NASA reports that coordinated observations by the Hubble Space Telescope and the James Webb Space Telescope have characterised 27 newly discovered, very faint trans-Neptunian objects (TNOs). The smallest was about 5 kilometres across.

The observations found that small bodies in two dynamically distinct TNO populations showed colour relationships similar to those seen in larger objects. NASA says this may indicate that collisions have not substantially altered their surfaces, although the reason remains unresolved. Webb also found fewer very small objects than some planet-formation models predicted.

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What the telescopes measured

Trans-Neptunian objects are small bodies that orbit the Sun beyond Neptune. Many occupy the Kuiper Belt, a distant region containing icy remnants from the Solar System’s formation.

NASA describes the observations as the first joint study of these far-flung bodies using Hubble and Webb. The two telescopes supplied different but complementary measurements: Hubble observed visible light, while Webb observed infrared light. Combining those observations gave the teams information that neither dataset could provide alone.

The researchers measured the objects’ colours, sizes and orbits. The objects were not resolved into visible worlds or mapped surface features. Instead, they appeared as extremely faint points of light, from which astronomers could extract photometric and orbital information.

Colour can provide clues about the composition and physical state of a surface, but it is not a complete compositional measurement by itself. Infrared observations add another part of the light spectrum, helping researchers examine these faint objects more fully.

The work was reported in two complementary papers published in The Astronomical Journal. A University of Victoria team led the analysis of the size distribution, while a Northern Arizona University team studied colour and composition.

Two populations, similar colour relationships

The researchers compared objects from dynamically hot and dynamically cold TNO populations.

In this context, “hot” and “cold” describe orbital histories rather than temperature. NASA describes hot TNOs as bodies that formed between the present-day orbits of Uranus and Neptune and were later pushed outward during the early migration of the outer giant planets. Cold TNOs are described as bodies that remain on relatively circular, original orbits in the plane of the Solar System.

Orbital dynamics can preserve clues about where a small body formed, even after gravitational interactions move it elsewhere. Comparing these populations therefore provides a way to examine bodies with different formation and migration histories.

The small TNOs in both populations showed the same broad colour relationships as their larger counterparts. The small objects did not show noticeably different colour relationships that would indicate substantial collision-related surface alteration.

NASA presents two possible explanations: the objects may have experienced fewer collisions than expected, or they may have retained material from their early, or primordial, surfaces. The observations do not yet determine which explanation is correct.

The result matters because TNOs are remnants of the planetesimal-building stage of Solar System formation. Planetesimals are solid bodies formed when material in the early Solar System coalesced; planetesimals can later combine to form planets. Small TNOs can therefore preserve evidence from an early stage of Solar System formation.

Fewer small objects than some models predicted

The observations also produced a constraint on the population’s size distribution—the number of objects found at different sizes.

Webb found fewer very small TNOs than expected from some planet-formation models. The overall size distributions of the dynamically hot and cold populations were also surprisingly similar.

That combination is relevant to models of the early Solar System. If bodies formed in regions with different conditions in the early disk, researchers might expect those regions to produce different distributions of planetesimal sizes. The reported observations instead suggest that comparable size outcomes may have emerged in the two populations.

This is an interpretation of the observations, not a direct reconstruction of the early Solar System. The supplied NASA summary does not identify the specific models, their predicted numbers or the statistical uncertainty around the observed deficit of very small objects.

What the findings do not establish

The sample contains 27 newly discovered objects, so it is not yet established that it represents the wider trans-Neptunian population or the entire Kuiper Belt.

The source also does not provide the survey area, detection completeness, statistical uncertainties or confidence intervals needed to assess the robustness of the reported size-distribution result.

Nor do the similar colours prove that collisions were rare. They are consistent with limited surface modification, but preservation of original material is another possible explanation. The observations do not resolve the balance between those possibilities.

Finally, colour measurements alone cannot identify the full composition of each object. The study provides photometric, infrared and orbital evidence, not complete compositional measurements of the surfaces.

What to watch next

Further analysis should test why small TNOs show colour relationships similar to larger bodies. Additional observations could show whether the low number of very small objects is confirmed in a broader sample or reflects the limits of the current observations.

Researchers will also need to test whether revised planet-formation and collision models can reproduce all of the reported features together: the colour relationships, the unexpectedly low number of very small bodies and the similar size distributions of the hot and cold populations.

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