The planets in the HIP 41378 system pass in front of their star (the transit method), resulting in an almost perfect alignment with Earth. However, only one in every 200 systems like HIP 41378 has this configuration. The HIP 41378 system is therefore a rare gem, enabling the study of planets with orbits comparable to those of the planets in the Solar System.
Five planets in the system were first discovered in 2015 by the Kepler telescope, but little was known about their nature and orbits. Understanding this system required a decade of observations using instruments mainly based in Chile, which measured variations in the star’s velocity caused by the planets (the radial velocity method).
HIP 41378 is thus one of the first multiple-planet systems in which long-period planets have been well characterised. It consists of three inner planets smaller than Neptune and three outer planets, which are low in density and may be as large as Saturn. A seventh, more distant planet has yet to be confirmed.
Among the exoplanets, HIP 41378 f is particularly intriguing: its extremely large radius, given its mass, defies planetary formation models. One hypothesis, currently being investigated at the LTE as part of the WRAPS project (“Where do Rings Appear in Planetary Systems?”), suggests that this planet is in fact surrounded by highly inclined, opaque rings, giving it the appearance of a larger planet (see Figure). The formation of such a ring could be the natural result of a moon’s evolution around this planet.
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Read the original article : A decade of monitoring the HIP 41378 planetary system: Masses and orbital periods of six planets and a planet candidate, Astronomy & Astrophysics n°710, A275, 2026.
Read a related article : Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii, Astronomy and Astrophysics n°675, A174, 2023.
