  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_{-0.029}^{+0.051} M_sun and 0.943 +/-
0.010 R_sun, the planet's mass, radius, and mean density are 8.63 +/- 0.35
Mearth, 2.00 +/- 0.14 Rearth, and 5.9_{-1.1}^{+1.5} 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.
