K2-155: A Bright Metal-poor M Dwarf with Three Transiting Super-Earths

We report on the discovery of three transiting super-Earths around K2-155 (EPIC 210897587), a relatively bright early M dwarf (V=12.81 mag) observed during Campaign 13 of the NASA K2 mission. To characterize the system and validate the planet candidates, we conducted speckle imaging and high-dispersion optical spectroscopy, including radial velocity measurements. Based on the K2 light curve and the spectroscopic characterization of the host star, the planet sizes and orbital periods are 1.55(+0.20)(−0.17)R⊕ and 6.34365±0.00028 days for the inner planet; 1.95(+0.27)(−0.22)R⊕ and 13.85402±0.00088 days for the middle planet; and 1.64(+0.18)(−0.17)R⊕ and 40.6835±0.0031 days for the outer planet. The outer planet (K2-155d) is near the habitable zone, with an insolation 1.67±0.38 times that of the Earth. The planet's radius falls within the range between that of smaller rocky planets and larger gas-rich planets. To assess the habitability of this planet, we present a series of 3D global climate simulations assuming that K2-155d is tidally locked and has an Earth-like composition and atmosphere. We find that the planet can maintain a moderate surface temperature if the insolation proves to be smaller than ∼1.5 times that of the Earth. Doppler mass measurements, transit spectroscopy, and other follow-up observations should be rewarding, since K2-155 is one of the optically brightest M dwarfs known to harbor transiting planets.