A world only 3 miles wide beyond Neptune still looks freshly made

The smallest object among 27 newly identified worlds beyond Neptune measures only about 3 miles across, roughly 5 kilometers, and it looks essentially the same as its far larger relatives, unweathered by the collisions astronomers expected to have reshaped it long ago. NASA announced the findings on 8 September 2026, the product of the first survey to point the Hubble and James Webb space telescopes at the same patch of sky simultaneously to study these distant, icy bodies together.

Trans-Neptunian objects, or TNOs, are small, faint bodies that orbit the sun beyond Neptune, and most are so dim that they sit more than 100 million times fainter than anything visible to the unaided eye. One of the 27 objects in the new survey was so faint that NASA compared detecting it to standing on Earth and spotting a small swarm of fireflies on the surface of the moon.

Twenty-Seven Worlds Too Faint to Catch Before

The survey combined Hubble’s sensitivity to visible light with Webb’s infrared vision, a pairing that let researchers measure the colors, sizes and orbits of TNOs that neither telescope could have fully characterized alone. Two teams, one led by a doctoral candidate at the University of Victoria working under Canada’s National Research Council, the other by a doctoral candidate at Northern Arizona University, published their results as companion papers in The Astronomical Journal, the deepest TNO survey conducted to date.

According to NASA’s account of the joint Hubble and Webb campaign, the smallest of the 27 newly found objects is about five times smaller than what even the most sensitive ground-based telescopes can detect, which is why a survey built around two space telescopes working in tandem was necessary to find it at all. Hubble supplied the visible-light measurements, Webb supplied the infrared data, and neither instrument alone could have delivered the combination of color, size and orbit information the teams needed.

NASA published the results as two companion papers in The Astronomical Journal on 8 September 2026, timing the release so the color-and-composition findings and the size-distribution findings would reach the astronomical community together rather than as two disconnected announcements months apart.

Building Blocks That Never Finished Building

TNOs matter to planetary scientists because they represent an arrested stage of planet formation. Beyond Neptune, the dust and pebbles that once orbited the young sun clumped into city-sized bodies called planetesimals, the solid building blocks that elsewhere went on to merge into full-sized planets, but in the outer solar system that merging never happened, leaving a frozen population of planetesimals largely untouched since.

Researchers sorted the objects they studied into two dynamical groups. Dynamically cold TNOs still follow close to their original, nearly circular orbits in the plane of the solar system, while dynamically hot TNOs formed closer in, between where Uranus and Neptune sit today, before being flung outward into steep, elliptical orbits when the giant planets migrated early in the solar system’s history.

Objects That Kept Their Original Surfaces

Before this survey, astronomers generally expected that small TNOs from both populations would have absorbed enough collisions over billions of years to visibly alter their surfaces compared with their larger counterparts. That is not what the data showed. Anastasia Morgan, the Northern Arizona University doctoral candidate who led the study of color and composition, said the smallest objects appear to be preserving the history of how they were made rather than showing the altered surface color a history of fragmenting collisions would produce, calling it fascinating that the tiniest bodies seem to remember their own formation.

David Trilling of Northern Arizona University, a co-author on that study, said the dynamically hot TNOs still carry a signature of where they originally formed even after being scattered into their current, more chaotic orbits. The teams have not settled on why the expected weathering never showed up, and are continuing to weigh whether the small bodies simply experience fewer collisions than models predicted or somehow retain their original composition despite the impacts they do absorb.

Fewer Small Worlds Than the Models Predicted

Webb’s data also let researchers count objects by size across both populations, and the count came up short of expectations. Marielle Eduardo, the University of Victoria doctoral candidate who led the size-distribution study, said the two populations ended up with surprisingly similar overall size distributions despite forming in different regions of the early solar system, describing the underlying planetesimal-formation process as producing similar outcomes whether the original disk of material was hot or cold, dense or sparse.

That similarity, paired with fewer very small TNOs than some planet-formation models anticipated, gives researchers a new constraint to test those models against. The two studies behind these findings are published as separate papers, one covering color and composition and the other covering size distribution, and neither would have been possible without Hubble and Webb observing the same targets together, since each telescope’s data filled a gap the other could not close alone.

Both teams describe the coordinated survey as a template rather than a one-time exercise, since the same pairing of visible-light and infrared observations could be pointed at other faint, distant populations in the solar system in the years ahead. For now, the 27 objects catalogued in these two papers stand as the deepest look yet at bodies that have sat largely undisturbed since the solar system’s earliest days, three miles across and, by the researchers’ own account, still looking much as they did when they first formed.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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