How do these six super-Earths resist the chaos of the universe?
A team of astronomers stumbled upon this rare instance of order — a group of at least six exoplanets (in the super-Earth to mini-Neptune regimes) following a rhythmic, cosmic dance around a host star.
The discovery could help shed light on how planets form around stars, and how they evolve over time.
The discovery is detailed in a study published Monday in the journal Astronomy and Astrophysics.
As captured by this video animation of the planets' orbit, five outer exoplanets follow a repeating pattern with the same planets aligning in their orbit every few rounds around their star.
HERE'S THE BACKGROUND — The team of astronomers behind the discovery were observing TOI-178, a star located around 200 light-years away from Earth in the constellation Sculptor.
At first, they thought there were two planets orbiting around this star at roughly the same distance. However, once they got a closer look, their observations revealed a total of six exoplanets orbiting the star.
Save for the planet located closest to its host star, the five outer planets follow a unique cosmic path as they orbit around the star with a pattern that repeats itself every few orbits.
WHAT'S NEW — This type of orbital configuration can also be observed around Jupiter, where three of its moons follow a rhythmic pattern as they orbit around the gas giant.
Jupiter has 79 confirmed moons, but three of them; Io, Europa and Ganymede, are locked in a special orbital configuration. For every orbit that Ganymede completes around Jupiter, Io, the closest of the three to Jupiter, completes four full orbits and Europa completes two full orbits.
However, the orbital patterns that the five exoplanets follow are much more complex, according to the researchers behind the study, and the longest one ever observed.
The Jupiter moons are in a 4:2:1 resonance, while the five exoplanets orbiting around TOI-178 follow a 18:9:6:4:3 chain.
Here's the breakdown: As the second planet from the star completes its 18 orbits, the third planet from the star completes nine orbits, while the fourth completes six, the fifth completes four, and the last one, the furthest away from the host star, completes three.
Every few orbits, the planets will align in a repeated pattern.
WHAT'S NEXT — The orbital pattern of these five outer planets is not only cosmically soothing, but it also provides scientists with insight on how these planets formed and evolved.
“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” Yann Alibert, a researcher at the University of Bern and co-author of the study, said in a statement.
Planets form a disc of gas and dust that surrounds their host star. However, as they start forming and following an orbit around the star, they are often disturbed by incoming asteroids or other flying objects that impact them, and throw them off their orbit.
However, the TOI-178 system reveals very little disturbance as these planets' cosmic dance has remained intact.
The researchers behind the study are hoping to make additional discoveries of exoplanets orbiting around their host star, revealing different patterns that can help them understand the history of planetary formation.
Abstract: Determining the architecture of multi-planetary systems is one of the cornerstones of understanding planet formation and evolution. Among these, resonant systems are especially important as the fragility of their orbital configuration ensure that no significant scattering or collisional event took place since the earliest formation phase, when the parent protoplanetary disk was still present. In this context, TOI-178 has been the subject of particular attention as the first TESS observations hinted at the possible presence of a near 2:3:3 resonant chain. Here we report the results of observations from CHEOPS, ESPRESSO, NGTS, and SPECULOOS with the aim of deciphering the peculiar orbital architecture of the system. We show that TOI-178 harbours at least six planets in the super-Earth to mini-Neptune regimes with radii ranging from 1.177±0.074 to 2.91±0.11 Earth radii, and periods of 1.91, 3.24, 6.56, 9.96, 15.23, and 20.71 d. All planets but the innermost one form a 2:4:6:9:12 chain of Laplace resonances, and the planetary densities show important variations from planet to planet, jumping from 0.90+0.16 −0.21 to 0.15+0.03 −0.04 times the Earth density between planets c and d. Using Bayesian interior structure retrieval models, we show that the amount of gas in the planets does not vary in a monotoneous way as one could expect from simple formation and evolution models, contrarily to other known systems in chain of Laplace resonances. The brightness of TOI-178 (H=8.76 mag, J=9.37 mag, V=11.95 mag) allows for precise characterisation of its orbital architecture as well as of the physical nature of the six presently known transiting planets it harbours. The peculiar orbital configuration and the diversity in average density among the planets in the system will enable the study of planetary interior structures and atmospheric evolution providing important clues on the formation of super-Earths and mini-Neptunes.