K2-161b: A low-density super-neptune on an eccentric orbit

Abstract

© 2018 The Author(s).We report the discovery of K2-161b, which was first identified as a planetary candidate from Kepler K2 photometry of Campaign 14, and whose planetary nature and orbital parameters were then confirmed with precision radial velocities. K2-161b is half as massive as Saturn (MP = 0.179 ± 0.021 MJ), and has a radius of RP = 0.840 ± 0.011 RJ, which translates into a bulk density of ρP = 0.37 ± 0.05 g cm-3. K2-161b transits its slightly evolved G-type host star (M∗ = 1.105 ± 0.019 M⊙, R∗ = 1.669 ± 0.022 R⊙) every 11.633 64 ± 0.000 10 d and presents a significantly eccentric orbit (e = 0.420 ± 0.034). We estimate a relatively short circularization time-scale of 1.8 Gyr for the planet, but given the advanced age of the system we expect the planet to be engulfed by its evolving host star in ∼1 Gyr before the orbit circularizes. The low density of the planet coupled to the brightness of the host star (J = 9.4) makes this system one of the best candidates known to date in the