Predictions of the LSST Solar System Yield: Neptune Trojans

TL;DR

This work provides the first LSST-era predictions for Neptune Trojans using the Sorcha survey simulator, integrating three plausible absolute-magnitude distributions, a color Gaussian mixture model, and detailed orbital distributions. It shows that LSST should discover on the order of 130–300 NTs over a decade, with a notable bias toward the L5 cloud driven by the survey footprint, and that roughly 60% will yield high-quality color/light-curve data, enabling robust population studies. The Science Validation (SV) phase offers early NT insight, while the authors carefully discuss uncertainties stemming from small current NT samples and potential linking/template limitations. Overall, the results indicate that LSST will enable a transformative expansion of the NT census, constraining formation and migration histories through large, well-characterized color and orbital data sets.

Abstract

The NSF-DOE Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), beginning full operations in late 2025, will dramatically transform solar system science by vastly expanding discoveries and providing detailed characterization opportunities across all small body populations. This includes the co-orbiting 1:1 resonant Neptune Trojans, which are thought to be dynamically hot captures from the protoplanetary disk. Using the survey simulator $\texttt{Sorcha}$, combined with the latest LSST cadence simulations, we present the very first predictions for the Neptune Trojan yield within the LSST. We forecast a model-dependent median number of $\sim130-300$ discovered Neptune Trojans, and infer a notable 2:1 detection bias toward the recently emerged L5 cloud near the galactic plane versus the L4 cloud, reflecting the lower-cadence coverage in the Northern Ecliptic Spur region that suppresses L4 detections. The additionally simulated Science Validation survey will offer the very first early insights into this understudied cloud. Around 60\% of detected main survey Neptune Trojans will meet stringent color light curve quality criteria, increasing the sample size more than fourfold compared to existing datasets. This enhanced sample will enable robust statistical analyses of Neptune Trojan color and size distributions, crucial for understanding their origins and relationship to the broader trans-Neptunian population. These comprehensive color measurements represent a major step forward in characterizing the Neptune Trojan population and will facilitate future targeted spectroscopic observations.

Figures (11)

• Figure 1: Skymaps of the number of visits across all filters of (left) the 10 year LSST survey cadence based on the one snap v4.3.1 simulation scocv3 and (right) the Science Validation survey based on the lsstcam_20250930 simulation claver25sv0903 — note the difference in color scale between the two. The main wide-fast-deep survey makes up $\sim$80% of the total main LSST survey time, receiving on average $\sim$800 visits per pointing. Also of note are the Northern Ecliptic Spur (NES) mini-survey (dark blue region, $+10^\circ$ ecliptic latitude) and the Deep Drilling Fields (yellow-white circles, regions with high temporal sampling cadence). For full details of the survey strategy, see scocv3. Overplotted on both in solid red is the ecliptic plane. In solid blue is the galactic plane, with the corresponding dashed cyan lines representing $\pm10^\circ$. On top of all of this in pink are the positions of both L4 and L5 clouds at the beginning of the survey for context.
• Figure 2: Differential histograms of all 3 absolute magnitude $H$ distributions used in modeling the NT population in this work. In all cases, the solid line represents the analytical model for the distribution, whilst the histograms are the uniformly sampled $N_{NT}$ objects from that model. The shaded regions in all cases represent the region beyond the LSST's nominal single-visit detection threshold ($H_r \approx 10$) for an object at Neptune's orbit. Represented here are (left panel) the single-component model, (middle) the two-component model, and (right) the rolling power law model. Note that the single- and two-component models have a logarithmic y-axis scale, whilst the rolling power law model is linear.
• Figure 3: $g-r$, $r-i$, and $r-z$ color histograms for the input model produced from GMM parameters in Table \ref{['tab:allparams']}. The leading diagonal represents the individual 1D histograms of each color, whilst the middle left, bottom left, and bottom middle 2D histograms highlight their correlation. Samples are drawn to ensure this correlation is kept.
• Figure 4: Orbital distributions in (top row) semimajor axis $a$, (middle row) eccentricity $e$, and (bottom row) inclination $i$ for the (left column) single-component, (middle column) two-component, and (right column) rolling power law models respectively.
• Figure 5: Example distributions of libration angle $L_{1:1}$ and resonance argument $\phi_{1:1}$ used for all 3 $H$ models. All three $H$ models share the same shape of distributions for $L_{1:1}$ and $\phi_{1:1}$, only differing in the number of objects sampled from them.