Hubble and James Webb Telescopes Discover Small Icy Bodies Beyond Neptune

Image: Illustrative image · ESA/Webb, NASA & CSA, Tazaki et al. · CC BY 4.0 · Source

Astronomers combined Hubble's optical and James Webb's infrared observations to study 27 tiny trans-Neptunian objects, including some as small as 5 km in diameter, revealing unexpected uniformity in color and composition.

Astronomers have combined observations from two of the most powerful space telescopes, Hubble and James Webb, to identify and study some of the smallest and dimmest known bodies in the Solar System beyond Neptune’s orbit. The two telescopes complement each other with Hubble observing in visible light and James Webb operating in the infrared spectrum. Together, they investigated 27 trans-Neptunian objects (TNOs), remnants from the early Solar System formation period.

Among these, the smallest body measures approximately 5 kilometers in diameter. Some objects are so faint that their brightness has been likened to the glow of fireflies on the lunar surface when viewed from Earth. This study enabled astronomers to probe a population of icy bodies that were previously out of reach for detailed observation.

A surprising key finding was that even the smallest icy bodies share very similar colors with far larger TNOs. Scientists had expected that frequent collisions and fragmentation over billions of years would have altered their surfaces enough to produce noticeable color differences. However, the observations suggest that these small objects have preserved the original material properties from which they formed.

Furthermore, both observing teams found fewer small TNOs than some theoretical models had predicted. The size distribution appeared similar between two groups of objects distinguished by their orbital characteristics: “cold” TNOs with relatively circular, less inclined orbits, and “hot” TNOs exhibiting more elongated and tilted orbits.

This similarity suggests that the process that formed planetesimals—the building blocks of planets—produced comparable results under different early Solar System conditions. Notably, the “hot” objects, which have undergone considerable orbital evolution over time, still retain compositional clues to their original formation locations. This implies that later collisions and orbital changes did not completely erase the chemical signatures imprinted at birth.

By enabling the study of both large and very small trans-Neptunian bodies, these observations offer astronomers a rare opportunity to better understand the early Solar System’s makeup and evolutionary history. It sheds light on the conditions present during planet formation and the preservation of primordial material in distant icy worlds.