Hubble and Webb Find Ancient Icy Worlds That Kept Their Origins

The takeaway: two telescopes, one patch of sky, and a sharper picture than either machine alone.

What did 'cold' and 'hot' objects reveal?

The researchers studied two distinct TNO populations, labeled dynamically "cold" and "hot" — names describing orbits, not temperatures.

• Cold TNOs stayed close to where they formed. Their orbits are relatively circular and hug the plane where most planets travel.
• Hot TNOs formed between the present-day orbits of Uranus and Neptune, then got scattered outward as the giant planets shifted positions early on. Their orbits are elongated and carry them above and below the main orbital plane.

Despite these radically different histories, the two groups showed an unexpected similarity. The smallest objects keep surface characteristics resembling their larger counterparts.

Before the survey, scientists expected billions of years of repeated collisions would have fractured the small bodies and exposed fresh material, changing their colors. That didn't show up. Collisions may have altered these surfaces far less than assumed — or the surfaces somehow keep their original makeup even after impacts. Researchers haven't yet worked out which explanation holds.

"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 Anastasia Morgan, a Northern Arizona University PhD candidate who led the color 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.

The takeaway: even after 4.5 billion years of orbital chaos, these objects still carry their birth certificates.

Why are there fewer small objects than predicted?

Webb's data also let scientists count how many objects exist at each size, and that produced a second surprise. Hot and cold TNOs formed in different parts of the early solar system, yet their size distributions look remarkably alike.

"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 Marielle Eduardo, a University of Victoria PhD candidate who led the size-distribution study.

The teams also found a shortage of very small TNOs compared with what some planet formation models predict. Explaining the deficit could help scientists refine how they think the solar system's original building blocks developed.

These are preliminary reads, and the researchers themselves flag open questions — the collision puzzle chief among them.

The takeaway: planetesimals may form the same way everywhere, and current models may simply predict too many small ones.

What changes for planet-formation science?

The discoveries hinged on complementarity. Hubble's visible-light vision and Webb's infrared sensitivity delivered a fuller portrait of these distant bodies than either telescope could manage alone.

Together, they've opened a window onto a largely unexplored population of ancient remnants. Their shared colors, preserved surfaces and unexpected numbers all feed into a better picture of how the planets took shape billions of years ago.

The findings appear in two companion papers in The Astronomical Journal (172: 187 and 172: 188, 2026), led by Eduardo and Morgan respectively, with co-authors including Wesley Fraser, Gary Bernstein, John Stansberry and William Grundy.

The takeaway: the solar system's smallest survivors just became some of its most informative witnesses.