KEPLER-18b, c, AND d: A SYSTEM OF THREE PLANETS CONFIRMED BY TRANSIT TIMING VARIATIONS, LIGHT CURVE VALIDATION, WARM-SPITZER PHOTOMETRY, AND RADIAL VELOCITY MEASUREMENTS

We report the detection of three transiting planets around a Sun-like star, which we designate Kepler-18. The transit signals were detected in photometric data from the Kepler satellite, and were confirmed to arise from planets using a combination of large transit-timing variations (TTVs), radial velocity variations, Warm-Spitzer observations, and statistical analysis of false-positive probabilities. The Kepler-18 star has a mass of 0.97M(circle dot), a radius of 1.1R(circle dot), an effective temperature of 5345 K, and an iron abundance of [Fe/H] = +0.19. The planets have orbital periods of approximately 3.5, 7.6, and 14.9 days. The innermost planet b is a super-Earth with a mass of 6.9 +/- 3.4M(circle plus), a radius of 2.00 +/- 0.10R(circle plus), and a mean density of 4.9 +/- 2.4 g cm(3). The two outer planets c and d are both low-density Neptune-mass planets. Kepler-18c has a mass of 17.3 +/- 1.9 M-circle plus, a radius of 5.49 +/- 0.26R(circle plus), and a mean density of 0.59 +/- 0.07 g cm(3), while Kepler-18d has a mass of 16.4 +/- 1.4 M-circle plus, a radius of 6.98 +/- 0.33 R-circle plus and a mean density of 0.27 +/- 0.03 g cm(.)(3) Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected TTVs.