A super-earth transiting a naked-eye star

We have detected transits of the innermost planet "e" orbiting 55Cnc (V = 6.0), based on two weeks of nearly continuous photometric monitoring with the MOST space telescope. The transits occur with the period (0.74 days) and phase that had been predicted by Dawson & Fabrycky, and with the expected duration and depth for the crossing of a Sun-like star by a hot super-Earth. Assuming the star's mass and radius to be 0.963_-0.029 ^+0.051 M ⊙and 0.943±0.010 R ⊙, the planet's mass, radius, and mean density are 8.63±0.35 M _⊕, 2.00±0.14 R_⊕, and 5.9_-1.1 ^+1.5gcm^-3, respectively. The mean density is comparable to that of Earth, despite the greater mass and consequently greater compression of the interior of 55Cnce. This suggests a rock-iron composition supplemented by a significant mass of water, gas, or other light elements. Outside of transits, we detected a sinusoidal signal resembling the expected signal due to the changing illuminated phase of the planet, but with a full range (168±70ppm) too large to be reflected light or thermal emission. This signal has no straightforward interpretation and should be checked with further observations. The host star of 55Cnce is brighter than that of any other known transiting planet, which will facilitate future investigations.