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

Journal article, 2022

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 (MRI)-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 ∼10 5 yr. Ho we ver, a relati vely massi ve secondary planet al w ays remains with ∼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 sho wn ho w oligarchic growth of a relatively massive secondary from the initial pebble ring can be a v oided. We discuss some potential additional physical processes that should be included in the modelling and explored as next steps.