A New Kind of Trojan: JWST Reveals Unusual Asteroids Near Jupiter
In their new study, Michael E. Brown and collaborators report on the discovery of a previously unknown surface type among Jupiter Trojans--asteroids that share Jupiter’s orbit around the Sun. Using the powerful James Webb Space Telescope (JWST), the team studied four small Trojans that are significantly more reflective, or “high albedo,” than most of their peers. Their analysis suggests these bright objects may belong to a completely new class of Trojans--possibly fragments from recent collisions, distinct from the two major classes previously known.
The Origins and Oddities of Trojans
Jupiter Trojans are thought to be relics of the early Solar System. According to models like the Nice model, they originated in the distant Kuiper Belt and were pulled into their current positions during the migration of the giant planets. However, the surfaces of Trojans today look very different from Kuiper Belt Objects (KBOs), lacking many of the ices and organics commonly found there. One way scientists can investigate their interiors is by studying objects that have been recently broken apart in collisions, revealing fresh material beneath the surface. Earlier studies had identified a few Trojans with unexpectedly high albedos, suggesting they might be fragments from such impacts.
Using JWST to Look Beneath the Surface
To investigate this idea, Brown and his team used JWST’s Near Infrared Spectrograph (NIRSpec) to collect spectra from 0.8 to 5 microns for four high albedo Trojans. These spectra capture how sunlight reflects off the surface and how much heat the asteroid emits. After correcting for thermal emission using a model that includes object size and surface temperature, they created detailed reflectance spectra to study the surface composition. The data showed that all four Trojans had remarkably similar spectra: red-sloped at shorter wavelengths, a noticeable flattening around 1.3 microns, and a broad absorption feature beyond 2.7 microns.
No Ice, No Problem?
Interestingly, none of the Trojans displayed strong signs of water ice, which might be expected if recent impacts had exposed fresh, icy layers. A modeling exercise showed that even small amounts of surface ice would have been detectable, but none were found. The lack of expected ice means that high reflectivity must be caused by something else--perhaps fine-grained dust or a different kind of surface material not seen in other Trojans.
Not Red, Not Less-Red--Something Else Entirely
When comparing these new observations with known Trojan groups--the “red” and “less-red” spectral classes--the researchers found that the high albedo Trojans didn’t fit into either. Their spectra were similar to the red Trojans up to about 1.3 microns but became much flatter at longer wavelengths, with a possible absorption feature not seen in other asteroid types. Even more surprising, the only previously observed Trojan with a similar spectral profile was Polymele, a small high albedo asteroid that will be visited by NASA’s Lucy spacecraft in 2027.
A New Class of Trojans Emerges
The similarities between these high albedo Trojans and Polymele--especially their shared size, brightness, and spectral features--suggest that they may form a third, previously unrecognized class of Trojan asteroids. These objects are all under ~40 km in size and could be the products of relatively recent disruptions, although no direct spectral evidence of impact remnants was found. Their mid-range colors and unusual features raise questions about how they formed and how their surfaces evolved.
Looking Forward to Lucy
With the Lucy mission’s upcoming flyby of Polymele, scientists will soon get their first close-up look at one of these enigmatic objects. This encounter could provide crucial clues to help explain the puzzling nature of high albedo Jupiter Trojans and the history of collisions in the outer Solar System.
Source: Brown
