2021AJ....162..292A

We present the discovery of a highly irradiated and moderately inflated ultrahot Jupiter, TOI-1431b/MASCARA-5 b (HD 201033b), first detected by NASA's Transiting Exoplanet Survey Satellite mission (TESS) and the Multi-site All-Sky Camera (MASCARA). The signal was established to be of planetary origin through radial velocity measurements obtained using SONG, SOPHIE, FIES, NRES, and EXPRES, which show a reflex motion of K = 294.1 ± 1.1 m s^–1. A joint analysis of the TESS and ground-based photometry and radial velocity measurements reveals that TOI-1431b has a mass of M_p = 3.12 ± 0.18 M_J (990 ± 60 M_⊕), an inflated radius of R_p = 1.49 ± 0.05 R_J (16.7 ± 0.6 R_⊕), and an orbital period of P = 2.650237 ± 0.000003 days. Analysis of the spectral energy distribution of the host star reveals that the planet orbits a bright (V = 8.049 mag) and young (0.29_–0.19^+0.32 Gyr) Am type star with T_eff=7690_–250⁺⁴⁰⁰ K, resulting in a highly irradiated planet with an incident flux of <F>=7.24_–0.64^+0.68×10⁹ erg s^–1 cm^–2 (5300_–470⁺⁵⁰⁰S_⊕) and an equilibrium temperature of T_eq = 2370 ± 70 K. TESS photometry also reveals a secondary eclipse with a depth of 127_–5⁺⁴ ppm as well as the full phase curve of the planet's thermal emission in the red-optical. This has allowed us to measure the dayside and nightside temperature of its atmosphere as T_day = 3004 ± 64 K and T_night = 2583 ± 63 K, the second hottest measured nightside temperature. The planet's low day/night temperature contrast (∼420 K) suggests very efficient heat transport between the dayside and nightside hemispheres. Given the host star brightness and estimated secondary eclipse depth of ∼1000 ppm in the K band, the secondary eclipse is potentially detectable at near-IR wavelengths with ground-based facilities, and the planet is ideal for intensive atmospheric characterization through transmission and emission spectroscopy from space missions such as the James Webb Space Telescope and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey.