Distribution of confirmed exoplanets in the semi-major axis vs. planetary mass plane (http://exoplanet.eu). The black points are exoplanets observed by the radial-velocity method. The red points are exoplanets observed by the radial-velocity + transit methods. As for masses of exoplanets indicated with the black points, the observed values of M_p sin i are used. The grey area shows the theoretical sweet spot for planetesimal capture given by Eqs. (61) and (62) with C = 10. The red dashed line shows the theoretical sweet spot for planetesimal capture with C = 10³, which corresponds to the case where collisional cascade have broken planetesimals down to R_pl ≲ 10⁵ cm.