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

Context. 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. Aims. In this article, our aim is to confirm the planetary nature of the transiting planet candidate TOI-674b using spectroscopic and photometric observations. Methods. 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. Results. 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 M = 0.420 +/- 0.010 M-circle dot, R = 0.420 +/- 0.013 R-circle dot, and T-eff = 3514 +/- 57 K; it is located at a distance d = 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 x 10(-6) days, a planetary radius of 5.25 +/- 0.17 R-circle plus, and a mass of 23.6 +/- 3.3 M-circle plus implying a mean density of rho(p) =0.91 +/- 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 found in the Neptunian desert, and is a promising candidate for atmospheric characterization using the James Webb Space Telescope.

Automated summary

This article confirms and characterizes the planetary nature of the transiting planet candidate TOI-674b using various observational methods. TOI-674b is identified as a low-density super-Neptune orbiting a nearby M dwarf in the Neptunian desert. It is a promising target for atmospheric characterization with the James Webb Space Telescope.