Free Astronomy Magazine May-June 2021

MAY-JUNE 2021 A rtist’s representation of the or- bits of the planets in the TOI- 178 system. [ESO/L. Calçada] But even if the arrangement of the orbits is neat and well-ordered, the densities of the planets “are much more disorderly,” says Nathan Hara from the Université de Genève, Switzerland, who was also involved in the study. “It appears there is a planet as dense as the Earth right next to a very fluffy planet with half the density of Neptune, followed by a planet with the density of Nep- tune. It is not what we are used to.” In our Solar System, for example, the planets are neatly arranged, with the rocky, denser planets closer to the central star and the fluffy, low- density gas planets farther out. “This contrast between the rhythmic har- mony of the orbital motion and the disorderly densities certainly chal- lenges our understanding of the for- mation and evolution of planetary systems,” says Leleu. To investigate the system’s unusual architecture, the team used data from the European Space Agency’s CHEOPS satellite, alongside the ground-based ESPRESSO instrument on ESO’s VLT and the NGTS and