Abstract

Multi-planet systems offer a unique opportunity for comparative exoplanetology, having originated from the same protoplanetary disk and evolved in the same environment. This makes them appealing targets to test theoretical planetary formation and evolution scenarios. In this seminar I will describe the multi-planet system orbiting the old, metal-poor star TOI -561. The system has been characterised using TESS photometry and high precision HARPS -N radial velocity (RV) data. We found that TOI -561 hosts four transiting planets, namely an ultra-short period (USP) super-Earth and three mini-Neptunes. In fact, after the identification of three transiting candidates by TESS , we discovered two additional external planets based on RV analysis. We confirmed and characterised the two inner TESS candidates, but we showed that the third candidate can not be confirmed by RVs, and would make the system dynamically unstable. We demonstrated that the two transits initially associated with this candidate are instead due to single transits of the two additional planets discovered in the RV data, that also have a predicted strong, anti-correlated transit time variation signal. With a density of ∼3 g cm−3, which is consistent with a water-dominated composition, the inner super-Earth is the lowest density USP planet known to date.