Presentation #107.04 in the session Modern Theories of Planetesimal Formation.
In the last two decades, we have experienced a significant shift in our thoughts about the early stages of planet formation, and in particular, the growth from dust and pebbles into ~100 km-scale planetesimals. The idea that gas-pebble interactions could drive super concentrations of solid material so as to collapse under self-gravity and thereby form large-scale bodies has indeed proven revolutionary. The outcomes of numerical simulations of formation via pebble cloud collapse do quite well in matching the observed properties of the telescopically accessible planetesimal populations that have yet to be matched in the various flavours of the more classical accretion mode of hierarchical growth. Critically, via virtue of the short ~100 yr collapse timescales that these clouds experience, formation via this route naturally avoids the scourge of hierarchical growth models: the metre-barrier. I argue that the evidence for this new formation route is overwhelming. In this talk I try to convince the audience of this, by highlighting some of the key observables of the asteroids and Kuiper Belt Objects that support pebble cloud collapse, including binarity of the bodies and the properties of those binary systems, and their size distributions. I will discuss how those observables constrain numerical models, and indeed, reject numerous others. In particular, I will highlight what aspects of cloud collapse models remain largely unconstrained, and point out experiments, both numerical and observable, that can improve those constraints. I will finish with a discussion of the contact binary Kuiper Belt Object Arrokoth, which is now held up as a glaring nail in the coffin of hierarchical accretion, at least for the outer Solar System’s denizens. While I agree with this sentiment, I will discuss the possibility that Arrokoth may not be a direct product – the central body – of cloud collapse. Rather, I suggest that Arrokoth is simply the detritus ejected during the collapse of a cloud, thereby avoiding accretion into a larger body.