Why Ultima Thule Was Not a (Total) Surprise: The Formation of (Binary!) Planetesimals

Andrew Youdin (University of Arizona)

January 28, 2019

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Abstract: Planetesimals, the 1-100 km scale building blocks of planets, are the lowest mass gravitationally bound objects in the universe. The origin of planetesimals is crucial to understanding the Solar System, extrasolar planets and circumstellar disks. I will overview the current status of planetesimal formation theory. Then I will discuss the latest developments from computer simulations, astronomical observations and Solar System exploration. Specifically, I will describe recent simulations of the streaming instability – a mechanism to concentrate pebbles and boulders, disk structures observed by ALMA, and the New Horizons flyby of MU69, i.e. Ultima Thule. I will explain why all planetesimals – not just Ultima Thule – likely formed as binaries.