The CARMENES search for exoplanets around M dwarfs - LP 714-47 b (TOI 442.01): populating the Neptune desert

We report the discovery of a Neptune-like planet (LP 714-47 b, mb = 30.8 ± 1.5 M⊕ , Rb = 4.7 ± 0.3 R⊕ ) located in the “hot Nep- tune desert.” The confirmation of the TESS Object of Interest (TOI 442.01) was achieved with radial-velocity follow-up using CARMENES, ESPRESSO, Keck/HIRES, iSHELL, and PFS, as well as from photometric data using TESS, Spitzer, and ground- based photometry from MuSCAT2, TRAPPIST-South, MONET-South, the George Mason University telescope, the Las Cumbres Observatory Global Telescope network, the El Sauce telescope, the TÜB ̇ITAK National Observatory, the University of Louisville Manner Telescope, and WASP-South. We also present high-spatial resolution adaptive optics imaging with the Gemini Near-Infrared Imager. The low uncertainty in the mass and radius determination place LP 714-47 b among the physically well characterised plan- ets, allowing for a meaningful comparison with planet structure models. The host star LP 714-47 is a slowly rotating early M dwarf (Teff = 3950 ± 51 K) with a mass of 0.59 ±0.02 M⊙ and a radius of 0.58 ±0.02 R⊙. From long-term photometric monitoring and spec- troscopic activity indicators, we determine a stellar rotation period of about 33 d. The stellar activity is also manifested as correlated noise in the radial velocity data. In the power spectrum of the radial velocity data we detect a second signal with a period of 16 days in addition to the 4-day signal of the planet. This could be a harmonic of the stellar rotation period or the signal of a second planet. It may be possible to tell the difference after more TESS data and radial-velocity data are obtained.