A unique planetary system located 200 light-years from Earth hosts five exoplanets with orbits locked together in a repeating pattern, despite their very different sizes and densities. The discovery is challenging astronomers’ notions of the kinds of planetary systems that can exist and how they form.
Five of six exoplanets in orbit around the star TOI-178 are in an 18:9:6:4:3 orbital resonance with each other, according to 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 period. The third will complete six orbits, and so on. The video below offers a demonstration of the process in action.
The innermost of the six exoplanets (shown with a blue orbital path) is not in resonance with the others, though it might have been in the past. In the animation above, rhythmic patterns are represented by red pulses and a chime sound (in the pentatonic scale), which get triggered 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 of them being in orbital resonance. The new study was led by Adrien Leleu, CHEOPS fellow at the University of Geneva.
When Leleu, a dynamicist (an expert in celestial mechanics) and his colleagues first observed the TOI-178 system, they thought they saw two planets orbiting around the host star in the same orbit, but this result was inconclusive. The scientists decided to make follow-up observations using the European Space Agency’s CHEOPS satellite and the ground-based ESPRESSO instrument on the European Southern Observatory’s Very Large Telescope, in addition to the Next Generation Transit Survey and SPECULOOS projects, both in Chile. All these instruments allowed the team to detect the six exoplanets and characterise their orbits, which they did using the transit method (looking at the dimming of the host star when a planet passes in front) and by measuring the wobble of the host star.
All six exoplanets are in close proximity to the central star, with the nearest planet taking around two days to make a complete orbit and the most distant orbiting in around 20 days. None are inside the habitable zone, the Goldilocks region around a star where liquid water (and thus life) would be possible. Five of the six exoplanets are locked in perfect resonance, such that some planets come into alignment every few orbits. The 18:9:6:4:3 chain is among the longest ever discovered.
Orbital resonance happens when orbiting bodies exert a periodic gravitational influence on each other. In our solar system, Jupiter’s moons Io, Europa, and Ganymede are in a 4:2:1 resonance.
The TOI-178 is interesting for a number of reasons, with the orbital resonance a sign of prolonged stability.
“From our understanding of planet formations, chain of resonances often occurs in the earliest phases of planetary system formation, when the star is still surrounded by a gaseous disc,” Leleu explained in an email. “However during the billions of years that follow the formation, many things can happen and most systems get out of the 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 TOI-178 is not only this orbital configuration, but also the planets’ composition,” said Leleu. This consequently presents a challenge to our understanding of how planets form and evolve.
Indeed, the planets are between one and three times the size of Earth but have masses ranging from 1.5 to 30 times the mass of Earth. So while their orbital configurations are neat and tidy, their compositions are not. For example, one planet is a super-Earth, but its immediate neighbour is a low-density ice giant similar to Neptune. We don’t see that sort of thing around here.
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According to Leleu, theory suggests that the planets should have lower density the farther they are from their star. But that’s not the case here. “In TOI-178, it’s only true for the two inner planets that are rocky, but then the third planet from the star has a very low density, then planet 4 and 5 are more dense, and then planet 6 is once again more fluffy,” he said.
Astronomers will now have to figure out how the system formed, including whether some of the planets formed farther out and slowly drifted inward.
Interestingly, TOI-178 could host other, more distant planets, but they just haven’t been detected. Looking ahead, ESO’s upcoming Extremely Large Telescope, which should become operational later this decade, might be able to to learn more about this odd star system.