The formation of ice giants in a packed oligarchy: Instability and aftermath

As many as five ice giants - Neptune-mass planets composed of similar to 90% ice and rock and similar to 10% hydrogen - are thought to form at heliocentric distances of similar to 10 - 25 AU on closely packed orbits spaced similar to 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 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 similar to 1 g cm similar to 2, the disk surface density similar to must fall below similar to 0.1 g cm similar to 2. Ejections are predominantly by Jupiter and occur within similar to 107 yr. To eject more than one oligarch requires similar to P0: 03 g cm similar to 2. For certain choices of sigma and initial semimajor axes of planets, systems starting with up to four oligarchs in addition to Jupiter and Saturn can readily yield solar system - like outcomes in which two 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 similar to P0: 1 similar to 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 greater than or similar to 20 similar to as is observed.