FIVE PLANETS TRANSITING A NINTH MAGNITUDE STAR

Abstract

ABSTRACT The Kepler mission has revealed a great diversity of planetary systems and architectures, but most of the planets discovered by Kepler orbit faint stars. Using new data from the K2 mission, we present the discovery of a five-planet system transiting a bright (V = 8.9, K = 7.7) star called HIP 41378. HIP 41378 is a slightly metal-poor late F-type star with moderate rotation (v sin i ≃ 7 km s − 1 ) and lies at a distance of 116 ± 18 pc from Earth. We find that HIP 41378 hosts two sub-Neptune-sized planets orbiting 3.5% outside a 2:1 period commensurability in 15.6 and 31.7 day orbits. In addition, we detect three planets that each transit once during the 75 days spanned by K2 observations. One planet is Neptune-sized in a likely ∼160 day orbit, one is sub-Saturn-sized, likely in a ∼130 day orbit, and one is a Jupiter-sized planet in a likely ∼1 year orbit. We show that these estimates for the orbital periods can be made more precise by taking into account dynamical stability considerations. We also calculate the distribution of stellar reflex velocities expected for this system, and show that it provides a good target for future radial velocity observations. If a precise orbital period can be determined for the outer Jovian planets through future observations, this system will be an excellent candidate for follow-up transit observations to study its atmosphere and measure its oblateness.