Some of the solar system’s smallest, faintest bodies may preserve clues to how its planets began. A NASA-supplied report published by ScienceDaily describes a survey in which the Hubble and James Webb Space Telescopes were used together to study 27 previously unknown trans-Neptunian objects, or TNOs, beyond Neptune’s orbit. The observations reveal similarities between small and large objects, alongside a shortage of tiny ones that challenges some planet-formation predictions.
Two studies in The Astronomical Journal examined the objects’ colors and sizes. The research teams were led by PhD candidates at the University of Victoria and Northern Arizona University, with guidance from the National Research Council of Canada. Anastasia Morgan of Northern Arizona University led the study of color and composition; Marielle Eduardo of the University of Victoria led the study of size distribution.
The survey paired Hubble’s observations in visible light with Webb’s infrared observations of the same region of sky. Together, the data helped researchers estimate the objects’ sizes, compare their colors and determine their orbits. Color can offer clues about surface chemistry, but the report describes it as an indicator rather than a direct account of each object’s composition.
The team studied two groups called dynamically “cold” and “hot” TNOs. Those labels describe their orbits, not their temperatures. Cold objects largely remain near the regions where they formed, following relatively circular paths close to the plane of the planets. Hot objects are thought to have formed between the present-day orbits of Uranus and Neptune, then been scattered outward as the giant planets shifted early in the solar system’s history. Their current orbits are more elongated and extend farther above and below the planetary plane.
Despite those different orbital histories, the two groups showed similar color relationships between their smallest members and larger objects. Before the observations, astronomers had expected repeated impacts over billions of years to fracture small bodies and expose fresh material, potentially changing their surface colors. The reported similarity suggests that such surface changes may have been less pronounced than expected, and that the objects may retain clues to their formation.
The reason remains uncertain. The report offers two possibilities: the objects may have experienced fewer impacts than astronomers assumed, or their surfaces may retain their original compositions even after collisions. The observations do not determine which explanation is correct. David Trilling of Northern Arizona University, a co-author, said that the dynamically hot objects retain a signature of where they formed despite their altered orbits.
Webb’s measurements also revealed that the hot and cold populations have remarkably similar size distributions, even though they formed in different parts of the early solar system. That resemblance suggests the processes that assembled planetesimals—the early building blocks of planets—may have worked similarly across those environments. It does not establish that conditions were identical or explain how the bodies formed.
The survey also found fewer very small TNOs than some existing planet-formation models predict. The smallest object measured was about 5 kilometers (3 miles) across, according to the report. Webb’s sensitivity made it possible to detect 27 exceptionally faint objects, including one whose brightness the report compares to spotting a small swarm of fireflies on the Moon from Earth. The shortage of small objects could help researchers refine models, but the report does not settle why fewer were found.
TNOs are difficult to observe: they are so faint that even Hubble and Webb see them only as points of light. Their value lies in what their patterns may preserve. If their colors and sizes retain traces of the early solar system, they offer astronomers a way to test ideas about the materials and conditions that preceded the planets. The combined observations open a view of that distant population, while leaving key questions about collisions and formation unresolved.