For decades, Neptune’s trailing asteroids were the missing members of a solar-system family—and now a telescope in Chile has helped astronomers find them. A team led by Queen’s University Belfast identified two asteroids and one candidate that appear to be long-lasting members of the distant group.
Asteroids that travel near a planet in a stable orbit are known as Trojans. The first such groups were found around Jupiter in 1906. Astronomers later found similar populations around Saturn and Uranus, but Neptune’s remained hard to detect. Its Trojans are icy and faint, and from Earth they can be lost against crowded, bright regions of the sky.
The new results, published in The Astrophysical Journal Letters, came from early observations by the Vera C. Rubin Observatory in Chile’s Andes. Led by Dr. Meg Schwamb, an astronomer and lecturer at Queen’s Astrophysics Research Center, the international team sifted through Rubin data to spot the objects moving near Neptune. Researchers from institutions including the Instituto de Astronomia, Geofísica e Ciências Atmosféricas in Brazil, the Planetary Science Institute and the DiRAC Institute also took part.
Rubin’s 8.4-meter Simony Survey Telescope is paired with the largest digital camera ever built, the LSST Camera. The observatory began operations in June 2025 and is carrying out the Legacy Survey of Space and Time, or LSST, a wide-ranging effort to repeatedly map the sky. Its rapid surveys and sharp images make it especially useful for finding small, dim objects that move over time.
The timing also helped. For about 10 years, Neptune’s trailing Trojans appeared in front of the Milky Way’s center from Earth’s point of view. That part of the sky is packed with stars, making tiny objects difficult to pick out. The trailing cloud has now started moving away from that crowded background. Combined with Rubin’s depth and broad coverage, the change helped the team double the known number of objects in Neptune’s trailing population that follow long-lived orbits.
Different color filters in Rubin’s images also let the researchers compare the surfaces of Neptune’s leading and trailing Trojans. The two groups appear similar, offering a new clue about these icy bodies and how they fit into the solar system’s history. Schwamb said the early data already gives scientists their clearest view yet of the trailing cloud.
The discovery is an early sign of what Rubin may add to astronomy. Over the next nine years of the LSST survey, researchers expect it to find tens to hundreds of Neptune Trojans across both the leading and trailing groups, as well as tens of thousands of icy objects in the distant trans-Neptunian region and Kuiper Belt. Each new point in that growing census can help show how the outer solar system formed and changed. For a population that stayed hidden for generations, the search is only just getting started.