Confirmation of the hot super-Neptune TOI-672 b with NIRPS and HARPS and Insights into the Neptunian desert around M dwarfs

Abstract

The Neptunian desert is a distinct lack of Neptune-sized planets at short orbital periods, purportedly carved by photoevaporation and tidal circularization following high-eccentricity migration. Constraining these processes and how they vary across different host-star spectral types requires the detailed characterization of 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 analyse photometry from TESS and ExTrA and precise radial velocity measurements taken with the recently commissioned Near-InfraRed Planet Searcher (NIRPS) and HARPS spectrographs. We measure the planetary orbital period, radius, and mass of 3.634 days, 5.31 +0.24 -0.26 Rearth, and 50.9 +4.5 -4.4 Mearth, respectively. 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 then use a novel approach to determine the desert boundaries in period-radius space and instellation-radius space, and, for the first time, compare the Neptunian desert boundaries for planets orbiting FGK versus M dwarf stars. We determine that the boundary ridge shifts slightly inward from 3.3 +- 1.4 days for FGK host stars to 2.2 +- 1.0 days for M dwarf host stars; these values do not statistically significantly differ from each other, and the shift to shorter periods for M dwarf planets is smaller than 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.

Figures (22)

• Figure 1: Period-radius diagram showing TOI-672 b (yellow star) against the known planet population (light grey dots, taken from the NASA exoplanet archive, where only those with periods and radii determined to better than 4 $\sigma$ are shown; the background is also shaded according to this population). Planets around M dwarf hosts are highlighted (as green circles with a black outline, cut on $T_{\rm eff} < 3900$ K, $M_{*}, R_{*} < 0.6$$M_{\odot}, R_{\odot}$). The Neptunian desert boundaries derived by Mazeh2016 (orange line) and CastroGonzalez2024 (red line) are shown. The desert, ridge, and savanna, as defined by CastroGonzalez2024, are labelled, and the higher-period cut-off of the ridge is shown (red dashed line).
• Figure 2: The TESS photometry for TOI-672 covering Sectors 9, 10, 36, and 63 (left to right, chronologically); the data are described in Section \ref{['sec:obs_tess']} and the fit to these data is described in Section \ref{['sec:fit_tess']}. Top panel: the PDCSAP flux (grey circles) and the GP model (green line) used for detrending. Middle panel: the detrended flux after the GP model is subtracted, showing the transit model (orange line). Bottom panel: the residuals left after the GP and transit models are subtracted from the photometry.
• Figure 3: Top panel: the detrended TESS photometry for TOI-672 from all sectors (grey circles, binned as dark brown circles), phase folded on the best-fit planetary period, with the transit model shown (orange line). Bottom panel: the residuals after the GP and planet models are subtracted from the photometry.
• Figure 4: The ExTrA photometry for TOI-672 from the night of 27 April 2024 using Telescope 2 (T2); the data are described in Section \ref{['sec:obs_extra']} and the fit to these data is described in Section \ref{['sec:fit_extra']}. Top panel: the extracted photometry (grey circles, binned as dark purple circles), and the GP model used to detrend it (green line). Middle panel: the detrended photometry, showing the transit model (purple line). Bottom panel: the residuals after the GP and transit models are subtracted from the photometry.
• Figure 5: The NIRPS and HARPS data for TOI-672; the data are described in Section \ref{['sec:obs_nirpsharps']} and the fit to these data are described in Section \ref{['sec:fit_nirpsharps']}. Top panel: the NIRPS RVs (red triangles), the GP model fit to the NIRPS RVs (green line, with the one and two standard deviations of the fit shaded), and the combined GP and planet model (grey line). The BERV crossing event described in Section \ref{['sec:obs_nirpsharps']} is shaded in blue. Middle panel: both sets of RVs, where NIRPS has been detrended with a GP and HARPS (blue circles) has no detrending, with the planet-only model is shown (grey line). Bottom panel: the RV residuals after the GP and planet models are subtracted.