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Early time-series photometry from NASA's Kepler spacecraft has revealed aplanet transiting the star we term Kepler-4, at RA = 19h02m27.68s, Dec =+50:08:08.7. The planet has an orbital period of 3.213 days and shows transitswith a relative depth of 0.87 x 10^{-3} and a duration of about 3.95 hours.Radial velocity measurements from the Keck HIRES spectrograph show a reflexDoppler signal of 9.3 (+1.1 -1.9) m/s, consistent with a low-eccentricity orbitwith the phase expected from the transits. Various tests show no evidence forany companion star near enough to affect the light curve or the radialvelocities for this system. From a transit-based estimate of the host star'smean density, combined with analysis of high-resolution spectra, we infer thatthe host star is near turnoff from the main sequence, with estimated mass andradius of 1.223 (+0.053 -0.091) solar masses and 1.487 (+0.071 -0.084) solarradii. We estimate the planet mass and radius to be 24.5 +/- 3.8 Earth massesand 3.99 +/- 0.21 Earth radii. The planet's density is near 1.9 g/cm^3; it isthus slightly denser and more massive than Neptune, but about the same size.

Kepler-4b: Hot Neptune-Like Planet of a G0 Star Near Main-Sequence Turnoff