An international team led by researchers at Queen's University Belfast has identified two asteroids and a third candidate as permanent members of Neptune's trailing Trojan population, resolving a mystery that has eluded astronomers for decades. Schwamb said the team had been able to double the number of known objects in this population on long-lived orbits.
The findings, published in The Astrophysical Journal Letters, come from data gathered during Rubin's early test observations last year. Rubin, a next-generation telescope perched in the Andes, began operations in June 2025 and is now conducting the largest survey of the southern sky, the Legacy Survey of Space and Time.
Asteroid populations trailing and leading Jupiter have been known since 1906, when telescope observations revealed groups that became known as the Trojan and Greek populations. Similar families have since been found around Saturn and Uranus, but none around Neptune, partly because these distant, faint objects are hard to spot as they move across the night sky.
For the past ten years, the Neptune Trojans have been crowded in a region of sky that placed them in front of the Milky Way's center as seen from Earth, an extremely dense and bright backdrop that obscured the small foreground objects.
The Queen's-led team, headed by astronomer and lecturer Dr Meg Schwamb, combined Rubin's sharp optics and rapid sky surveys with its unmatched depth and coverage. "The Trailing Cloud has now begun to move out of this region of sky and combined with the Rubin Observatory's unmatched depth and sky coverage, we've been able to double the number of known objects in this population on long-lived orbits," Schwamb said.
Using images taken with different filters, the team also showed that the Trojans leading and trailing Neptune have similar surfaces.
"Early data from Rubin Observatory is already expanding our knowledge of the solar system's 'small bodies,' including asteroids, comets, and other minor planets," Schwamb said. "And these early observations have provided our clearest view yet of Neptune's trailing Trojan cloud."
Schwamb was joined by researchers from the Instituto de Astronomia, Geofísica e Ciências Atmosféricas, the Planetary Science Institute, the DiRAC Institute, the Michigan Institute for Data and AI in Society, JHUAPL, the NSF's NOIRLab and multiple universities worldwide.
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, plus tens of thousands of icy bodies in the Trans-Neptunian Region and Kuiper Belt.
Analysis
Why This Matters
- The discovery resolves a decades-long puzzle: Neptune was the only giant planet without a known Trojan population, despite such families being found around Jupiter, Saturn and Uranus.
- The find doubles the known sample of Neptune's trailing Trojans on long-lived orbits, giving scientists more material to study the surfaces and origins of these distant icy bodies.
- It demonstrates the power of the Vera C. Rubin Observatory early in its mission, suggesting the survey will transform the census of small objects in the Solar System.
Background
Trojan asteroids share a planet's orbit, clustering around the gravitationally stable Lagrange points ahead of and behind the planet. Jupiter's Trojans have been known since 1906, and similar families were later found around Saturn and Uranus. Neptune's trailing Trojans were particularly hard to find because they are faint, distant and, for the past decade, have appeared against the bright and dense glare of the Milky Way's center. Rubin's depth and rapid survey capability, combined with the cloud now moving out of that crowded region of sky, finally made the discovery possible.
What to Watch
- Rubin's LSST survey continues for nine more years, with predictions of tens to hundreds of new Neptune Trojans in both leading and trailing populations.
- The survey is also expected to turn up tens of thousands of icy bodies in the Trans-Neptunian Region and Kuiper Belt, which could reshape understanding of the outer Solar System.
- Further filter-based observations of the newly found Trojans may clarify how similar the leading and trailing populations truly are.
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12 claims in this story checked against 1 source: 12 supported.
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- SupportedNeptune’s elusive Trojan asteroids were found in Rubin Observatory data.Universe Today: “the team identified two asteroids and a candidate as permanent members of Neptune's trailing Trojans.”
- SupportedA Queen’s University Belfast-led team doubled the known number of trailing Trojans on long-lived orbits.Universe Today: “we've been able to double the number of known objects in this population on long-lived orbits.”
- SupportedThe team identified two asteroids and a third candidate as permanent members of Neptune’s trailing Trojan population.Universe Today: “the team identified two asteroids and a candidate as permanent members of Neptune's trailing Trojans.”
- SupportedThe discovery resolved a mystery that had eluded astronomers for decades.Universe Today: “Thanks to the Queen's-led team, the mystery of the missing Trojans has now been resolved!”
- SupportedSchwamb said the trailing cloud is moving out of the region and that Rubin’s depth and coverage helped the team double the known objects on long-lived orbits.Universe Today: “The Trailing Cloud has now begun to move out of this region of sky and combined with the Rubin Observatory's unmatched depth and sky coverage, we've been able to double the number of known objects in this population on long-lived orbits.”
- SupportedThe findings were published in The Astrophysical Journal Letters.Universe Today: “As detailed in a paper that appeared in The Astrophysical Journal Letters”
- SupportedThe data came from Rubin’s early test observations last year.Universe Today: “This last objective was what drew Schwamb and her team to analyze data from Rubin's early test observations last year.”
- SupportedRubin began operations in June 2025 and is conducting the largest survey of the southern sky, the LSST.Universe Today: “Having commenced operations in June 2025, Rubin is currently conducting the largest survey of the southern sky - the Legacy Survey of Space and Time (LSST).”
- SupportedObservations beginning in 1906 revealed Jupiter’s leading and trailing asteroid populations, known as the Trojan and Greek populations.Universe Today: “Beginning in 1906, telescope observations revealed a population of asteroids leading and trailing Jupiter, which came to be known as the "Trojan" and "Greek" populations.”
- SupportedFor the past ten years, Neptune’s Trojans were crowded in front of the Milky Way’s center, whose brightness obscured the faint objects.Universe Today: “For the past ten years, the Trojans have been crowded in a region of the sky that placed them in front of the Milky Way's center (relative to Earth-based observers), an extremely dense and bright part of the night sky that obscured any tiny light sources in the foreground.”
- SupportedImages taken with different filters allowed the team to show that leading and trailing Neptune Trojans have similar surfaces.Universe Today: “The Rubin Observatory obtained images of these objects using different filters, which allowed Dr. Schwamb and her team to show that the Trojans leading and trailing Neptune have similar surfaces.”
- SupportedOver the next nine years, Rubin is expected to discover tens to hundreds of Neptune Trojans in both populations and tens of thousands of icy bodies in the Trans-Neptunian Region and Kuiper Belt.Universe Today: “As Rubin continues the LSST survey over the next nine years, it is expected to discover tens to hundreds of Neptune Trojans in both its leading and trailing population, plus tens of thousands of icy bodies in the Trans-Neptunian Region and Kuiper Belt.”
