The mass of TOI-1883 b: A low-density super-Neptune in the ridge regime transiting an early-M dwarf

Abstract Recent large-scale transit surveys conducted by space telescopes such as Kepler and TESS have revealed a vast number of exoplanets, uncovering the diversity of their population. One of the remarkable findings is the presence of a deficiency region in the period–radius distribution of short-period (<10 d) Neptune-sized planets (4–8 $R_\oplus$). This region is classified into the Neptune desert (<3.2 d), the ridge (3.2–5.7 d), and the savanna (>5.7 d) based on orbital period, each likely reflecting distinct evolutionary pathways. In this study, we used the InfraRed Doppler (IRD) instrument on the Subaru Telescope to determine the mass of the super-Neptune TOI-1883 b, which resides in the ridge region ($P \sim 4.51~{\rm d}$) orbiting an M dwarf. We measured a planetary mass of $M_p = 13.7^{+6.8}_{-6.5}\, M_\oplus$ and a mean density of $\rho _p = 0.4^{+0.3}_{-0.2}\, \mathrm{g\, cm^{-3}}$, with $3\sigma$ upper limits of $34.1~M_\oplus$, and $5\sigma$ upper limits of $47.7~M_\oplus$. These results suggest that TOI-1883 b is likely a low-density super-Neptune. We also find that the boundary of the Neptune desert defined by planets orbiting FGK-type stars exhibits a similar distribution for planets around M-type stars. According to the population-based argument of Bourrier et al. (2025, A&A, 701, A190), this suggests that TOI-1883 b may have undergone disk-driven migration to reach its current orbit and experienced early atmospheric photoevaporation driven by strong stellar XUV irradiation. The derived planetary mass is comparable to or exceeds the conventional critical core mass. We suggest that the high metallicity of the host star ($[\mathrm{Fe/H}] = 0.32 \pm 0.18$) may have suppressed the onset of runaway gas accretion. Furthermore, TOI-1883 b has a high Transmission Spectroscopy Metric (TSM > 140), making it an excellent target for future atmospheric characterization via transmission spectroscopy.