KEPLER-21b: A 1.6 R-Earth PLANET TRANSITING THE BRIGHT OSCILLATING F SUBGIANT STAR HD 179070

We present Kepler observations of the bright (V = 8.3), oscillating star HD 179070. The observations show transit-like events which reveal that the star is orbited every 2.8 days by a small, 1.6 R-Earth object. Seismic studies of HD 179070 using short cadence Kepler observations show that HD 179070 has a frequency-power spectrum consistent with solar-like oscillations that are acoustic p-modes. Asteroseismic analysis provides robust values for the mass and radius of HD 179070, 1.34 +/- 0.06 M-circle dot and 1.86 +/- 0.04 R-circle dot, respectively, as well as yielding an age of 2.84 +/- 0.34 Gyr for this F5 subgiant. Together with ground-based follow-up observations, analysis of the Kepler light curves and image data, and blend scenario models, we conservatively show at the >99.7% confidence level (3 sigma) that the transit event is caused by a 1.64 +/- 0.04 R-Earth exoplanet in a 2.785755 +/- 0.000032 day orbit. The exoplanet is only 0.04 AU away from the star and our spectroscopic observations provide an upper limit to its mass of similar to 10 M-Earth (2 sigma). HD 179070 is the brightest exoplanet host star yet discovered by Kepler.