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 _ { \odot } and 0.943 \pm 0.010 R _ { \odot } , the planet ’ s mass , radius , and mean density are 8.63 \pm 0.35 M _ { \oplus } , 2.00 \pm 0.14 R _ { \oplus } , 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 \pm 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 .