Astronomers combining NASA’s Hubble and James Webb space telescopes have discovered 27 tiny trans-Neptunian objects orbiting beyond Neptune. The faint, icy bodies fell short of some planet-formation model predictions, yet their surface colors and size distributions preserve unexpected clues about the early solar system’s origins.
Hunting the Smallest Worlds Beyond Neptune With Hubble and Webb
Deep past the orbit of Neptune, astronomers expected to find a crowded realm of tiny, icy debris left over from the construction of the planets. Instead, a joint observation campaign utilizing NASA’s Hubble and James Webb Space Telescopes turned up fewer of these objects than some formation models predicted.
The survey targeted distant bodies known as trans-Neptunian objects, or TNOs. Most TNOs are more than 100 million times dimmer than anything visible to the unaided human eye, appearing in deep-space imaging as faint points of light. By pointing both space observatories simultaneously at the same patch of sky, researchers captured visible light with Hubble and infrared data with Webb to calculate orbits, estimate sizes, and evaluate surface composition.
Among the discoveries were 27 tiny TNOs, including some of the smallest and dimmest ever directly observed. One newly detected object was so faint that spotting it from Earth was comparable to seeing a small swarm of fireflies on the Moon. The smallest measured about three miles across, roughly five times smaller than what the most sensitive ground-based telescopes can detect.
Cold Orbits and Migrating Giants: Two Distinct Populations
The research teams split their sample into two distinct groups based on orbital behavior. Dynamically cold TNOs still travel along their original, nearly circular paths close to the plane of the solar system. By contrast, dynamically hot TNOs formed closer to the Sun between the current locations of Uranus and Neptune before being shoved outward during the early migration of the outer gas giants, leaving them on tilted, elongated orbits.
Despite their different birthplaces and orbital histories, Eduardo’s team found that both populations shared nearly identical distributions of sizes. That parity provides a fresh benchmark for testing how planetesimals—the solid building blocks of planets—formed across varying regions of the primordial disk.
“It’s very interesting that the process of planetesimal formation ends up producing the same distribution of sizes for both cold and hot populations, despite forming in different regions of the early solar system. The process seems to be insensitive to disk conditions, producing similar planetesimal sizes whether the disk is hot or cold, and dense or fluffy.”
Marielle Eduardo, University of Victoria PhD candidate, via Scitechdaily
Surviving Billions of Years of Collisions Without Losing Their Colors
Before the joint Hubble and Webb observations, planetary scientists assumed that small TNOs in both groups would have undergone continuous collisions over billions of years. Researchers theorized that this constant battering and fragmentation would have shattered the smallest bodies and altered their surface compositions, making their colors distinct from their larger siblings.
Instead, the color measurements—which serve as a chemical fingerprint for surface makeup—matched the patterns seen on much larger TNOs. The small, cold TNOs clustered tightly around a single color family, while the hot TNOs maintained a wider spread that still aligned with larger hot bodies.
“You could imagine a scenario where getting knocked around and fragmented would change the surface composition, and then you would see a different surface color for tiny TNOs compared to their larger siblings. So it’s really fascinating to see that the smallest objects are somehow ‘remembering’ and preserving the history of how they were made.”
Anastasia Morgan, Northern Arizona University PhD candidate, via FOX Weather
Co-author David Trilling of Northern Arizona University noted that these dynamically hot TNOs retain a signature of where they were born, even though their orbits have been thoroughly scrambled over time. While the surveys leaves open questions—such as whether fewer collisions occurred out past Neptune than models assumed, or whether the objects simply resist surface alteration—the findings demonstrate that these distant worlds preserve pristine records of early planetary assembly.
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