Company for an ultra-short period sub-Earth: discovery of two additional planets orbiting GJ 367

Ultra-short period small planets (R < 2 R⊕, P < 1 d) are believed to be naked cores of former Neptunian planets that have lost their atmospheres during the migration process from beyond the ice line to their current close-in orbits. How this migration occurred is currently unclear and needs to be explored on a case-by-case basis once the inner architecture of the hosting system is known. GJ 367 is an M1 V star that has been recently found to host a transiting ultra-short period sub-Earth on a 8 h orbit (Lam et al. 2021). With the aim of unveiling the architecture of the system, we performed an intensive RV follow-up campaign with the HARPS spectrograph collecting nearly 300 high-precision radial velocities. We discovered two additional low mass companions with orbital periods of ~11.5 and 34 days and minimum masses of ~4.5 and 6.6 M⊕, respectively, and explored the different secular migration scenarios that could account for the current architecture of the planetary system.