We report the discovery and the Rossiter-McLaughlin effect of Kepler-8b , a transiting planet identified by the NASA Kepler Mission . Kepler photometry and Keck-HIRES radial velocities yield the radius and mass of the planet around this F8IV subgiant host star .
The planet has a radius R _ { P } = 1.419 R _ { J } and a mass , M _ { P } = 0.60 M _ { J } , yielding a density of 0.26 g cm ^ { -3 } , among the lowest density planets known .
The orbital period is P = 3.523 days and orbital semimajor axis is 0.0483 ^ { +0.0006 } _ { -0.0012 } AU .
The star has a large rotational v \sin { i } Â of 10.5 \pm 0.7 Â km s ^ { -1 } and is relatively faint ( V \approx 13.89Â mag ) , both properties deleterious to precise Doppler measurements .
The velocities are indeed noisy , with scatter of 30 m s ^ { -1 } , but exhibit a period and phase consistent with the planet implied by the photometry .
We securely detect the Rossiter-McLaughlin effect , confirming the planet ’ s existence and establishing its orbit as prograde .
We measure an inclination between the projected planetary orbital axis and the projected stellar rotation axis of \lambda = -26.9 \pm 4.6 ^ { \circ } , indicating a moderate inclination of the planetary orbit .
Rossiter-McLaughlin measurements of a large sample of transiting planets from Kepler will provide a statistically robust measure of the true distribution of spin-orbit orientations for hot jupiters in general .