Kepler-4b: A hot neptune-like planet of a g0 star near main-sequence turnoff

Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at R.A. = 19^h02 ^m2768, δ = +50°08′087. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 × 10^-3 and a duration of about 3.95 hr. Radial velocity (RV) measurements from the Keck High Resolution Echelle Spectrometer show a reflex Doppler signal of 9.3^+1.1 _-1.9 m s^-1, consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the RVs for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223^+0.053 _-0.091 M _⊙ and 1.487^+0.071 _-0.084 R _⊙. We estimate the planet mass and radius to be {M _P, R _P} = {24.5 3.8 M _⊕, 3.99 0.21 R _⊕}. The planet's density is near 1.9 g cm^-3; it is thus slightly denser and more massive than Neptune, but about the same size.