  Super-Earths transiting nearby bright stars are key objects that
simultaneously allow for accurate measurements of both their mass and radius,
providing essential constraints on their internal composition. We present here
the confirmation, based on Spitzer transit observations, that the super-Earth
HD 97658 b transits its host star. HD 97658 is a low-mass
($M_*=0.77\pm0.05\,M_{\odot}$) K1 dwarf, as determined from the Hipparcos
parallax and stellar evolution modeling. To constrain the planet parameters, we
carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST
and Spitzer photometry. HD 97658 b is a massive ($M_P=7.55^{+0.83}_{-0.79}
M_{\oplus}$) and large ($R_{P} = 2.247^{+0.098}_{-0.095} R_{\oplus}$ at 4.5
$\mu$m) super-Earth. We investigate the possible internal compositions for HD
97658 b. Our results indicate a large rocky component, by at least 60% by mass,
and very little H-He components, at most 2% by mass. We also discuss how future
asteroseismic observations can improve the knowledge of the HD 97658 system, in
particular by constraining its age. Orbiting a bright host star, HD 97658 b
will be a key target for coming space missions TESS, CHEOPS, PLATO, and also
JWST, to characterize thoroughly its structure and atmosphere.
