Limits on the number of primordial Scattered Disk objects at Pluto mass and higher from the absence of their dynamical signatures on the present day trans-Neptunian Populations

TL;DR

This study constrains the initial number of large primordial objects in the trans-Neptunian region by analyzing their potential dynamical signatures, finding that the primordial size distribution must have been less top-heavy than some models suggest.

Contribution

It provides the first statistical limits on the primordial population of Pluto-mass and larger objects based on their absence in current dynamical structures.

Findings

Primordial population of >Pluto-mass objects was limited by current dynamical structures.

The initial size distribution must have declined sharply above Pluto mass.

These limits challenge certain planetesimal formation models.

Abstract

Today, Pluto and Eris are the largest and most massive Trans-Neptunian Objects respectively. They are believed to be the last remnants of a population of planetesimals that has been reduced by >99% since the time of its formation. This reduction implies a primordial population of hundreds or thousands of Pluto-mass objects, and a mass-number distribution that could have extended to hundreds of Lunas, dozens of Mars, and several Earths. Such lost protoplanets would have left signatures in the dynamics of the present-day Trans-Neptunian Populations, and we statistically limit their primordial number by considering the survival of ultra-wide binary TNOs, the Cold Classical Kuiper belt, and the resonant population. We find that if the primordial mass-number distribution extended to masses greater than Pluto (~1e-3 Earth masses), it must have turned downwards to be no more top-heavy than roughly equal mass per log size, a significant deviation from the distribution observed between 1e-5 and 1e-3 Earth masses. We compare these limits to the predicted mass-number distribution of various planetesimal and proto-planet growth models. The limits derived here provide a test for future models of planetesimal formation.