As many as 5 ice giants—Neptune-mass planets composed of \sim 90 % ice and rock and \sim 10 % hydrogen—are thought to form at heliocentric distances of \sim 10–25 AU on closely packed orbits spaced \sim 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 \sim 1 { g } / { cm } ^ { 2 } , the disk surface density \sigma must fall below \sim 0.1 { g } / { cm } ^ { 2 } .
Ejections are predominantly by Jupiter and occur within \sim 10 ^ { 7 } { yr } .
To eject more than 1 oligarch requires \sigma \lesssim 0.03 { g } / { cm } ^ { 2 } .
For certain choices of \sigma and initial semi-major axes of planets , 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 findings 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 \sigma \lesssim 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 .
Whether evidence of the instability can be found in Kuiper belt objects ( KBOs ) is unclear , since in none of our simulations do marauding oligarchs excite as large a proportion of KBOs having inclinations \gtrsim 20 ^ { \circ } as is observed .