A planetary system 200 light years away recently found locked in a very rare orbital dance. Of the 6 exoplanets orbiting a star named TOI-178, the 5 outermost follow rhythmic orbital periods, with each-linked to the planets on either side.
Such a dance is known as a chain of resonance and it is only rarely seen. But it might reveal something about how planetary systems form.
Orbital resonances are not uncommon. Basically, they mean that the orbital periods of 2 bodies can be often described as a ratio.
Some examples can be found in the solar system. There’s Pluto & Neptune, for each two orbits of Pluto around the Sun, Neptune goes around three times, that’s a 2:3 resonance. And 3 moons of Jupiter are in a resonance chain. For each one orbit of Ganymede, Europa goes around three times & Io four times; that’s a 1: 2: 4 resonance.
But the 5 outer exoplanets of the TOI-178 system have one among the most complex resonance chains ever seen is 3: 4: 6: 9: 18 (where the innermost exoplanet in the chain is completing 18 orbits for each 3 orbits of the outermost).
This complexity, consistent with a team of researchers led by Adrien Leleu from Université de Genève & University of Bern in Switzerland, is a clue to the system’s 7 billion years history.
“The orbits in this system are well ordered, which tells us that this system has evolved quite-gently since its birth,” said astronomer Yann Alibert of University of Bern.
The TOI-178 system isn’t the only such with a complex chain of resonance. Last year, astronomers announced the discovery of 6 exoplanet system, HD 158259 system, in a near-perfect resonance chain, with each pair of planets near to a 2:3 resonance.
The Kepler-80 system has 5 exoplanets in a resonance chain of 4: 6: 9: 12: 18. And famous TRAPPIST-1 system has-a 7 exoplanet resonance chain of 2: 3: 4: 6: 9: 15: 24.
The TOI-178 system is different. Because the arrangement of the exoplanets in the system is a higgledy-piggledy-mess that does not seem to make a lick of sense.
Here in the solar system, for example, the planets are all neatly grouped, with the denser rocky worlds on the inside, the fluffy gas giants in the middle & the ice giants on the outside. And HD 158259 has one rocky super-Earth on the inside & the 5 outer exoplanets are mini Neptunes. TRAPPIST-1’s exoplanets are all similar sizes & densities.
Compare & contrast with TOI-178:
“It appears there’s a planet as dense as the Earth right next to a fluffy planet with-half the density of Neptune, followed by a planet with the density of Neptune,” explained astronomer Nathan Hara of Université de Genève. “It isn’t what we are used to.”
Moreover, it doesn’t jibe at all with our understanding of resonant systems or with our understanding-of how planetary systems form.
“This isn’t what we expected and is the first time that we observe such a set-up in a planetary system,” Leleu said. “In the few systems, we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star and it’s also what we expect from theory.”
It is unclear, what exactly gives, but the discovery certainly highlights a gap in our knowledge. The harmonious orbits suggest that the system hasn’t been significantly disturbed since formation, since the exoplanets have had time to settle into a strong resonance as they exert a cumulative gravitational effect on one another.
Such complexity is delicate, any disruptive influence on the system could easily-render the orbits chaotic. That is difficult to resolve with the apparent disorder of the planets themselves. Working with models & simulations could help to find out how such a system is possible.
Meanwhile, the team believes there might be even more exoplanets farther-out in the system, also linked in the resonance chain. By calculating the likely resonance, astronomers could figure-out where these exoplanets should be in relation to the others & appearance for them. This too, could render some clues.
The system is so-peculiar that the team believes it could become one among the “Rosetta stone“ systems for studying planet formation & evolution.
“We might find more planets that could-be in the habitable zone, where liquid water might be present on the surface of a planet, which begins outside of the orbits of the planets that we discovered to date,” Leleu said.
“We also want to figure out what happened to the innermost planet that’s not in resonance with the others. We suspect that it broke-out of resonance due to tidal forces.”