  Identification of the main planet formation site is fundamental to
understanding how planets form and migrate to the current locations. We
consider the heavy-element content trend of observed exoplanets derived from
improved measurements of mass and radius, and explore how this trend can be
used as a tracer of their formation sites. Using gas accretion recipes obtained
from detailed hydrodynamical simulations, we confirm that the disk-limited gas
accretion regime is most important for reproducing the heavy-element content
trend. Given that such a regime is specified by two characteristic masses of
planets, we compute these masses as a function of the distance ($r$) from the
central star, and then examine how the regime appears in the mass-semimajor
axis diagram. Our results show that a plausible solid accretion region emerges
at $r \simeq 0.6$ au and expands with increasing $r$, using the conventional
disk model. Given that exoplanets that possess the heavy-element content trend
distribute currently near their central stars, our results imply the importance
of planetary migration that would occur after solid accretion onto planets
might be nearly completed at $r \geq 0.6$ au. Self-consistent simulations would
be needed to verify the predictions herein.
