Kepler-20: A Sun-like Star with Three Sub-Neptune Exoplanets and Two Earth-size Candidates

Abstract

We present the discovery of the Kepler-20 planetary system, which we initially identified through the detection of five distinct periodic transit signals in the Kepler light curve of the host star 2MASSJ19104752+4220194. We find a stellar effective temperature T_(eff)=5455±100K, a metallicity of [Fe/H]=0.01±0.04, and a surface gravity of log(g)=4.4±0.1. Combined with an estimate of the stellar density from the transit light curves we deduce a stellar mass of M_*=0.912±0.034 M_⊙ and a stellar radius of R_*=0.944^(+0.060)_(-0.095) R_⊙. For three of the transit signals, our results strongly disfavor the possibility that these result from astrophysical false positives. We conclude that the planetary scenario is more likely than that of an astrophysical false positive by a factor of 2 x 10^5 (Kepler-20b), 1 x 10^5 (Kepler-20c), and 1.1 x 10^3 (Kepler-20d), sufficient to validate these objects as planetary companions. For Kepler-20c and Kepler-20d, the blend scenario is independently disfavored by the achromaticity of the transit: From Spitzer data gathered at 4.5µm, we infer a ratio of the planetary to stellar radii of 0.075±0.015 (Kepler-20c) and 0.065±0.011 (Kepler-20d), consistent with each of the depths measured in the Kepler optical bandpass. We determine the orbital periods and physical radii of the three confirmed planets to be 3.70d and 1.91^(+0.12)_(-0.21) R_⊕ for Kepler-20b, 10.85 d and 3.07^(+0.20)_(-0.31) R_⊕ for Kepelr-20c, and 77.61 d and 2.75^(+0.17)_(-0.30) R_⊕ for Kepler-20d. From multi-epoch radial velocities, we determine the masses of Kepler-20b and Kepler-20c to be 8.7±2.2 M_⊕ and 16.1±3.5 M_⊕, respectively, and we place an upper limit on the mass of Kepler-20d of 20.1 M_⊕ (2 σ).

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