Webb and Hubble Find 27 Tiny Worlds Beyond Neptune Stay Pristine

New Webb and Hubble observations uncovered 27 small trans-Neptunian objects whose surfaces remain untouched despite collisions models predicted would alter them.

The Hubble and James Webb space telescopes teamed up for the deepest survey yet of the solar system's far edge, discovering 27 new Trans-Neptunian Objects, all less than 25 miles (40 kilometers) across, with the smallest only about 6 miles (10 kilometers) in diameter. TNOs are small, faint, icy bodies orbiting the sun beyond Neptune; some formed there at the dawn of the solar system as planetesimals that never grew into planets. Hubble observed them in visible light while Webb observed them in infrared, and the objects shine between magnitudes 24.1 and 29.3 — space.com describes this faintness as equivalent to seeing a swarm of fireflies on the moon from Earth.

Models of how these tiny objects formed predicted they should have been peppered with impacts that mixed up their surface material, so their composition and color would differ from larger TNOs. The new observations, led by PhD candidates Anastasia Morgan of Northern Arizona University and Marielle Eduardo of the University of Victoria, found the opposite: the little TNOs still look as pristine as the day they formed. "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," Morgan said. "So it's really fascinating to see that the smallest objects are somehow 'remembering' and preserving the history of how they were made."

The survey split the objects into two types. Dynamically cold TNOs move on their original near-circular orbits in the plane of the solar system and have not budged since they formed, while dynamically hot TNOs formed between Uranus and Neptune but were ejected by gravitational resonances into elongated, inclined orbits in a 'Scattered Disk'. According to space.com, even the hot TNOs seem to have resisted any changes to their surface composition — "These dynamically hot TNOs retain a signature of where they were born, even though they've been orbitally scrambled since then," said David Trilling of Northern Arizona University. That leaves two possibilities: either far fewer impacts occur far from the sun than astronomers thought, or the impacts happen but do not tear up the small bodies' surfaces as expected.

In visible light, a TNO's apparent brightness depends partly on how reflective its surface is, its albedo, so a larger dark body can look fainter than a smaller icy one. At infrared wavelengths, however, brightness depends mostly on size, which allowed Webb's infrared vision to yield accurate diameters for all 27 objects. Surprisingly, the team found fewer of the very small TNOs than formation models predict. "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," Eduardo said, adding that the process seems insensitive to whether the planet-forming disk was hot or cold, dense or fluffy.

TNOs are the solid building blocks of planet formation — city-sized planetesimals that coalesced from a disk of dust and pebbles orbiting the young sun, and some clumped together to form planets. NASA and space.com both note the discovery offers a glimpse into those early planet-building stages, because the tiny bodies appear to be preserving their primordial, pre-collision state rather than showing the surface alteration astronomers expected.

According to space.com, the research was published in The Astronomical Journal as two separate papers on Sept. 8, one on the objects' color and composition and the other on their size distribution. Deeper surveys should find more of these faint bodies and clarify their role in the solar system's history. New hardware is arriving to help: The Ticker reports that NASA's Nancy Grace Roman Space Telescope, launched Aug. 30, 2026, will combine Hubble-quality imaging with a field of view more than 200 times larger, and the telescope's wide, repeated imaging of huge star fields — including a microlensing survey of hundreds of millions of stars toward the galactic center, described by The Conversation — shows how space telescopes increasingly survey populations rather than single targets. History suggests such wide-field instruments often make discoveries nobody anticipated.