Dancing exoplanets challenge theories on planet formation

Astronomers have revealed a system consisting of six exoplanets, five of which are locked in a rare rhythm around their central star. The researchers believe the system could provide important clues about how planets, including those in the Solar System, form and evolve.

The Swedish research contribution in this study has been significant, with the participation of, among others, Malcolm Fridlund and Carina Persson at Chalmers University of Technology.


The first time the team observed TOI-178, a star some 200 light-years away in the constellation of Sculptor, they thought they had spotted two planets going around it in the same orbit. However, a closer look revealed something entirely different. 

– Through further observations we realised that there were not two planets orbiting the star at roughly the same distance from it, but rather multiple planets in a very special configuration, says Adrien Leleu from the Université de Genève and the University of Bern, Switzerland, who led a new study of the system published today in Astronomy & Astrophysics.

The new research has revealed that the system boasts six exoplanets and that all but the one closest to the star are locked in a rhythmic dance as they move in their orbits. In other words, they are in resonance. This means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits. 

The five outer exoplanets of the TOI-178 system follow a complex chain of resonance, one of the longest yet discovered in a system of planets. The five outer planets in the TOI-178 system follow a 18:9:6:4:3 chain: while the second planet from the star (the first in the resonance chain) completes 18 orbits, the third planet from the star (second in the chain) completes 9 orbits, and so on. 

The six exoplanets found are very close to the star, with orbital periods ranging from 2 to 21 days, which is closer than the the star's so called habitable zone. But the researchers suggest that, by continuing the resonance chain, they might find additional planets that could exist in or very close to this zone.

– For a planet to be in the habitable zone where liquid water can be found on the surface, the orbital period in this system must be at least 40 days. The fact that the planets around TOI-178 have orbits so extremely close to their star means that any water on their surfaces would boil away, even though the star is cooler than our Sun, says Carina Persson, at the department of Space, Earth and Environment.

Page manager Published: Wed 27 Jan 2021.