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 0.97 M _⊙, a radius of 1.1 R _⊙, an effective temperature of 5345K, 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 ± 3.4 M _⊕, a radius of 2.00 ± 0.10 R _⊕, and a mean density of 4.9 ± 2.4gcm³. 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 _⊕, a radius of 5.49 ± 0.26 R _⊕, and a mean density of 0.59 0.07gcm ³, while Kepler-18d has a mass of 16.4 ± 1.4 M _⊕, a radius of 6.98 ± 0.33 R _⊕ and a mean density of 0.27 ± 0.03gcm³. Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected TTVs.