Free Astronomy Magazine May-June 2021

43 MAY-JUNE 2021 ASTRO PUBLISHING chain) completes 18 orbits, the third planet from the star (second in the chain) completes 9 orbits, and so on. In fact, the scientists initially only found five planets in the system, but by following this resonant rhythm they calculated where in its orbit an additional planet would be when they next had a window to observe the system. More than just an orbital curiosity, this dance of resonant planets pro- vides clues about the system’s past. “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 signif- icantly disturbed earlier in its life, for example by a giant impact, this fragile configuration of orbits would not have survived. T his artist’s impression shows the view from the planet in the TOI- 178 system found orbiting furthest from the star. New research by Adrien Leleu and his colleagues with several telescopes, including ESO’s Very Large Telescope, has revealed that the system boasts six exoplan- ets and that all but the one closest to the star are locked in a rare rhythm as they move in their orbits. But while the orbital motion in this system is in harmony, the physical properties of the planets are more disorderly, with significant varia- tions in density from planet to planet. This contrast challenges as- tronomers’ understanding of how planets form and evolve. This artist’s impression is based on the known physical parameters for the planets and the star seen, and uses a vast database of objects in the Universe. [ESO/L. Calçada /spaceengine.org ]