HAT-P-11b: A Super-Neptune Planet Transiting a Bright K Star in the Kepler Field

We report on the discovery of HAT-P-11b, the smallest radius transiting extrasolar planet (TEP) discovered from the ground, and the first hot Neptune discovered to date by transit searches. HAT-P-11b orbits the bright (V = 9.587) and metal rich ([Fe/H] = +0.31 ± 0.05) K4 dwarf star GSC 03561-02092 with P = 4.8878162 ± 0.0000071 days and produces a transit signal with depth of 4.2 mmag, the shallowest found by transit searches that is due to a confirmed planet. We present a global analysis of the available photometric and radial velocity (RV) data that result in stellar and planetary parameters, with simultaneous treatment of systematic variations. The planet, like its near-twin GJ 436b, is somewhat larger than Neptune (17 M _⊕, 3.8 R _⊕) both in mass M_p = 0.081 ± 0.009 M _J(25.8 ± 2.9 M _⊕) and radius R_p = 0.422 ± 0.014 R _J(4.73 ± 0.16 R _⊕). HAT-P-11b orbits in an eccentric orbit with e = 0.198 ± 0.046 and ω = 355fdg2 ± 17fdg3, causing a reflex motion of its parent star with amplitude 11.6 ± 1.2 m s^-1, a challenging detection due to the high level of chromospheric activity of the parent star. Our ephemeris for the transit events is T_c = 2454605.89132 ± 0.00032 (BJD), with duration 0.0957 ± 0.0012 days, and secondary eclipse epoch of 2454608.96 ± 0.15 days (BJD). The basic stellar parameters of the host star are M _sstarf = 0.809^+0.020 _-0.027 M _sun, R _sstarf = 0.752 ± 0.021 R _sun, and T _effsstarf = 4780 ± 50 K. Importantly, HAT-P-11 will lie on one of the detectors of the forthcoming Kepler mission; this should make possible fruitful investigations of the detailed physical characteristic of both the planet and its parent star at unprecedented precision. We discuss an interesting constraint on the eccentricity of the system by the transit light curve and stellar parameters. This will be particularly useful for eccentric TEPs with low-amplitude RV variations in Kepler's field. We also present a blend analysis, that for the first time treats the case of a blended transiting hot Jupiter mimicking a transiting hot Neptune, and proves that HAT-P-11b is not such a blend.

Based in part on observations obtained at the W. M. Keck Observatory, which is operated by the University of California and the California Institute of Technology. Keck time has been granted by NOAO (A285Hr) and NASA (N128Hr).