Subaru Telescope solves the color puzzle of small Jupiter Trojan asteroids

A new study using one of the world’s most powerful telescopes has overturned a long-standing assumption about the color behavior of Jupiter Trojan asteroids — and in doing so, may have rewritten part of the story of how the outer solar system was built.

Observations carried out with the Subaru Telescope in Hawaii, using its first-generation wide-field camera Suprime-Cam, have produced the most detailed color and size survey of small Jupiter Trojans to date. The results, published in The Astronomical Journal, reveal that these minor bodies behave in a fundamentally different way from their larger counterparts — a discovery with deep implications for our understanding of planetary formation.

What are Jupiter Trojans?

Jupiter shares its orbital path around the Sun with a vast population of asteroids trapped in two gravitational sweet spots — one roughly 60 degrees ahead of the planet, the other 60 degrees behind. These are the Jupiter Trojans, and they are among the most primitive objects in the solar system. Scientists consider them frozen relics from the earliest epoch of planetary formation, preserved in orbital amber for billions of years.

Because Jupiter Trojans are believed to have formed in or migrated from the distant, cold outer regions of the protoplanetary disk, studying their physical properties — particularly their colors — offers a rare window into the raw material that gave rise to the giant planets and their companions.

A tale of two colors — but only for the big ones

Decades of observations had established a consistent picture for large Jupiter Trojans: their surfaces fall into two distinct color categories. One group, known as D-type, appears distinctly reddish. The other, called P-type or C-type, is comparatively less red. This bimodal color distribution was thought to reflect the different formation regions of these two populations — some having originated closer to the Sun, others in the frigid outer reaches of the solar system, before being swept into Jupiter’s orbital zone during a period of large-scale planetary migration in the early solar system.

The assumption was that smaller Trojans would follow the same pattern.

They do not.

Small Trojans break the mold

To study small Jupiter Trojans — those near the kilometer scale — a research team from the University of Occupational and Environmental Health, the National Astronomical Observatory of Japan, and Kobe University turned to Suprime-Cam during its final operational night in May 2017 on the Subaru Telescope.

Spectroscopy is typically too insensitive for such faint, small targets even with large instruments, so the team employed multicolor photometry — measuring how much light each asteroid reflects across multiple color filters in rapid succession. This approach was essential because asteroids rotate, and their reflected light intensity changes with rotation. Measuring through multiple filters as quickly as possible minimized the error introduced by that rotation.

Surveying a region approximately 60 degrees ahead of Jupiter, the team detected and analyzed 120 Jupiter Trojan asteroids, focusing on color and size relationships among kilometer-sized objects.

Key findings that challenge conventional models

The survey produced two major results that directly contradict previously held assumptions.

First, there is no clear color bimodality among small Jupiter Trojans. While large Trojans separate neatly into red and less-red groups, small Trojans show a continuous, uniform distribution of colors. Less-red objects are overall more abundant. The sharp color divide seen in larger bodies simply disappears at smaller sizes.

Second, the size distribution of red and less-red small Trojans is virtually identical. Earlier studies had suggested that the two color groups among large Trojans show different size distributions — a finding interpreted as evidence that red asteroids fragment into less-red objects through collisions. The new data show no such difference in size distribution at smaller scales. Both color populations fragment in the same way, suggesting that collisional processes alone cannot account for the color transition observed in the larger population.

Together, these findings challenge the leading hypothesis linking Trojan colors directly to formation region, and suggest instead that some additional physical process — perhaps surface chemistry driven by space weathering, or the nature of the parent bodies from which small Trojans were chipped — plays a more important role than previously thought.

Suprime-Cam’s final night in service

There is a quiet historical significance woven into this research. The observations were conducted during the final night of Suprime-Cam’s scientific operations on the Subaru Telescope, before the instrument was retired to make way for its successor, Hyper Suprime-Cam — a second-generation wide-field camera offering a field of view seven times larger.

«Suprime-Cam was indispensable for this study, which required rapid multicolor observations over a wide area of the sky,» said lead researcher Fumi Yoshida of the University of Occupational and Environmental Health and the Chiba Institute of Technology, who has worked with Suprime-Cam since 2000. The instrument’s capacity for rapid filter changes made it uniquely suited for photometric work on faint, rotating objects.

What comes next

The implications of this study will be tested and refined by ongoing and upcoming space missions. The European Space Agency’s JUICE mission is currently exploring the Jovian system, while NASA’s Lucy mission is on a trajectory to conduct the first-ever flybys of Jupiter Trojan asteroids. The in-situ data returned by Lucy will be directly compared against the color and compositional predictions that ground-based surveys like this one have produced.

Combining spacecraft observations with the statistical framework of the Subaru survey will significantly sharpen our understanding of how and where Jupiter Trojans formed — and what that tells us about the turbulent early history of our planetary system.

Publication: Fumi Yoshida et al., Color and Size Distributions of Small Jupiter Trojans, The Astronomical Journal (2026). DOI: 10.3847/1538-3881/ae47e4

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. Todos los derechos reservados. Prohibida la reproducción total o parcial sin autorización expresa. Fuente original: Subaru Telescope / Phys.org — Yoshida et al., The Astronomical Journal (2026), DOI: 10.3847/1538-3881/ae47e4.