Astronomers have studied 27 small icy objects orbiting beyond Neptune, finding that some may preserve clues to the conditions in which the Solar System’s planetary building blocks formed.
The objects are called trans-Neptunian objects, or TNOs. Some are only about five kilometres across, making them among the faintest and smallest objects of their kind to be observed directly, according to coverage published by BBC Sky at Night Magazine on November 8.
The researchers combined observations from the Hubble Space Telescope and the James Webb Space Telescope. Their goal was to examine how the objects’ surfaces vary in colour, which can offer clues about their composition and history.
Small objects that may retain an old signature
Small bodies in the outer Solar System are expected to have experienced many collisions over billions of years. Impacts can break up surfaces, expose new material and mix different substances.
That led researchers to expect the smallest TNOs to have more varied colours than larger ones. Instead, the small objects largely followed colour patterns seen among larger TNOs.
The result suggests that their surfaces may have changed less than expected. Lead author Anastasia Morgan of Northern Arizona University described the objects as preserving a memory of how they formed.
That interpretation is intriguing, but colour is an indirect clue. It does not provide a complete chemical inventory of an object, and further observations will be needed to understand why the patterns persist.
A population smaller than some models predict
The researchers also found fewer tiny TNOs than some planet-formation models predicted. That difference could help scientists refine their ideas about how small bodies formed, grew and were reshaped in the distant Solar System.
These objects are remnants of an era when dust and small pieces of ice gradually gathered into larger bodies. Some of those bodies eventually became planets and moons. The present-day population beyond Neptune therefore offers a record of processes that took place early in the Solar System’s history.
The objects are extremely difficult to study because they are far away and faint. Even powerful telescopes can detect only a limited number, and measuring their properties requires careful observation.
What comes next
The team is planning deeper observations. Future work with Webb could look for TNOs as small as one kilometre across, potentially allowing researchers to compare an even wider range of sizes.
A larger sample could help determine whether the colour patterns are typical of small objects throughout the outer Solar System or whether the observed group has unusual characteristics.
The findings also show how combining observatories can answer questions that one telescope may not address on its own. Hubble and Webb can contribute complementary measurements, helping astronomers investigate objects that are both small and remote.
For now, the study offers a new view of how icy bodies may preserve evidence of their beginnings. It does not settle how the planets formed, but it gives researchers another piece of the puzzle: a population of tiny worlds whose surfaces appear to retain more of their past than collision models once suggested.