A planetary system approximately two hundred light-years away from us was discovered in a weird rare orbital dance. Of the six exoplanets orbiting a star called TOI-178, the five outermost follow rhythmic orbital periods, with each linked to the planets on all sides.
Such a movement is known as a chain of resonance, and it’s a very rare happening. It can help provide some insight into how planetary systems form.
Orbital resonances themselves aren’t that rare. They mean that a ratio relates to the orbital periods of two bodies.
There are some examples in our solar system – Pluto and Neptune!
For every two orbits Pluto completes around the Sun, Neptune goes around thrice, so it’s a 2:3 ratio (resonance).
Three moons of Jupiter also form a resonance chain. For every orbit of Ganymede, Europa revolves twice and Io four times, forming a 1:2:4 resonance.
However, the five outer exoplanets of the TOI-178 system present one of the most complex resonance chains ever discovered – 3:4:6:9:18. The innermost exoplanet of the chain completes 18 orbits for every three orbits of the outermost one.
A team of researchers from the Université de Genève and the University of Bern in Switzerland, under Adrien Leleu, found that the resonance complexity is a clue to the system’s 7-billion-year rich history.
Astronomer Yann ALibert from the University of Bern said:
“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth.”
TOI-178 is not the only system with such a complex chain of resonance. In 2020, astronomers discovered a six-exoplanet system, called HD 158259, in a near-perfect resonance chain with each pair of planets experiencing a close to 2:3 resonance.
The Kepler-80 system includes five exoplanets in a resonance chain of 4:6:9:12:18.
Also, the popular TRAPPIST-1 system features a seven-exoplanet resonance chain of 2:3:4:6:9:15:24.
TOI-178 is unique, though. The arrangement of the exoplanets in the system is a total mess that initially doesn’t seem to make sense.
For example, in our Solar System, planets are all neatly grouped, with the dense rocky ones on the inner side, gas giants in the middle, and ice giants on the outer side.
HD 158259 includes on rocky super-Earth on its inner side, and the five outer exoplanets are like mini Neptunes. TRAPPIST-1’s exoplanets are comparable in sizes and densities.
Astronomer Nathan Hara compared it to TOI-178:
“It appears there is a planet as dense as the Earth right next to a very fluffy planet with half the density of Neptune, followed by a planet with the density of Neptune.”
“It is not what we are used to,” he added.
Additionally, it does not jibe at all with our current understanding of resonant systems or with the present understanding of how planetary systems are born.
“This is not what we expected, and is the first time that we observe such a setup in a planetary system […] In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory,” said Leleu.
It is yet to be known precisely what fives, but the discovery indeed highlights a gap in the astronomical knowledge scientists share.
Some additional research will be conducted to uncover the system’s secrets and how it can enrich the scientists’ experiences.