Confirmation of the Hot Super-Neptune TOI-672 B with NIRPS and HARPS: Insights into the Neptunian Desert Around M Dwarfs

The Neptunian desert is a distinct lack of Neptune-sized planets at short orbital periods, purportedly carved by photoevaporation and tidal circularisation following high-eccentricity migration. Constraining these processes and how they vary across different hoststar spectral types requires detailed characterisation of the planets in the desert and around its boundaries. In this study, we confirm the planetary nature of a massive super-Neptune identified by TESS around the M0 dwarf TOI-672. We analysed photometry from TESS and ExTrA and precise radial velocity measurements taken with the recently commissioned Near-InfraRed Planet Searcher (NIRPS) and HARPS spectrographs. We measured a planetary orbital period of 3.634 days, a radius of 5.31(-0.26)(+0.24) R-circle plus, and mass of 50.9(-4.4)(+4.5) M-circle plus. Our findings place TOI-672 b within the Neptunian ridge, a pile-up of planets from 3-5 days at the Neptunian desert boundary. We used a novel approach to determine the desert boundaries in period-radius space and instellation-radius space, and for the first time, we compared the Neptunian desert boundaries for planets orbiting FGK versus M dwarf stars. We determined that the boundary ridge shifts slightly inwards from 3.3 +/- 1.4 days for FGK host stars to 2.2 +/- 1.0 days for M dwarf host stars. Statistically, these values do not significantly differ from each other, and the shift to shorter periods for M dwarf planets is smaller than what theoretical photoevaporation models predict. We also find that TOI-672 b is a single-planet system within the sensitivity limits of our RV and TTV datasets.