A Super-earth Transiting a Naked-eye Star

We have detected transits of the innermost planet “e” orbiting 55 Cnc (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 d) 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+0.051 −0.029 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+1.5 −1.1 g cm −3. The mean density is comparable to that of Earth, despite the greater mass and consequently greater compression of the interior of 55 Cnc e. 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± 70 ppm) 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 55 Cnc e is brighter than that of any other known transiting planet, which will facilitate future investigations. Subject headings: planetary systems — planets and satellites: formation, interiors — stars: individual (55 Cnc)