  When a planet forms a deep gap in a protoplanetary disk, dust grains cannot
pass through the gap. As a consequence, the density of the dust grains can
increase up to the same level of the density of the gas at the outer edge. The
feedback on the gas from the drifting dust grains is not negligible, in such a
dusty region. We carried out two-dimensional two-fluid (gas and dust)
hydrodynamic simulations. We found that when the radial flow of the dust grains
across the gap is halted, a broad ring of the dust grains can be formed because
of the dust feedback and the diffusion of the dust grains. The minimum mass of
the planet to form the broad dust ring is consistent with the pebble-isolation
mass, in the parameter range of our simulations. The broad ring of the dust
grains is good environment for the formation of the protoplanetary solid core.
If the ring is formed in the disk around the sun-like star at $\sim 2\
\mbox{AU}$, a massive solid core ($\sim 50M_{\oplus}$) can be formed within the
ring, which may be connected to the formation of Hot Jupiters holding a massive
solid core such as HD 149026b. In the disk of the dwarf star, a number of
Earth-sized planets can be formed within the dust ring around $\sim 0.5\
\mbox{AU}$, which potentially explain the planet system made of multiple
Earth-sized planets around the dwarf star such as TRAPPIST-1.
