Inside-out planet formation: VI. oligarchic coagulation of planetesimals from a pebble ring?

Inside-Out Planet Formation (IOPF) is a theory addressing the origin of Systems of Tightly-Packed Inner Planets (STIPs) via in situ formation and growth of the planets. It predicts that a pebble ring is established at the pressure maximum associated with the dead zone inner boundary (DZIB) with an inner disc magnetorotational instability (MRD-active region. Using direct N-body simulations, we study the collisional evolution of planetesimals formed from such a pebble ring, in particular, examining whether a single dominant planet emerges. We consider a variety of models, including some in which the planetesimals are continuing to grow via pebble accretion. We find that the planetesimal ring undergoes oligarchic evolution, and typically turns into 2 or 3 surviving oligarchs on nearly coplanar and circular orbits, independent of the explored initial conditions or form of pebble accretion. The most massive oligarchs typically consist of about 70 per cent of the total mass, with the building-up process typically finishing within similar to 10(5) yr. However, a relatively massive secondary planet always remains with similar to 30-65 per cent of the mass of the primary. Such secondary planets have properties that are inconsistent with the observed properties of the innermost pairs of planets in STIPs. Thus, for IOPF to be a viable theory for STIP formation, it needs to be shown how oligarchic growth of a relatively massive secondary from the initial pebble ring can be avoided. We discuss some potential additional physical processes that should be included in the modelling and explored as next steps.

Automated summary

This study investigates the formation process of planetesimals in the theory of Inside-Out Planet Formation (IOPF) through collisional evolution simulations. The research finds that the planetesimal ring undergoes oligarchic evolution, resulting in 2 or 3 surviving oligarchs on nearly coplanar and circular orbits. However, a relatively massive secondary planet always remains, which is inconsistent with the observed properties of innermost planet pairs.