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

Abstract

<jats:p><jats:italic>Context.</jats:italic> The NASA mission TESS is currently doing an all-sky survey from space to detect transiting planets around bright stars. As part of the validation process, the most promising planet candidates need to be confirmed and characterized using follow-up observations.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> In this article, our aim is to confirm the planetary nature of the transiting planet candidate TOI-674b using spectroscopic and photometric observations.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> We use TESS, <jats:italic>Spitzer</jats:italic>, 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.</jats:p> <jats:p><jats:italic>Results.</jats:italic> We confirm and characterize TOI-674b, a low-density super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, <jats:italic>V</jats:italic> = 14.2 mag, <jats:italic>J</jats:italic> = 10.3 mag) is characterized by its M2V spectral type with <jats:italic>M</jats:italic><jats:sub>⋆</jats:sub> = 0.420 ± 0.010 <jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>, <jats:italic>R</jats:italic><jats:sub>⋆</jats:sub> = 0.420 ± 0.013 <jats:italic>R</jats:italic><jats:sub>⊙</jats:sub>, and <jats:italic>T</jats:italic><jats:sub>eff</jats:sub> = 3514 ± 57 K; it is located at a distance <jats:italic>d</jats:italic> = 46.16 ± 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 ± 3 × 10<jats:sup>−6</jats:sup> days, a planetary radius of 5.25 ± 0.17 <jats:italic>R</jats:italic><jats:sub>⊕</jats:sub>, and a mass of 23.6 ± 3.3 <jats:italic>M</jats:italic><jats:sub>⊕</jats:sub> implying a mean density of <jats:italic>ρ</jats:italic><jats:sub>p</jats:sub> =0.91 ± 0.15 g cm<jats:sup>−3</jats:sup>. 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 found in the Neptunian desert, and is a promising candidate for atmospheric characterization using the <jats:italic>James Webb</jats:italic> Space Telescope.</jats:p>