A Search for Slowly Moving Distant Solar System Objects in the Catalina Sky Survey

We present a search for faint, slowly moving bodies in the outer Solar System, with particular interest in large objects that may still remain undiscovered at heliocentric distances beyond 100 AU. The motivation for this work comes from models of the primordial planetesimal disk, which suggest that the early Solar System could have produced many Pluto-sized bodies, and perhaps even a small number of objects with sizes comparable to Mars. If any of these bodies have survived until today, they would be very difficult to detect because they are expected to be both faint and extremely slow-moving on the sky. In recent years, several surveys have explored this type of search, focusing on faint sources with very small apparent motions as a way to identify large TNOs beyond 100 AU.

Our search is based on deep, wide-field images obtained during the last four years with the 0.68-m Schmidt telescope at Mt. Bigelow and the 1.5-m Cassegrain telescope at Mt. Lemmon, both operated as part of the Catalina Sky Survey. We align and co-add unfiltered images taken on different nights in order to improve the detectability of faint distant sources, while keeping sensitivity to the small displacements expected for very slow objects. The image alignment is performed with Tycho Tracker, and the source detection is carried out with Python routines based on Astropy, together with a matched-filter technique. With this procedure, we reach a limiting magnitude of about V≈21.5V.

The candidates are then inspected over multiple epochs and checked with preliminary orbital fits using Find_Orb. This step helps to separate real distant Solar System objects from artifacts and false detections, which can be frequent in sparse, low-cadence datasets. Applying this pipeline, we discovered two new outer Solar System objects: the Centaur 2020 BO71 and the trans-Neptunian object 2020 TZ411, both later confirmed by the Minor Planet Center. These results show that Catalina Sky Survey archival images can be effectively used to search for faint, slow-moving objects, and that the method can be applied to other wide-field multi-epoch surveys.