TOI-2196 b: a 24 M⊕ planet in the hot Neptune desert transiting a G-type star discovered by TESS

The hot Neptune desert is a dearth of short-period (sub-)Neptunes in the radius-period diagram. Highly irradiated planets are usually either small (R < 2 Earth radii) and rocky, or Hot Jupiters with low densities. Here we report on the discovery of TOI-2196 b (TIC 372172128.01), a sub-Neptune planet on a 1.2-day orbit transiting a G-type star (V = 12.0, [Fe/H] = 0.24) discovered by TESS in sector 27. We collected 42 high-precision radial velocity measurements with the HARPS spectrograph to confirm the planetary nature of the transit signal and determine the planet mass. TOI-2196 b has a radius of 3.4 ± 0.3 Earth radii, which combined with the mass of 25.6 ± 1.3 Earth masses results in a mean density of 3.5 ± 1.0 g/cm3. A significant trend in the HARPS radial velocity measurements points to the presence of a more distant companion in this system. The short period of TOI-2196 b implies a high equilibrium temperature of 1850 K, thereby establishing the planet as one of few well-characterized (sub-)Neptunes in the hot Neptune desert that bridges the gap between small, rocky planets and gas giants. Depending on the stellar activity during its youth, the planet can have lost up to 50% of its primordial atmosphere and has thus been able to retain a large fraction of its atmosphere.