  Through photometric monitoring of the extended transit window of HD 97658b
with the MOST space telescope, we have found that this exoplanet transits with
an ephemeris consistent with that predicted from radial velocity measurements.
The mid-transit times are $5.6\sigma$ earlier than those of the unverified
transit-like signals reported in 2011, and we find no connection between the
two sets of events. The transit depth together with our determined stellar
radius ($R_\star = 0.703^{+0.039}_{-0.034} R_\odot$) indicates a
2.34$^{+0.18}_{-0.15}$ $R_{\oplus}$ super-Earth. When combined with the radial
velocity determined mass of 7.86 $\pm 0.73$ $M_{\oplus}$, our radius measure
allows us to derive a planet density of 3.44$^{+0.91}_{-0.82}$ g cm$^{-3}$.
Models suggest that a planet with our measured density has a rocky core that is
enveloped in an atmosphere composed of lighter elements. The star of the HD
97658 system is the second brightest known to host a transiting super-Earth,
facilitating follow-up studies of this not easily daunted, warm and likely
volatile-rich exoplanet.
