BEYOND NEPTUNE — Working together for the first time, NASA's Hubble and James Webb space telescopes have found 27 tiny, dim icy worlds orbiting the Sun beyond Neptune, the smallest only about 5 kilometres across — and, remarkably, these ancient bodies still carry the colours of the material from which they formed more than 4 billion years ago, according to two studies published in The Astronomical Journal.
The objects, known as trans-Neptunian objects (TNOs), are the frozen leftovers of planet formation: city-sized "planetesimals" that never merged into full planets in the outer solar system. In the deepest survey of this population ever made, Webb discovered the 27 new bodies, and Hubble followed up in visible light, giving astronomers colours spanning from visible to infrared wavelengths — a fingerprint of surface composition. The findings were published on 8 September 2026 and highlighted in new reporting on 2 November 2026.
Illustration: The Hubble and James Webb telescopes observing tiny icy worlds far beyond Neptune. This is an illustration, not a photograph of a real event.
What Happened
For the first time, astronomers used Hubble and Webb together to study TNOs in the same patch of sky. Webb, observing in infrared light, scanned a 0.05-square-degree field in three epochs between 24 January and 4 February 2023, imaging a 20-tile mosaic with its NIRCam instrument three times, about five days apart. Using a shift-and-stack technique and a machine-learning network to pick out moving objects, the team found 27 previously unknown TNOs, most more than 100 million times dimmer than objects visible to the unaided eye.
Hubble then observed the same field in visible light. The second team used 99 Hubble orbits at nine pointings, covering 65 percent of Webb's field of view, and recovered 13 of Webb's discoveries in Hubble images. Combining the two telescopes' data gave colours spanning 0.35 to 3.2 micrometres, letting the teams measure sizes, colours and orbits for these faint bodies.
The smallest object measured about 3 miles (5 kilometres) across — roughly five times smaller than the faintest object detectable with the most sensitive ground-based telescopes, which reach only about 50 kilometres, while the deepest previous Hubble search reached about 30 kilometres. One body was so faint that researchers compared spotting it to standing on Earth and seeing a small swarm of fireflies on the Moon.
The work was led by doctoral candidates: Marielle Eduardo of the University of Victoria in Canada, under the guidance of the National Research Council of Canada, led the study of the size distribution, titled "The Luminosity Function of Ultra-Faint Trans-Neptunian Objects Detected by JWST." Anastasia Morgan of Northern Arizona University in Flagstaff led the companion study of colours and composition, titled "Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects." Both appeared in The Astronomical Journal.
Illustration: Small icy planetesimals drifting in the dark outer solar system beyond Neptune. This is an illustration, not a photograph of a real event.
Background
TNOs are small, faint, icy bodies orbiting the Sun beyond Neptune. They offer the best surviving view of an early stage of planet-building, when a disk of dust and pebbles around the young Sun coalesced into planetesimals — the solid building blocks that clump together to form planets. Closer to the Sun, those blocks merged into full-sized worlds. Beyond Neptune, that second stage never happened, leaving a frozen population of planetesimals largely untouched.
Astronomers divide TNOs into two dynamical families. "Cold" TNOs still travel on their original, relatively circular orbits in the plane of the solar system. "Hot" TNOs formed between the present locations of Uranus and Neptune but were pushed outward when the giant planets migrated early in solar system history; today they follow highly elliptical orbits that carry them in and out of the solar system's plane.
Before these observations, researchers expected small TNOs in both families to look different from their larger relatives. Over billions of years, collisions should have battered their surfaces and changed their colours. The new survey tested that expectation at sizes never probed before — and found the opposite.
Why It Matters
The most surprising result is that even the smallest objects kept the colour patterns of much larger bodies. The cold classical bodies, on near-circular orbits, showed a limited colour range matching the same colour sequence seen in bigger bodies, and the authors found no evidence of a colour change at smaller sizes. In other words, these tiny worlds "remember" how they were made.
"You could imagine a scenario where getting knocked around and fragmented would change the surface composition, and then you would see a different surface color for tiny TNOs compared to their larger siblings. So it's really fascinating to see that the smallest objects are somehow 'remembering' and preserving the history of how they were made," said Morgan, who led the colour and composition study.
"These dynamically 'hot' TNOs retain a signature of where they were born, even though they've been orbitally scrambled since then," said co-author David Trilling of Northern Arizona University. Both hot and cold populations, the teams found, seem to keep the colours they had when they formed, with little change since the birth of the solar system.
The size distribution held a second surprise. Eduardo's team found that the number of objects at each size follows a single power law, with a slope of 0.29, and that the hot and cold subgroups follow the same power law even though they are thought to have formed at different distances from the Sun. That suggests planetesimal formation produced similar size distributions under different conditions in the early disk.
"It's very interesting that the process of planetesimal formation ends up producing the same distribution of sizes for both cold and hot populations, despite forming in different regions of the early solar system. The process seems to be insensitive to disk conditions, producing similar planetesimal sizes whether the disk is hot or cold, and dense or fluffy," said Eduardo.
The teams also found fewer very small bodies than some planet-formation models predicted. That shortfall is itself a clue: it may mean collisions are rarer than expected in the outer solar system, or that the tiniest planetesimals formed differently than models assumed — either way, models of how planets grow will need to account for it.
Illustration: Astronomers studying telescope images of faint distant solar system objects. This is an illustration, not a photograph of a real event.
What Happens Next
Researchers say further observations will help explain why these ancient objects retained their original characteristics and what their populations reveal about planet formation. The same combined Hubble-Webb approach can now be extended to other fields, and future surveys — including the growing census from the Vera C. Rubin Observatory, which listed 8,291 trans-Neptunian objects as of late October 2026 — will test whether the size and colour patterns found in this small, deep field hold across the wider outer solar system.
For now, the 27 new worlds stand as a reminder of how much of solar system history is still written in its smallest, faintest members. They never became planets, but in their preserved surfaces they carry a record of the disk from which every planet — including Earth — was built.
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FAQ
What is a trans-Neptunian object?
A trans-Neptunian object (TNO) is a small, icy body that orbits the Sun beyond Neptune. Most are far too faint to see with the unaided eye, and they are the leftover planetesimals from the solar system's formation.
How many new objects did Hubble and Webb find?
Webb discovered 27 previously unknown TNOs in the deepest survey of this population to date, and Hubble recovered 13 of them in visible light, giving combined colours from 0.35 to 3.2 micrometres.
How small is the smallest object found?
About 3 miles (5 kilometres) across — roughly five times smaller than the faintest object detectable with the most sensitive ground-based telescopes.
Why do the colours of these objects matter?
Colour is a fingerprint of surface composition. Because the smallest TNOs share the colours of larger ones, researchers conclude their surfaces have changed little since formation, preserving a record of where and how they formed.
What is the difference between "hot" and "cold" TNOs?
Cold TNOs remain on their original, near-circular orbits in the solar system's plane. Hot TNOs formed closer in, between Uranus and Neptune, and were scattered outward by migrating giant planets into elliptical, tilted orbits — yet both groups show similar size distributions.
Sources
NASA, "NASA's Hubble, Webb Find Far-out Solar System Objects 'Remember' Past," 8 September 2026; ScienceDaily / Shafaqna English, "Hubble and Webb discover 27 tiny objects preserving clues to the solar system's origins," 2 November 2026; Eduardo et al., "The Luminosity Function of Ultra-Faint Trans-Neptunian Objects Detected by JWST," The Astronomical Journal (2026), DOI: 10.3847/1538-3881/ae907f; Morgan et al., "Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects," The Astronomical Journal (2026), DOI: 10.3847/1538-3881/ae9084.
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