the coordinated dance of five planets around a star

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A team of astronomers announces the discovery of a system housing six planets, five of which are in an unusually regular orbital configuration. Details of the study are published in the journal Astronomy and astrophysics.

Initially, astronomers believed that this star named TOI-178 was coiled by two planets both having the same orbits. Further study of the system ultimately revealed not two, but six planets, five of which are locked in a rhythmic dance with each other. And if other systems in perfect resonance have already been discovered in the past, this one stands out even more insofar as the planets involved are very different from each other.

An incredible cosmic ballet

The system is made up of planets one to three times the size of Earth, with masses ranging from 1.5 to thirty times that of our planet. Some are rocky, Super-Earths, and some are made of gas, like the outer planets of our own system, but much much smaller. They are then called “Mini-Neptunes” or gaseous dwarf planets.

All these exoplanets except the innermost have a resonance 18: 9: 6: 4: 3. In other words, these five planets line up at regular intervals as they complete a tour of their parent star. The first exoplanet in the chain (the second closest to the star) loops eighteen orbits while the second in the chain ends nine, the third completes six, the fourth completes four and the fifth (the outermost planet) finishes three.

Note that a similar, but much less complex resonance can also be found in our system with three of Jupiter’s moons. Io performs four full orbits for each orbit of Ganymede, while also completing two full orbits for each orbit of Europe. So here we have a resonance 4: 2: 1.

This cosmic ballet bears witness to a star system that has hardly been disturbed since birth. If it had been the case (because of a giant impact or by the gravitational influence of another system for example), the fragile configuration of its orbits would have been erased.

Credit: ESO / L. Calçada /

In tight formation

This study, supported by data from the CHEOPS satellite, the European Space Agency (ESA), and the VLT (in Chile), also found that the TOI-178 exoplanets revolve around their star. much faster and at a much closer distance than Earth around the Sun.

The innermost planet, the one that does not integrate the resonant chain, is the fastest, completing a turn of TOI-178 in just a few days, while the slowest finishes an orbit in just a few weeks. Finally, none of these worlds seems to evolve in the habitable zone of their star, the one allowing water to exist in liquid form.

Astronomers plan to continue analyzes of this exceptional system based on the giant european telescope (ELT) of ESO, which is due to be inaugurated in 2025. The ELT should be able to directly image these exoplanets and probe their atmosphere in detail.

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