The Captured and the Crossing: Predictions for the LSST Discovery Yields for Neptune Trojans and Centaurs

The giant planet region of the Solar System (5.2 au < a < 30.1 au) can be a highly chaotic and dynamic environment of small body science, whose resident minor bodies can offer unique insights into both early Solar System formation and present-day evolution processes. Co-orbiting with Neptune in a 1:1 resonance, the Neptune Trojans (NTs) are a family of objects that can be dynamically stable on the order of billions of years (e.g., Lin et al., 2022). Numerical studies show that Neptune’s outwards migration is capable of capturing and retaining NTs (e.g., Gomes & Nesvorný, 2016), implying that the present day observed population may retain some information about the primordial protoplanetary disk conditions. Further in from Neptune, the Centaurs are a class of objects on giant-planet-crossing orbits, evolving inwards due to their many gravitational perturbations. They are an intermediate population, with colour distributions similar to smaller trans-Neptunian Objects (TNOs) (e.g., Wong & Brown, 2017), and physical sizes more typical of the nuclei of Jupiter family comets (e.g., Fernández et al., 2013). Understanding Centaur properties in ensemble is therefore a means of providing insight into the evolution of dynamically scattering TNOs into present day comets, as well as probing the evolution of how their surfaces are processed.

Over decades of observations, the Minor Planet Center (MPC) only notes <30 NTs, and∼300 Centaurs to date. These small sample sizes are set to be revolutionised with dawning of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). With first light having already occurred in July, the LSST is scheduled to commence operations in late 2025. One of its main science goals is to ”create an inventory of the Solar System”, cataloguing as many objects as possible across all orbital classes. This is achieved through its unprecedented combination of depth (m_r ∼24.7), coverage in 6 broadband optical/NIR filters (ugrizy), and its observing cadence of 30s visits to 18,000 deg² of the southern hemisphere every three nights over its full decadal observational baseline. Early estimates for the LSST’s performance predicted an order of magnitude increase in Near Earth Objects, Main Belt Asteroids, Jupiter Trojans, and TNOs (Ivezić et al., 2007; LSST Science Collaboration et al., 2009; Jones et al., 2015; Ivezć et al., 2019), with more refined estimates detailed in Kurlander et al. (2025a). Predictions for the LSST’s discovery metrics, including the total number of observations available per object, when they are discovered, and how many will be discovered, are vitally important in understanding the potential for small body science within the LSST through light curve, phase curve, and surface colour studies. Further, such predictions are crucial in understanding the gaps in the LSST’s observation cadence, which will enable the design of follow-up observational campaigns to supplement and bolster the LSST.

In this work, we present the very first estimates for the Neptune Trojan and Centaur discoveries within the LSST. We use the best available dynamical models for both populations (Nesvornŷ et al., 2019; Lin et al., 2021), and absolute magnitude and colour distributions drawn from Dark Energy Survey Neptune Trojan discoveries (Bernardinelli et al., 2025) and Pan-STARRS1 Centaur discoveries (Kurlander et al., 2025b). We use the new high-fidelity survey simulator Sorcha (Merritt et al., In Press; Holman et al., In Press) in order to generate ephemerides using the in-built N-body integrator ASSIST (Holman et al., 2023) (itself an extension of REBOUND, Rein & Liu, 2012; Rein & Spiegel, 2015). Sorcha is able to take the most recent LSST cadence simulations (SCOC, 2024), in order to forward bias these objects and simulate LSST discoveries. We highlight the year 1 science potential for both populations, including discovery numbers, and the prospects for light/phase curve and surface colour studies. The implications for understanding the Neptune Trojan colour-inclination dependency, as well as assumptions of symmetry of the L4 and L5 clouds are also discussed. Finally, we will discuss Centaur activity searches in the era of the LSST.