Neptune's Trailing Trojans Found: Rubin Observatory Doubles a Swarm Hidden for 15 Years

For more than fifteen years, one corner of the solar system kept a conspicuous blank spot. Neptune's leading Trojan cloud, the swarm of icy bodies that runs sixty degrees ahead of the planet in its orbit, held about thirty known objects. Its trailing cloud, the swarm that follows sixty degrees behind, had yielded only two confirmed members. That asymmetry bothered astronomers, because the physics of the two clouds should be roughly mirror images of each other. Now the blank is starting to fill in. A team led by Dr Meg Schwamb of Queen's University Belfast, working through early data from the NSF-DOE Vera C. Rubin Observatory in Chile, has identified two new long-term stable members of the trailing cloud, 2025 MH348 and 2025 NN80, plus one further candidate. The paper, published in The Astrophysical Journal Letters, doubles the known stable population of Neptune's trailing Trojans at a stroke.

A Trojan is a small body locked into a planet's orbit, held in place by a gravitational balance point sixty degrees ahead of or behind the planet. Jupiter has thousands of them; Neptune's are far rarer and far fainter, small icy relics that have kept their orbits for billions of years. That longevity is what makes them valuable. Unlike planets, which have migrated, collided and been reshaped, primordial Trojans are close to original solar system material, and the differences between the leading and trailing clouds carry a record of how Neptune moved outward early in the solar system's history and swept smaller bodies up along the way. With only two trailing objects to study, that comparison was barely possible. With four confirmed and a candidate, it has finally begun.

The reason the trailing cloud stayed empty for so long was not that it was empty, but that it was hidden. For over fifteen years it sat almost directly in front of the Milky Way's galactic plane and centre as seen from Earth, a backdrop of the densest star fields in the sky. Faint, slow-moving icy bodies were lost in the crowd. Two things changed. The cloud has begun to drift out of that crowded region, and a new instrument arrived that could cope with the crowd: Rubin's 8.4-metre Simonyi Survey Telescope, paired with the LSST Camera, the largest digital camera ever built, which can survey the southern sky every few nights. Using colours measured in Rubin's different filters, Schwamb's team found that the new trailing objects look similar in surface composition to the leading Trojans, which matches the expectation that the two clouds share an origin rather than being assembled from different material.

The wider context is why I find this discovery exciting rather than merely tidy. Rubin's decade-long Legacy Survey of Space and Time formally began in June 2026, after first images in June 2025, and these Trojans came from commissioning-phase data gathered before the survey proper had even started. That early data had already yielded more than 11,000 new asteroids and roughly 380 new trans-Neptunian objects, and Schwamb has said hundreds more Trojans are likely to be found over the survey's ten years. If that pace holds, the outer solar system is about to go from a sketch with a few dozen named landmarks to a census, and the census will test our story of how the giant planets moved and how the Kuiper Belt was assembled.

That is the prediction I am making with this article: that by 31 December 2027, the known population of long-term stable Neptune L5 (trailing) Trojans will reach at least 10 confirmed members. The survey is now running at full pace, the cloud is moving out of the worst of the star fields, and the pipeline has already proved it can pull stable Trojans out of test data. But detection is not confirmation: each candidate needs a long enough observed arc and a stability analysis before it counts, and the one candidate in this paper may yet fail that test. Reaching 10 means six net new confirmations in about fourteen months, which is demanding even for Rubin. That is why I put it at 50%, not higher: the instrument is capable of it, but the count will only move when the follow-up work keeps up with the detections.