  As many as 5 ice giants--Neptune-mass planets composed of 90% ice and rock
and 10% hydrogen--are thought to form at heliocentric distances of 10-25 AU on
closely packed orbits spaced ~5 Hill radii apart. Such oligarchies are
ultimately unstable. Once the parent disk of planetesimals is sufficiently
depleted, oligarchs perturb one another onto crossing orbits. We explore both
the onset and the outcome of the instability through numerical integrations,
including dynamical friction cooling of planets by a planetesimal disk whose
properties are held fixed. To trigger instability and the ejection of the first
ice giant in systems having an original surface density in oligarchs of Sigma ~
1 g/cm^2, the disk surface density s must fall below 0.1 g/cm^2. Ejections are
predominantly by Jupiter and occur within 10 Myr. To eject more than 1 oligarch
requires s < 0.03 g/cm^2. Systems starting with up to 4 oligarchs in addition
to Jupiter and Saturn can readily yield solar-system-like outcomes in which 2
surviving ice giants lie inside 30 AU and have their orbits circularized by
dynamical friction. Our numerical simulations support the idea that planetary
systems begin in more crowded and compact configurations, like those of
shear-dominated oligarchies. In contrast to previous studies, we identify s <
0.1 Sigma as the regime relevant for understanding the evolution of the outer
solar system, and we encourage future studies to concentrate on this regime
while relaxing our assumption of a fixed planetesimal disk.
