An international team led by Salomé Grouffal of the Marseille Astrophysics Laboratory (Aix-Marseille University/ CNES/CNRS) and the Grenoble Institute of Planetology and Astrophysics (CNRS/Grenoble Alpes University) has spent ten years studying an exceptional planetary system comprising at least six planets with long orbital periods, the outermost of which reaches the orbit of Mars. Such a rare system offers a new window into understanding the diversity of planetary systems. The planets in the HIP 41378 system pass in front of their star^[1], creating an almost perfect alignment with Earth. However, only one in 200 systems like HIP 41378 has this configuration, and most known transiting planets are therefore very close to their star. HIP 41378 is therefore essential for studying planets in an environment comparable to that of the Solar System.

Discovered in 2015 by the Kepler telescope, HIP 41378 was already known to host five planets, but their nature and orbits remained poorly understood. Understanding this system has required a decade of observations using instruments based mainly in Chile^[2], which measured variations in the star’s velocity caused by the planets^[3]. These signals, weaker than a pedestrian’s walking speed, played hide-and-seek with the astronomers.

HIP 41378 thus becomes one of the first multiple systems where long-period planets are well characterised. It consists of three inner planets smaller than Neptune and three outer planets, which are low-density and can reach the size of Saturn. A seventh, more distant planet remains to be confirmed.
The outer planet HIP 41378 f is intriguing: its extremely low density defies planetary formation models. Could it be surrounded by giant rings or extensive opaque clouds? Further observations are needed to elucidate its origin.

This system provides a valuable laboratory for comparative planetology and opens up new avenues for understanding the formation and diversity of planetary systems.