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)ischaracterizedbyitsM2Vspectraltypewith$$\mathrm{M}$\star=0.420\pm 0.010$M$\odot$,$$\mathrm{R}$\star = 0.420\pm 0.013$R$\odot$,and$$\mathrm{T}${$\mathrm{e}\mathrm{f}\mathrm{f}$} = 3514\pm 57$ 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$[gcm$^{-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.