Interstellar Interlopers: Number Density and Origins of 'Oumuamua-like Objects

Abstract

We provide a calculation of Pan-STARRS' ability to detect objects similar to the interstellar object 1I/2017 U1 (hereafter 'Oumuamua), including the most detectable approach vectors and the effect of object size on detection efficiency. Using our updated detection cross-section, we infer an interstellar number density of such objects ($n_{IS} \approx 0.2~\rm{au}^{-3}$). This translates to a mass density of $\rho_{IS} \approx 4M_\oplus~\rm{pc}^{-3}$ which cannot be populated unless every star is contributing. We find that given current models, such a number density cannot arise from the ejection of inner solar system material during planet formation. We note that a stellar system's Oort cloud will be released after a star's main sequence life time and may provide enough material to obtain the observed density. The challenge is that Oort cloud bodies are icy and \OBJECT was observed to be dry which necessitates a crust generation mechanism.