A unique planetary system located at 200 lightsyears from Earth it hosts five exoplanets with orbits locked together in a repetitive pattern, despite their very different sizes and densities. The discovery causes astronomers notions about the types of planetary systems that may exist and how they form.
Five of the six exoplanets orbiting the star TOI-178 are in an 18: 9: 6: 4: 3 orbital resonance with each other, according to 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 in the exact same period. The third will complete six orbits and so on. The video below provides a demonstration of the process in action.
The innermost of the six exoplanets (presented with a blue orbital path) does not resonate with the others, although it may have been in the past. In the above animation, the rhythmic patterns are represented by red pulses and a sound (on the pentatonic scale), which is triggered when each exoplanet completes either a full or half orbit. As the video shows, two or more exoplanets trigger the bell quite often, their result being in orbital resonance. The new study was led by Adrien Leleu, a CHEOPS member at the University of Geneva.
When Leleu, a dynamist (an expert in celestial mechanics) and his colleagues first observed the TOI-178 system, thought they saw two planets orbiting the host star in the same orbit, but this result was inconclusive. The scientists decided to make further observations using the of the European Space Agency The CHEOPS satellite and the ESPRESSO ground instrument of the Southern European Observatory Very large telescope, beside Next → Generation Transit Study and SPECULOUS project, 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 (analyzing fading of the host star when a planet passes in front) and by measurement the oscillation of the host star.
All six exoplanets are in the immediate vicinity of the central star, with the nearest planet taking about two days to make a complete orbit. the farthest orbiting in about 20 days. None are in 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 aligns every few orbits. The 18: 9: 6: 4: 3 chain is one of the longest ever discovered.
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Orbital resonance occurs when orbiting bodies exert a periodic gravitational influence on each other. In our solar system, the months of Jupiter, Io, Europa and Ganymede are in a 4: 2: 1 resonance.
TOI-178 is interesting for several reasons, with orbital resonance being a sign of prolonged stability.
“From the understanding of planetary formations, the chain of resonances often appears in the early stages of the formation of the planetary system, when the star is still surrounded by a gas disk “, Leleu explained in an e-mail. “However, over the billions of years of formation, many things can happen and most systems resonate. It can happen slowly because of it [gravitational] tidal effects, for example, or violent, due to instability and the collision / ejection of the planet. ”
Only five other star systems have resonant chains involving four or more planets, “which is not much,” he added. Astronomers believe that these planetary systems are rare and quite young.
“What is unique about the TOI-178 is not only this orbital configuration, but also the composition of the planets,” Leleu said. Consequently, this presents a challenge for our understanding of how planets form and evolve.
Indeed, the planets are between one and three tit is the size of the Earth but they have masses ranging from 1.5 to 30 times the mass of the Earth. So while their orbital configurations are neat and tidy, their compositions are not. For example, a planet is a super-Earth, but its immediate neighbor is athe density of the ice giant similar to Neptune. We don’t see anything like that around here.
According to Leleu, the theory suggests that planets should have a lower density the farther they are from their star. But this is not the case here. “In TOI-178, it is true only for the two inner planets that are rocky, but then the third planet from the star has a very low density, then planets 4 and 5 are denser, and then planet 6 is again fluffier. ,” he said.
Astronomers will now have to figure out how the system was formed, including whether some of the planets format farther out and slowly drifting inwards.
Interestingly, the TOI-178 could accommodate more remote ones planets, but they were simply not detected. Looking to the future, ESO’s extremely large future telescope, which should become operational later this decade, could learn more about this strange star system.