A targeted, parallax-based search for Planet Nine

Abstract

The hypothesized Planet Nine is thought to reside in the distant outer solar system, potentially explaining various anomalies in the orbits of extreme trans-Neptunian objects (ETNOs). In this work, we present a targeted observational search for Planet Nine in a field of approximately 98 square-degrees. This field is close to the highest probability region of finding Planet Nine, according to simulations, but poorly constrained by previous searches. Our observations and search methodology, based on the detection of parallax position shifts between consecutive nights, work well in these conditions. We provide 85% confidence exclusion limits for objects with Sloan r-band magnitudes brighter than between 21.0 and 21.4, with an average sensitivity limit of 21.3. No credible Planet Nine candidates were identified within this field and magnitude limits. A caveat to our approach is that it would miss a candidate if its position were affected by scattered light from bright stars in at least one of the nights. However, we estimate that the probability for this is very low, around 0.4%. We discuss several possible reasons for our Planet Nine non-detection. Our study complements prior searches, particularly those using archival survey data that are limited in the Galactic plane or at fainter brightness limits. While our consecutive-night observation approach offers high sensitivity to minimal motion, extending the search for Planet Nine to fainter magnitudes (which may be crucial, according to recent predictions), will require higher sensitivity instrumentation