TOI-674b: an oasis in the desert of exo-Neptunes transiting a nearby M dwarf

We use TESS, Spitzer, ground-based light curves and HARPS spectrograph radial velocity measurements to establish the physical properties of the transiting exoplanet candidate TOI-674b. We perform a joint fit of the light curves and radial velocity time series to measure the mass, radius, and orbital parameters of the candidate. We confirm and characterize TOI-674b, a low-density super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, \(V = 14.2\) mag, \(J = 10.3\) mag) is characterized by its M2V spectral type with \(\mathrm{M}_\star=0.420\pm 0.010\) M\(_\odot\), \(\mathrm{R}_\star = 0.420\pm 0.013\) R\(_\odot\), and \(\mathrm{T}_{\mathrm{eff}} = 3514\pm 57\) K, and is located at a distance \(d=46.16 \pm 0.03\) pc. Combining the available transit light curves plus radial velocity measurements and jointly fitting a circular orbit model, we find an orbital period of \(1.977143 \pm 3\times 10^{-6}\) days, a planetary radius of \(5.25 \pm 0.17\) \(\mathrm{R}_\oplus\), and a mass of \(23.6 \pm 3.3\) \(\mathrm{M}_\oplus\) implying a mean density of \(\rho_\mathrm{p} = 0.91 \pm 0.15\) [g cm\(^{-3}\)]. A non-circular orbit model fit delivers similar planetary mass and radius values within the uncertainties. Given the measured planetary radius and mass, TOI-674b is one of the largest and most massive super-Neptune class planets discovered around an M type star to date. It is also a resident of the so-called Neptunian desert and a promising candidate for atmospheric characterisation using the James Webb Space Telescope.