This planetary system has one of the longest resonant chains researchers have ever seen

The special system has six planets which, with the exception of the innermost planet, are entangled in a rhythmic dance.

Scientists have discovered a special planetary system using various telescopes. The galaxy is made up of six exoplanets, five of which revolve around their parent star in a very special rhythm. The five outer planets follow a complex “resonance chain” which, upon examination, turns out to be one of the longest researchers have ever seen.

YOU-178
The six exoplanets orbit the star TOI-178; a star about 200 light years away in the constellation Sculptor. When researchers first focused their telescopes on this star, they thought there were only two planets orbiting it. But on closer inspection, it turned out not to be quite right. “The follow-up observations have taught us that not two planets orbit the star at roughly the same distance, but that there are more planets in a very specific configuration,” said researcher Adrien Leleu.

Rhythmic dance
The researchers eventually managed to identify six planets that, except for the inner planet, got entangled in a rhythmic dance. In other words, they are in resonance, which means that the planets have entered synchronous orbits. There is a recurring pattern in the movements of the planets around their star, with a few planets aligned every few orbits. A similar resonance can be observed in three of the moons of the planet Jupiter in our solar system: Io, Europa and Ganymede. The interior of the three completes four full orbits around Jupiter as the exterior, Ganymede, completes one, and two as Europe completes one.

Animation of the orbits of the TOI-178 planets and their resonances.

Resonantieketen
However, the situation with the planets around TOI-178 is a little different. The five outer planets follow a much more complicated chain of resonance; one of the greatest researchers ever. Where the three moons of Jupiter are in a resonance of 4: 2: 1, the five outer planets of TOI-178 form a chain of 18: 9: 6: 4: 3. How does it work exactly? By the time the star’s second planet (the first in the chain) orbits eighteen, the star’s third planet (the second in the chain) completes nine, and so on. In fact, scientists had initially only discovered five planets in the system and were unable to locate the sixth planet until they calculated, based on this resonant rhythm, where this additional planet should be when their next observation session.

Comparison
Upon examination, the planets appear to orbit their parent star at a much closer distance than Earth around the sun. The fastest (and innermost) planet only takes a few days to complete an orbit around its star, while the slowest takes about ten times as long. If we look at the size of the planets, it is striking that the six planets have dimensions ranging from one to three times the Earth. And this while their masses amount to one and a half to thirty land masses. But that’s not all. Some planets are rocky, but larger than the earth; planets also referred to by the term super-earth. Others are gaseous planets (similar to the outer planets of our solar system) but several times smaller; these are also called mini-Neptunes.

The discovery of this dance of the resonating planets is not only very remarkable, but it also gives us a better understanding of the history of the special planetary system. “The tasks of this system are very well organized,” says researcher Yann Alibert. “This tells us that the system has evolved peacefully since its inception.” Had the system been disrupted earlier in its history – for example, by a major impact – this vulnerable configuration of jobs would not have survived.

Density
Where the arrangement of the orbits of the planets is therefore perfectly ordered, the densities of the planets are however much less harmonized with each other. “It seems that a planet with the density of Earth is followed by a very ‘fluffy’ planet with half the density of Neptune, and this in turn by a planet with the density of Neptune”, summarizes the researcher Nathan. Hare. “It’s not what we’re used to.” For example, in our solar system the planets are carefully sorted by density: high density rocky planets are close to the sun while low density gas giants are much further away. “This contrast between the rhythmic harmony of orbital motions and disordered densities tests our understanding of the formation and evolution of planetary systems,” Leleu said.

The results the study provides us with important new clues about how planets, including those in our own solar system, form and evolve. None of the six exoplanets discovered are located in the so-called “habitable zone” (the area around a star where liquid water is possible on a rotating planet). But what is not can still come. By extending the resonance chain, the researchers hope to encounter more planets that are in or near this area. ESO’s Very Large Telescope (ELT) should reveal that. This powerful telescope is expected to enter service during this decade. Observations with the ELT should allow the team to take direct images of rocky planets in a star’s habitable zone. What’s more, astronomers can even study the atmospheres of these hypothetical planets. And this offers the possibility of knowing even more in detail systems such as TOI-178.

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