This result surprised us, as previous observations with the Transiting Exoplanet Survey Satellite (TESS) of NASA pointed toward a three planets system, with two planets orbiting very close together. We therefore observed the system with additional instruments, such as the ground-based ESPRESSO spectrograph at the European Southern Observatory (ESO)’s Paranal Observatory in Chile, but the results were inconclusive”, Leleu remembers. When he and his colleagues first proposed to investigate the system more closely, they were therefore not sure what they would find. The high precision and target-pointing agility of CHEOPS was required to bring clarity, but that turned out to be more difficult than expected. “After analyzing the data from eleven days of observing the system with CHEOPS, it seemed that there were more planets than we had initially thought”, Leleu says. The team identified a possible solution with five planets and decided to invest another day of precious observation time on the system to confirm. They found that there were indeed five planets present with orbital periods of around 2, 3, 6, 10 and 20 days respectively.

While a system with five planets would have been quite a remarkable finding in itself, Leleu and his colleagues noticed that there might be more to the story: the system appeared to be in harmony. “Our theory implied that there could be an additional planet in this harmony; however its orbital period needed to be very nearly 15 days”, Leleu explains. To check if their theory was in fact true, the team scheduled yet another observation with CHEOPS, at the exact time that this missing planet would pass by – if it existed. But then, an accident threatened to cancel their plans.

Prediction confirmed despite near-collision

“Just before the time of the observation, a piece of space debris threatened to collide with the CHEOPS satellite”, as co-author and Professor of Astrophysics at the University of Bern, Yann Alibert,
recalls. Therefore, the control center of the European Space Agency (ESA) initiated an evasive maneuver of the satellite and all observations were interrupted. “But to our great relief, this maneuver was done very efficiently and the satellite could resume observations just in time to capture the mysterious planet passing by”, as Nathan Hara, co-author and astrophysicist from the University of Geneva reports. “A few days later, the data clearly indicated the presence of the additional planet and thus confirmed that there were indeed six planets in the TOI-178 system”, Hara explains.

A system that challenges current understanding

Thanks to the precision of CHEOPS’ measurements as well as previous data from the TESS mission, the ESO’s spectrograph ESPRESSO, and others, the scientists could not only measure the periods
and sizes of the planets of 1.1 to 3 times the radius of the Earth, but also estimate their densities. With that came another surprise: compared to the harmonic, orderly way the planets orbit around their star, their densities appear to be a wild mixture.

“It is the first time we observe something like this”, as ESA Project Scientist Kate Isaak points out and adds that “in the few systems we know with such a harmony, the density of planets steadily decreases as we move away from the star. In the TOI-178 system, a dense, terrestrial planet like Earth appears to be right next to a very fluffy planet with half the density of Neptune followed by one very similar to Neptune”.

As Adrien Leleu concludes, “the system therefore turned out to be one that challenges our understanding of the formation and evolution of planetary systems”.

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