The Smallest Things Remember

Astronomy Daily companion blog · S05E189 · Wednesday, 9 September 2026 · astronomydaily.io
Objects five kilometres across, four and a half billion kilometres away, should be broken fragments of something bigger. Hubble and Webb have just found twenty-seven of them — and they are still wearing the colours they were born with.
Everything past Neptune is a leftover. The icy bodies of the Kuiper Belt are material that never got assembled into a planet, and out there — forty or fifty degrees above absolute zero, empty enough that objects mostly leave each other alone — they have sat undisturbed since the solar system was built. Earth melted. Mars melted. The asteroid belt has been ground down for four and a half billion years. The Kuiper Belt is the only place the original building blocks are still lying around in close to their original state.
There is a structure to it that matters. The objects astronomers call dynamically cold sit on calm, nearly circular, barely tilted orbits, almost certainly formed where we find them, and are deeply, uniformly red — irradiated organic material accumulated over billions of years. The dynamically hot population is on elliptical, tilted, scattered orbits: those formed closer to the Sun and were flung outward when the giant planets migrated, and they show a wider range of colours because they came from a range of starting distances. Colour, out there, works as a rough birth certificate.
The problem with the big ones
The difficulty is that all of that was learned from large objects. Ground-based telescopes lose these things at around twenty-five kilometres across, and below that limit the field has been working on assumption. The assumption was reasonable: an object a few kilometres across is unlikely to be an original, and far more likely to be shrapnel — a fragment left when two larger bodies collided.
That has a testable consequence. The red surface out there is a rind only a few metres deep; break a body open and you expose fresh ice that is much less red. A population made mostly of fragments should look noticeably bluer, and messier, than the large objects it came from.
Fireflies on the Moon
Two papers published on 8 September in The Astronomical Journal put that to the test, using an observing programme that pointed Hubble and Webb at the same patch of sky at the same moment — one in visible light, one in the infrared. The simultaneity is not a nicety: these are faint objects moving against the background stars, and a meaningful colour needs both measurements taken at the same instant.
The programme turned up twenty-seven previously unknown trans-Neptunian objects, the faintest ever directly detected. The smallest is about five kilometres across, roughly five times below the ground-based limit. NASA’s description of the faintest is hard to improve on: the equivalent of standing on Earth and picking out a small swarm of fireflies on the Moon.
The colour study, led by Anastasia Morgan at Northern Arizona University, found the small objects do not look like fragments at all. Within each population they carry the same colour relationship as their large counterparts — the smallest objects, in Morgan’s phrasing, are somehow remembering and preserving the history of how they were made. It holds even for the hot population, the objects thrown outward from somewhere else entirely. As David Trilling puts it, these dynamically hot objects retain a signature of where they were born, even though they have been orbitally scrambled since.
Two results, one conclusion
The companion paper, led by Marielle Eduardo at the University of Victoria, looked at the size distribution — how many small objects there are for every large one, which is a fingerprint of how a population was assembled. Gradual accretion leaves one shape; rapid formation leaves another. Eduardo found the same distribution in both populations, despite their having formed in different regions of the disc. Different neighbourhoods, the same recipe.
Read together, the results point away from small trans-Neptunian objects being rubble and towards their having been made small and stayed that way — which fits a model gaining ground for fifteen years, in which planetesimals form quickly, at large sizes, when a cloud of pebbles collapses under its own gravity. New Horizons saw one up close: Arrokoth, on New Year’s Day 2019, two lobes resting against each other, having come together at walking pace. Nothing about it looked violent.
The caveats are the ordinary ones: twenty-seven objects is a small sample, the conclusions are statistical rather than measurements of individual bodies, the colours come from filters rather than spectra, and all of it is a single deep line of sight.
What fixes that is southern
The fix is volume, and the machine built to deliver it sits on Cerro Pachón in Chile. The Vera Rubin Observatory’s premise is repeatedly imaging the whole southern sky, and it should find trans-Neptunian objects in numbers that make the current catalogue look like a pilot study. Rubin finds them and gives you orbits; Hubble and Webb are what you point at the interesting ones. There is also a third contribution Australia and New Zealand are unusually good at — stellar occultations, where you predict the moment a tiny object passes in front of a background star, spread telescopes along the shadow path and time the blink. It is how you get a real size and shape for something you cannot resolve, many of those shadow paths cross the southern oceans, and a great deal of that work is done by people standing in a paddock at three in the morning.
Twenty-seven new objects, the smallest the size of a suburb, still carrying the colours of a solar system that stopped existing four and a half billion years ago. You can hear the full discussion on Astronomy Daily, Series 5 Episode 189.