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 \pm 0.05 ) K4 dwarf star GSC 03561-02092 with P = 4.8878162 \pm 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 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 _ { \earth } , 3.8 R _ { \earth } ) both in mass M _ { p } = 0.081 \pm 0.009 M _ { J } ( 25.8 \pm 2.9 M _ { \earth } ) and radius R _ { p } = 0.422 \pm 0.014 R _ { J } ( 4.73 \pm 0.16 R _ { \earth } ) .
HAT-P-11b orbits in an eccentric orbit with e = 0.198 \pm 0.046 and \omega = 355.2 \pm 17.3 \arcdeg , causing a reflex motion of its parent star with amplitude 11.6 \pm 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 \pm 0.00032 ( BJD ) , with duration 0.0957 \pm 0.0012 d , and secondary eclipse epoch of 2454608.96 \pm 0.15 d ( BJD ) .
The basic stellar parameters of the host star are M _ { \star } = 0.809 _ { -0.027 } ^ { +0.020 } M _ { \sun } , R _ { \star } = 0.752 \pm 0.021 R _ { \sun } and T _ { eff \star } = 4780 \pm 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 mimicing a transiting hot Neptune , and proves that HAT-P-11b is not such a blend .