If one leaves multi-body systems to themselves, sometimes a strange order appears. The distances of the planetary orbits are integer multiples of a basic value, moons and planets move in unison, celestial bodies always turn to the same side – what we then perceive as cosmic order are all no miracles, but merely results of the effect of gravity in a system built up in a certain way.

This is also true of TOI-178, a star about 200 light-years away in the constellation Sculptor. When researchers first observed the star, they initially suspected they had discovered two planets orbiting it on the same path. That would have been a real sensation. So-called co-orbital planets orbiting their star at the same distance are theoretically possible, but rather unlikely. In the case of Kepler-223, something like this had also been suspected, but then it turned out that the four planets of the system orbit in a rhythmic chain.

This seems to be the case for TOI-178 as well, but to a greater extent. The new research has revealed that the system has six exoplanets, and that all but the one closest to the star are locked in a rhythmic dance as they move along their orbits. In other words, they are in resonance. This means that there is a pattern that repeats as the planets orbit the star, with some planets aligning every few orbits. A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa, and Ganymede. Io, the closest of the three moons to Jupiter, completes four full orbits around Jupiter for every orbit made by the most distant Ganymede, and two for every orbit made by Europa.

The five outer exoplanets of the TOI-178 system follow an even more complex resonance chain, one of the longest yet discovered in a planetary system:  While Jupiter’s three moons are in a 4:2:1 resonance, the five outer planets of the TOI-178 system follow an 18:9:6:4:3 chain. Thus, the second planet seen from the star (the first in the resonance chain) completes 18 orbits in the same time, the third planet 9, the fourth 6, and so on. In fact, scientists initially found only five planets in the system, but by following this resonance rhythm, they calculated where an additional planet would be in its orbit the next time they had a window of time to observe the system.

This dance of resonating planets hears clues about the system’s past. “The orbits in this system are very well ordered, which tells us that this system has evolved quite smoothly since its birth,” explains co-author Yann Alibert of the University of Bern. If the system had been significantly perturbed, for example by a huge impact, this fragile configuration of orbits would not have survived.

But even if the orbital configuration is clean and orderly, the densities of the planets are “much more disordered,” says Nathan Hara of the University of Geneva, Switzerland, who was also involved in the study. “It seems that there is a planet as dense as Earth right next to a very light planet with half the density of Neptune, followed by a planet with the density of Neptune. That’s not what we’re used to.” In our solar system, for example, the planets are neatly arranged, with the denser rocky planets closer to the central star and the looser-textured, low-density gas planets farther out.

“This contrast between the rhythmic harmony of orbital motion and the disordered densities certainly challenges our understanding of how planetary systems form and evolve,” Leleu points out.

Although none of the six exoplanets found lie in the star’s habitable zone, the researchers suspect that by following up on the resonance chain, they could find other planets that might exist in or very close to that zone. ESO’s Extremely Large Telescope (ELT), scheduled to be operational later this decade, will be able to directly image rocky planets in a star’s habitable zone and even characterize their atmospheres. This provides the opportunity to study systems like TOI-178 in even greater detail.

The cosmic dance shown in a video (turn on the sound!):