KELT-10b: The first transiting exoplanet from the KELT-South survey - a hot sub-Jupiter transiting a V = 10.7 early G-star

We report the discovery of KELT-10b, the first transiting exoplanet discovered using the KELT-South telescope. KELT-10b is a highly inflated sub-Jupiter mass planet transiting a relatively bright V = 10.7 star (TYC 8378-64-1), with Teff = 5948 ± 74 K, log g = 4.319-0.030+0.020 and [Fe/H] = 0.09-0.10+0.11, an inferred mass M* = 1.112-0.061+0.055 M⊙ and radius R* = 1.209-0.035+0.047 R⊙. The planet has a radius Rp = 1.399-0.049+0.069 RJ and mass Mp = 0.679-0.038+0.039 MJ. The planet has an eccentricity consistent with zero and a semimajor axis a = 0.052 50-0.000 97+0.000 86 au. The best-fitting linear ephemeris is T0 = 2457 066.720 45 ± 0.000 27 BJDTDB and P = 4.166 2739 ± 0.000 0063 d. This planet joins a group of highly inflated transiting exoplanets with a larger radius and smaller mass than that of Jupiter. The planet, which boasts deep transits of 1.4 per cent, has a relatively high equilibrium temperature of Teq = 1377-23+28 K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817-0.054+0.068 × 109 erg s-1 cm-2, which places it far above the insolation threshold above which hot Jupiters exhibit increasing amounts of radius inflation. Evolutionary analysis of the host star suggests that KELT-10b may not survive beyond the current subgiant phase, depending on the rate of in-spiral of the planet over the next few Gyr. The planet transits a relatively bright star and exhibits the third largest transit depth of all transiting exoplanets with V < 11 in the Southern hemisphere, making it a promising candidate for future atmospheric characterization studies.