What’s TOI-178? 5 planets found in strange rhythmic dance around a star 200 light-years away baffle scientists
It challenges theories about how planets form and change over time
Astronomers have discovered a ‘special’ system with multiple planets that are locked in an unusual rhythm around their central star, called TOI-178. The planetary system boasts of six exoplanets and all but the one closest to the star are locked in the rare “rhythmic dance” as they move in their orbits.
Located some 200 light-years away in the constellation of Sculptor, the discovery has puzzled researchers. They believe the system could provide important clues about how planets, including those in the solar system form and evolve. “Six-exoplanet system with rhythmic movement challenges theories of how planets form,” suggest scientists in their analysis published in Astronomy & Astrophysics.
The detection was made using a combination of telescopes, including the Very Large Telescope of the European Southern Observatory (ESO’s VLT). Initially, the team 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 realized 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 the study.
The authors were able to gather key information about the system and its planets, which orbit their central star much closer and much faster than the Earth orbits the Sun.
The fastest (the innermost planet) completes an orbit in just a couple of days, while the slowest takes about ten times longer. The six planets have sizes ranging from about one to about three times the size of Earth while their masses are 1.5 to 30 times the mass of Earth. Some of the planets are rocky but larger than Earth, and these planets are known as super-Earths. Others are gas planets, like the outer planets in our solar system, but they are much smaller and these are nicknamed mini-Neptunes.
What are the implications?
The five planets 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.
A similar resonance has been observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. Io, the closest of the three to Jupiter, completes four full orbits around Jupiter for every orbit that Ganymede, the furthest away, makes two full orbits for every orbit Europa makes.
“The five outer exoplanets of the TOI-178 system follow a much more complex chain of resonance, one of the longest yet discovered in a system of planets. While the three Jupiter moons are in a 4:2:1 resonance, 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 findings state.
The scientists initially found just five planets in the system, but by following this resonant rhythm they calculated where in its orbit an additional planet would be when they next had a window to observe the system.
According to the investigators, this dance of resonant planets provides clues about the system’s past. “The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explains co-author Yann Alibert from the University of Bern. This suggests that if the system had been significantly disturbed earlier in its life, for example by a giant impact, this fragile configuration of orbits would not have survived.
However, while the dance of the planets in their orbits is neat and well-choreographed, their densities are much more disorderly unlike in our solar system. “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,” writes Nathan Hara from the Université de Genève, Switzerland, who was involved in the research.
In our solar system, the planets are neatly arranged, with the rocky, denser planets closer to the central star and the fluffy, low-density gas planets farther out. “This contrast between the rhythmic harmony of the orbital motion and the disorderly densities certainly challenges our understanding of the formation and evolution of planetary systems,” says Leleu.
While none of the six exoplanets found lies in the star’s habitable zone, the researchers suggest that, by continuing the resonance chain, they might find additional planets that could exist in or very close to this zone. ESO’s Extremely Large Telescope (ELT), which is set to begin operating this decade, will be able to directly image rocky exoplanets in a star’s habitable zone and even characterize their atmospheres, presenting an opportunity to get to know systems like TOI-178 in even greater detail, concludes the team.