For more than 15 years, one of the most interesting groups of small bodies in the outer Solar System has been hidden in plain sight. Neptune's trailing Trojan cloud, the swarm of objects that follows the planet around the Sun 60 degrees behind it, has been sitting almost directly in front of the Galactic plane and the Galactic centre. Those are the densest star fields in the sky. A faint, slow-moving point of light against that background is very easy to miss.
That changed in 2025. The cloud drifted far enough away from the plane to overlap with the Science Validation (SV) survey the NSF-DOE Vera C. Rubin Observatory ran during commissioning. A team led by Dr. Meg Schwamb of the Astrophysics Research Centre at Queen's University Belfast searched that data. In a paper submitted to arXiv on 28 October 2026, they report two new confirmed long-term stable members of the trailing cloud and one more candidate. The authors say this doubles the known sample.
What a Trojan Is, and Why the Trailing Side Lagged Behind
Trojans share a planet's orbit and sit near one of two gravitationally balanced Lagrange points: L4, 60 degrees ahead of the planet, or L5, 60 degrees behind it. Objects there are locked in a 1:1 mean-motion resonance with the planet. They complete one orbit around the Sun for every orbit the planet makes, and they never catch up to it or fall away from it.
The idea is far from new. As Universe Today notes, Jupiter's Trojans have been known since 1906, and Trojan populations have also been found for Saturn and Uranus. Neptune's trailing cloud was the gap. The obstacle, according to the paper's abstract, was the sky position: for more than 15 years the trailing cloud has lined up with the Galactic plane and centre, and the high stellar density hindered discoveries.
The two objects confirmed in the new work matter because they are classified as long-term stable, or primordial. That is a stronger claim than "currently near L5." Some objects are captured into resonance only temporarily and leave again. The authors describe these long-term stable members as primordial, meaning they are original members of the cloud rather than recent arrivals.
The Objects
The paper names three bodies:
- 2025 MH348: a confirmed long-term stable L5 Trojan
- 2025 NN80: a confirmed long-term stable L5 Trojan
- 2025 MD138: a candidate L5 member, found in what the authors call a preliminary search
Public orbit data from NASA/JPL's Small-Body Database fit the picture. The SBDB solution for 2025 MH348 gives a semi-major axis of 29.9 au, an eccentricity of 0.0495 and a perihelion of 28.4 au, based on 22 observations. The earliest of those dates to 16 June 2018, so the object had been recorded years before Rubin flagged it. The last observation used is from 24 July 2025, and the orbit solution is dated 5 June 2026.
The record for 2025 NN80 gives a semi-major axis of 30 au, an eccentricity of 0.0775 and a perihelion of 27.7 au. That solution draws on 40 observations, the last on 17 August 2025.
Both orbits are nearly circular and sit at about Neptune's distance from the Sun, about 30 au, which is what a Neptune Trojan should look like. One detail is worth flagging. JPL's automatic orbit-class label for 2025 MH348 reads "Centaur." Those labels come from simple orbital-element cutoffs and do not test for resonance. Identifying the object as a stable Trojan requires the kind of dynamical analysis in the paper. The Trojan classification is the research team's finding, not something the database label confirms.
Rubin's First Real Test on Moving Objects
The paper is not only a discovery report. The authors call it the first characterization of objects found by Rubin's moving-object pipelines using an LSST-like cadence, and they describe it as a "blueprint for LSST-era analyses."
That matters because finding faint moving objects is one of the main jobs of the Legacy Survey of Space and Time (LSST). Rubin pairs the 8.4-metre Simonyi Survey Telescope with the LSST Camera. According to Universe Today, operations began in June 2025. The Science Validation data used here came from commissioning, before the full survey was running. It was, in effect, a dress rehearsal for the software that will link detections into orbits over the next decade.
Neptune's L5 cloud is a hard test case. The targets are distant and faint, they move slowly across the sky, and in this period they sat near some of the most crowded star fields there are. If the pipelines can pull stable Trojans out of that, it is a good sign for the much larger catalogue of distant objects LSST is expected to produce. Universe Today quotes Schwamb, from the Queen's University Belfast release, as saying that early Rubin data gave "our clearest view yet of Neptune's trailing Trojan cloud."
The collaboration includes researchers from IAG, the Planetary Science Institute, DiRAC, MIDAS, the Johns Hopkins University Applied Physics Laboratory, NSF NOIRLab and other institutions. Universe Today reports that the work is published in The Astrophysical Journal Letters.
Why It Matters
Primordial Trojans are, by the authors' description, original members of Neptune's Trojan clouds, so they are a natural place to look for clues about the outer Solar System's early history. Comparing the properties of both clouds is part of that work, and astronomers need both of Neptune's clouds, not just one.
Until now, that comparison has been lopsided. The paper compares the L5 sample with the well-studied L4 cloud, the leading Trojans, but the trailing side has had very few members to work with. Using Rubin's images in different filters, Schwamb's team showed that the leading and trailing Trojans have similar surfaces, according to Universe Today. Doubling the known stable L5 sample does not settle whether the two clouds are alike. It does begin to make a real comparison possible.
The timing also helps. Universe Today reports that the trailing cloud has begun to move out of the crowded region of sky, and that as Rubin continues the LSST survey over the next nine years it is expected to discover tens to hundreds of Neptune Trojans in both the leading and trailing populations. The authors present their methods as a blueprint for LSST-era analyses.
The headline numbers are modest: two confirmed objects and one candidate. But they come from commissioning data, before the survey was fully running, in one of the hardest parts of the sky to search. That suggests the objects were always there and the main obstacle was the background stars.
