The enigmatic star system has 5 planets locked in perfect harmony

A unique planetary system located at 200 lightsYears of Earth is home to five exoplanets whose orbits are locked together in a repeating pattern, despite their very different sizes and densities. Discovery challenges astronomers notions about the types of planetary systems that may exist and how they are formed.

Five of the six exoplanets orbiting the star TOI-178 are in 18: 9: 6: 4: 3 orbital resonance with each other, according to the new research published today in Astronomy & Astrophysics. So, for every 18 orbits made by the innermost of these five exoplanets, the next planet along the chain will complete nine orbits during the exact same time period. The third will perform six orbits, and so on. The video below provides a demonstration of the process in action.

The innermost of the six exoplanets (shown with a blue orbital path) does not resonate with the others, although it may have been in the past. In the animation above, the rhythmic patterns are represented by red pulses and a chime sound (in the pentatonic scale), which trigger when each exoplanet completes either a full orbit or a half-orbit. As the video shows, two or more exoplanets trigger the chime quite often, the result being orbital resonance. The new study was conducted by Adrien Leleu, CHEOPS Fellow at the University of Geneva.

When Leleu, a dynamist (expert in celestial mechanics) and his colleagues first observed the TOI-178 system, they believed they had seen two planets orbiting the host star in the same orbit, but this result was inconclusive. The scientists decided to make follow-up observations using tof the European Space Agency The CHEOPS satellite and the ESPRESSO ground instrument on the European Southern Observatory Very large telescope, in addition to Next Generation Transit Survey and SPECULOOS projects, both in Chile. All tThese instruments allowed the team to detect the six exoplanets and characterize their orbits, which they did using the transit method (watching the gradation of the host star as a planet passes by) and measuring the oscillation of the host star.

All six exoplanets are in close proximity to the central star, with the nearest planet taking about two days to make a full orbit and farthest in orbit about 20 days. None are inside the habitable zone, the Goldilocks region around a star where liquid water (and therefore life) would be possible. Five of the six exoplanets are locked in perfect resonance, so some planets line up every few orbits. The 18: 9: 6: 4: 3 chain is among the longest ever discovered.

Orbital resonance occurs when orbiting bodies exert a periodic gravitational influence on each other. In our solar system, the moons of Jupiter Io, Europa, and Ganymede are in a 4: 2: 1 resonance.

The TOI-178 is interesting for several reasons, orbital resonance being a sign of prolonged stability.

“From our understanding of planetary formations, the chain of resonances often occurs in the early stages of the formation of the planetary system, when the star is always surrounded by a gaseous disc, ”Leleu explained in an e-mail. “However, over the billions of years of formation, a lot can happen and most systems emerge from resonances. It can happen slowly, due to [gravitational] tidal effects for example, or violently, due to instability and planet collision / ejection. “

Only five other star systems have resonant chains involving four or more planets, “which is not a lot,” he added. Astronomers consider these planetary systems to be rare and quite young.

“What is unique to the TOI-178 is not only this orbital configuration, but also the makeup of the planets,” Leleu said. This therefore presents a challenge to our understanding of how planets form and evolve.

Indeed, the planets are between one and three times the size of the earth but have masses ranging from 1.5 to 30 times the mass of the Earth. So, although their orbital configurations are clean and orderly, their compositions are not. For example, a planet is a super-Earth, but its immediate neighbor is agiant ice of density similar to Neptune. We don’t see that sort of thing here.

According to Leleu, the theory suggests that planets should have a lower density the farther they are from their star. But it is not the case here. “In TOI-178, this is only true for the two inner planets which are rocky, but then the third planet of the star has a very low density, then planets 4 and 5 are denser, then planet 6 is fluffier again, “he said.

Astronomers will now need to determine how the system formed, including whether some of the planets formed farther out and slowly derivative towards the inside.

Interestingly, TOI-178 could accommodate other planets, but they just haven’t been detected. In the future, ESO’s next extremely large telescope, which is expected to become operational later this decade, may be able to learn more about this strange star system.