A search for Planet Nine using the Zwicky Transient Facility public archive

Abstract

Recent estimates of the characteristics of Planet Nine have suggested that it could be closer than originally assumed. Such a Planet Nine would also be brighter than originally assumed, suggesting the possibility that it has already been observed in wide-field moderate-depth surveys. We search for Planet Nine in the Zwicky Transient Facility public archive and find no candidates. Using known asteroids to calculate the magnitude limit of the survey, we find that we should have detected Planet Nine throughout most of the northern portion of its predicted orbit -- including within the galactic plane -- to a 95% detection efficiency of approximately $V=20.5$. To aid in understanding detection limits for this and future analyses, we present a full-sky synthetic Planet Nine population drawn from a statistical sampling of predicted Planet Nine orbits. We use this reference population to estimate that this survey rules out 56% of predicted Planet Nine phase space, and we demonstrate how future analyses can use the same synthetic population to continue to constrain the amount of parameter space effectively searched for Planet Nine.