27 trans-Neptunian objects shed light on the early Solar System

Hubble and James Webb have detected 27 small objects beyond Neptune. Their colours and sizes offer clues about collisions and the formation of the Solar System’s first solid bodies.

Beyond Neptune, icy bodies so faint that they generally elude ground-based telescopes have offered a glimpse of the Solar System’s youth. Combining observations from Hubble and the James Webb Space Telescope, two teams have identified 27 previously unknown trans-Neptunian objects. Their significance lies less in their number than in the clues they provide about the formation of the first building blocks of matter from which the planets emerged.

NASA’s announcement, published on 8 September 2026, draws on two studies published in “The Astronomical Journal”. The researchers examined the colours, sizes and orbits of these bodies. The findings put some expectations about the effects of collisions to the test, without turning these objects into new planets or providing a direct picture of their internal composition.

Seeing an object too faint for ground-based telescopes

Trans-Neptunian objects, or TNOs, orbit the Sun beyond Neptune. They are small, icy and very distant. Many are more than a hundred million times fainter than objects visible to the naked eye. In this survey, the smallest object observed is about five kilometres in diameter, making it difficult to detect from the ground.

The method relies on the complementary capabilities of two space observatories. Hubble measured the objects’ visible light; James Webb observed their infrared radiation. By studying the same region of the sky, the teams were able to combine these measurements to characterise the bodies they had detected. The coordinated observations made it possible to determine their colours, sizes and orbits.

Colour is not, by itself, a complete chemical analysis. It offers a clue about surface properties, like a fingerprint that helps compare objects with one another. It is this comparison, combined with measurements at multiple wavelengths and other data, that allows researchers to discuss their history. The observed colour should therefore not be read as a direct indication of temperature, or as stand-alone proof of the presence of a particular material.

Two populations, two orbital histories

The teams studied objects belonging to two groups known as “dynamically cold” and “dynamically hot”. These terms describe their orbits, not their temperature. Cold TNOs follow relatively circular paths, close to the plane in which the planets orbit the Sun. Hot TNOs have more elongated and more inclined orbits.

According to the scenario described by NASA, some objects now classed as hot may have formed between the former positions of Uranus and Neptune, before being pushed outwards as the giant planets migrated. Their current orbits thus preserve traces of disturbances that occurred early in the Solar System’s history. The cold TNOs studied, by contrast, follow relatively circular orbits close to where they formed.

The groups therefore followed different paths, but the researchers found that their size distributions were surprisingly similar. A size distribution describes the proportions of objects of different dimensions in a population. This similarity could indicate that the formation of the first solid bodies produced comparable sizes in regions with different conditions. It offers a line of enquiry for planet-formation models, but is not yet a definitive conclusion about all the conditions in the disc where the planets formed.

Surfaces that seem to retain a memory

The observations also yielded a surprise concerning collisions. Over billions of years, small bodies are exposed to impacts that can fragment them or alter their surfaces. Some models had led researchers to expect that these impacts would have changed the surface properties of small TNOs, making them different from their larger neighbours.

Yet the small objects observed have colours comparable to those of the larger bodies in their respective populations. The teams infer that collisions may not have transformed the surfaces as much as previously thought, or that some objects have retained properties inherited from their formation. The two possibilities have yet to be distinguished. The similarity in colour is a clue to continuity, but on its own it does not tell the detailed story of the impacts experienced by each body.

This “memory” is particularly useful because TNOs are remnants of material that did not go on to form a planet. At the beginning of the Solar System, a disc of dust and small grains orbiting the Sun coalesced into solid bodies called planetesimals. These provided the starting material from which larger objects could form. Beyond Neptune, this process did not lead to the construction of planets comparable to those in the inner region; a population of small icy bodies remains there.

Fewer small bodies than some models predicted

The observations also revealed fewer very small objects than some planet-formation models had predicted. This apparent deficit, together with the similarity in size distributions between populations, could lead researchers to reassess how the first solids assembled from dust and larger particles.

However, an observational finding must be distinguished from a general explanation. The telescopes detected 27 new TNOs and measured their properties as part of this study. Comparison with the models raises questions about the formation and evolution of these populations; it is not enough to establish a single history of the Solar System. The observed objects are few in number compared with the populations orbiting beyond Neptune, and interpretations will be refined as new observations are made.

The significance of this survey also lies in the combination of the two instruments: Hubble provides visible light, while Webb provides infrared. Together, they make it possible to detect and characterise bodies too faint for even the most sensitive ground-based telescopes. These are measurements of light and motion, which researchers use to derive properties and develop scenarios. At the edge of the Solar System, these small objects do not directly reveal every stage in the birth of the planets, but they allow models to be tested against material preserved since the earliest times.