Neptune's Missing Trojans Found at Last, Thanks to a New Telescope
Some asteroids do not wander alone. They share a planet's orbit, gathered in swarms ahead of and behind it, held in place by gravity. Astronomers call them Trojans. Jupiter's Trojans have been known since 1906. Neptune's trailing swarm, though, stayed missing for decades. Now it has been found.
An international team led by researchers at Queen's University Belfast reports two new long-term stable Trojans in Neptune's trailing cloud, plus a candidate member, in a paper in The Astrophysical Journal Letters. The team, led by Dr Meg Schwamb, an astronomer and lecturer at Queen's Astrophysics Research Center, found the distant icy bodies by analysing data from the Vera C. Rubin Observatory in Chile.
Why Neptune's Trojans were so hard to see
Trojan asteroids sit near two stable points in a planet's orbit, one leading the planet and one trailing it, where the gravity of the Sun and the planet balance out. Similar families had been found orbiting Saturn and Uranus, but Neptune's trailing Trojans proved elusive.
There were two problems. First, the objects are distant and faint, moving slowly across the sky. Second, for the past ten years the trailing Trojans have been crowded, as seen from Earth, in front of the centre of the Milky Way — an extremely dense and bright part of the night sky that obscured tiny light sources in the foreground.
Two things have now changed. The trailing cloud has begun to move out of that crowded region of sky. And Rubin Observatory, which began operations in June 2025, brings unmatched depth and sky coverage: its 8.4-metre Simony Survey Telescope works with the LSST Camera, the largest digital camera ever built, rapidly surveying the night sky. Schwamb said the early Rubin data have provided "our clearest view yet of Neptune's trailing Trojan cloud".
What was found
By combining Rubin's early test observations, the team identified the objects as primordial members of the trailing Neptune Trojan population — bodies that have been there since the early Solar System, on long-lived orbits. Schwamb said the discoveries have "doubled the number of known objects in this population on long-lived orbits". The paper's title records two new long-term stable Trojans, with a third object reported as a candidate.
Rubin imaged the objects through different filters, which allowed the team to show that the Trojans leading and trailing Neptune have similar surfaces. That similarity is a clue about where these bodies formed and how they came to share Neptune's orbit.
Schwamb expects much more to come. As Rubin continues its ten-year Legacy Survey of Space and Time, it is expected to discover tens to hundreds of Neptune Trojans in both the leading and trailing populations, plus tens of thousands of icy bodies in the trans-Neptunian region and Kuiper Belt beyond Neptune.
Why old, icy bodies matter
Trojans are leftovers from the birth of the Solar System, about 4.6 billion years ago. Because they have changed relatively little since they formed, they preserve clues about the raw materials and movements that shaped the planets. Finding Neptune's missing swarm does not just complete a list — it gives astronomers a new sample of ancient material to compare with the Trojans of Jupiter, Saturn and Uranus, and a test of models of how the outer planets moved into their present orbits.
For young astronomers, the story is encouraging: a mystery that lasted decades was solved not by a bigger guess, but by a new instrument looking carefully at a patch of sky that had just become visible. Sometimes the answer was there all along, waiting for the right telescope — and for the cloud to drift out from behind the stars.
If your child looks up at the night sky and asks how we know what is out there, a first lesson in astronomy or physics can start with Neptune's travelling companions and end with gravity holding asteroids in step with a planet. Book your first lesson
Sources:
- Schwamb, M. E. et al., "NSF-DOE Vera C. Rubin Observatory's First View of the Neptune Trojan L5 (Trailing) Cloud: Two New Long-term Stable Trojans", The Astrophysical Journal Letters (2026), DOI: 10.3847/2041-8213/aead9a, as reported 2 November 2026: https://phys.org/news/2026-11-queen-university-belfast-students-neptune.html
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