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A puzzling line up of six-exoplanets moving in a rhythmic dance as they orbit their star have been discovered in a system 200 light years from Earth.
Researchers used the European Space Agency CHEOPS space telescope to detect the unusual explanatory formation – and it could shed light on how systems form.
University of St Andrews astronomers found five of the six planets are locked in a harmonic rhythm, where their orbits align in a consistent pattern to one another – which the team says suggests no major collisions when the system first formed.
The outer planets of the TOI-178 system have formed a ‘resonance’ – a type of harmony where the planets orbital periods regularly attract each other.
The density of the planets in the system is also unusual – in the Solar System the dense rocky planets are closer to the Sun, followed by lighter gas giants.
In the case of the TOI-178 system a dense Earth-like planet is right next to a very fluffy planet with half the density of Neptune – followed by one similar to Neptune.
This unusual layout and orbital resonance ‘challenges what we know about how planetary systems form,’ according to the team behind the discovery.
This artist’s impression shows the view from the planet in the TOI-178 system found orbiting furthest from the star
A planetary system can form a resonance – a kind of harmony between the orbits and gravitational pull of the planets within that system.
It is when planets, whose orbital period ratios form simple fractions, regularly attract each other with their gravity when one planet takes three days to orbit its star and its neighbour takes two days, for example.
This means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits.
A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede.
Io, the closest of the three to Jupiter, completes four full orbits around Jupiter for every orbit that Ganymede, the furthest away, makes, and two full orbits for every orbit Europa makes.
The five outer exoplanets of the TOI-178 system follow a much more complex chain of resonance, one of the longest yet discovered in a system of planets.
While the three Jupiter moons are in a 4:2:1 resonance, the five outer planets in the TOI-178 system follow a 18:9:6:4:3 chain.
As an example – the second planet from the star completes 18 orbits, the third planet completes 9 orbits and so on down to the sixth planet from the star.
Researchers, including experts from the University of Geneva who manage the telescope, had believed there were only three planets orbiting the star TOI-178, but subsequently found another three, with five out of the six in a harmonic rhythm.
Initially observations from NASAs Transiting Exoplanet Survey Satellite (TESS) revealed the first three planets orbiting the star – follow up observations from CHEOPS revealed the next three.
Initially observations from NASAs Transiting Exoplanet Survey Satellite (TESS) revealed the first three planets orbiting the star – follow up observations from CHEOPS revealed the next three
Dr Thomas Wilson, at the University of St Andrews, said they had 11 days of high-quality data from CHEOPS and confirmed at least five planets were orbiting in a ‘special resonance chain’ within the system.
‘This rare configuration prompted us to search for another planet to complete the chain, and with additional data we were able to confirm the presence of a sixth planet with the orbital period we predicted,’ added Dr Wilson.
Professor Andrew Collier Cameron said the chain of resonances acts as a ‘fossil clue’ to the entire system’s formation process.
‘This orderliness has allowed these closely-packed planets to avoid catastrophic interactions with each other for billions of years,’ Cameron said.
“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explains co-author Yann Alibert from the University of Bern.
‘If the system had been significantly disturbed earlier in its life, for example by a giant impact, this fragile configuration of orbits would not have survived.’
Researchers found the planets to be in a unique planetary system configuration which challenges current understanding of planet formation and evolution.
Before now, closely-packed planets orbiting a star in such a unique, resonance manner decrease in density the further away they are from the star.
However, in this case the density of the planets around TOI-178 appears to be a wild mixture despite the harmonic, orderly way they orbit around their star.
They confirmed this using data from ESPRESSO on the Very Large Telescope (VLT) at the European Southern Observatory’s (ESO)’s Paranal Observatory in Chile.
ESA Project Scientist Kate Isaak said: ‘It is the first time we have observed something like this. In the few systems we know with such a harmony, the density of planets steadily decreases as we move away from the star.
‘In the TOI-178 system, a dense, terrestrial planet like Earth appears to be right next to a very fluffy planet with half the density of Neptune followed by one very similar to Neptune.’
Combining the fragile nature of the orbits of these closely-packed planets with the unprecedented mixture of planet densities, means that the TOI-178 system raises many questions.
Dr Adrien Leleu, the astrophysicist from the University of Geneva who led the team, said: ‘The system therefore turned out to be one that challenges our understanding of the formation and evolution of planetary systems.’
Details of the discovery have been published in the journal Astronomy and Astrophysics.
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