Kepler-210: An active star with at least two planets

Abstract

We report the detection and characterization of two short-period, Neptune-sized planets around the active host star Kepler-210. The host star's parameters derived from those planets are (a) mutually inconsistent and (b) do not conform to the expected host star parameters. We furthermore report the detection of transit timing variations (TTVs) in the O-C diagrams for both planets. We explore various scenarios that explain and resolve those discrepancies. A simple scenario consistent with all data appears to be one that attributes substantial eccentricities to the inner short-period planets and that interprets the TTVs as due to the action of another, somewhat longer period planet. To substantiate our suggestions, we present the results of N-body simulations that modeled the TTVs and that checked the stability of the Kepler-210 system.

Differential rotation of stars with multiple transiting planets

“…A total of 302 transits were used for the first planet, and 86 for the second one. The fitted parameters resulted in an increase in the semi-major axis and in the orbital inclination angle with respect to those reported by Ioannidis et al (2014). On the other hand, the fits resulted in a decrease in the planet radius of 1.6% and 7.0% for the Kepler-210b and Kepler-210c, respectively.…”

“…The five parameter that were fitted are planet radius, semi-major axis, inclination angle for the planets and limb darkening coefficients for the star. Different values resulted from this fit with respect to the values reported by Ioannidis et al (2014). The same procedure was applied to the 86 transits of the larger planet, Kepler-210c.…”

“…Kepler-210 is an active star, late K star with an estimated age of 350 ± 50 Myrs. With a radius of 0.69 R , mass of 0.63 M and an average rotation period of 12.33 days, it hosts two transiting Neptune-like planets (Ioannidis et al 2014).…”

Glimpses of stellar surfaces

“…The Kepler-210 system consists of a K dwarf with at least two planets orbiting around it (Ioannidis et al 2014). The Kepler light curve of the system was obtained from the STDADS 1 archive and contains the long cadence data from quarters Q1 to Q17.…”

“…In Fig. 1, we show the approximately 1400 days long light curve of Kepler-210 with each quarter normalized by its mean and the planetary transits removed (using the parameters calculated by Ioannidis et al 2014). The light curve shows clear modulations with an amplitude of ∼2%, similar to photospherically active stars, e.g., CoRoT-2 (cf., Alonso et al 2008;Huber et al 2009).…”