  Though ~10 Earth mass rocky/icy cores are commonly held as a prerequisite for
the formation of gas giants, theoretical models still struggle to explain how
these embryos can form within the lifetimes of gaseous circumstellar disks. In
recent years, aerodynamic-aided accretion of "pebbles," objects ranging from
centimeters to meters in size, has been suggested as a potential solution to
this long-standing problem. While pebble accretion has been demonstrated to be
extremely effective in local simulations that look at the detailed behavior of
these pebbles in the vicinity of a single planetary embryo, to date there have
been no global simulations demonstrating the effectiveness of pebble accretion
in a more complicated, multi-planet environment. Therefore, we have
incorporated the aerodynamic-aided accretion physics into LIPAD, a Lagrangian
code that can follow the collisional / accretional / dynamical evolution of a
protoplanetary system, to investigate how pebble accretion manifests itself in
the larger planet formation picture. We find that under generic circumstances,
pebble accretion naturally leads to an "oligarchic" type of growth in which a
large number of planetesimals grow to similar-sized planets. In particular, our
simulations tend to form hundreds of Mars- and Earth-mass objects between 4 and
10 AU. While the merging of some oligarchs may allow them to grow massive
enough to form giant planet cores, leftover oligarchs lead to planetary systems
that cannot be consistent with our own solar system. We investigate various
ideas presented in the literature (including evaporation fronts and planet
traps) and find that none easily overcome this tendency toward oligarchic
growth.
