A joint Hubble–Webb survey found 27 faint objects beyond Neptune, including one only about 5 kilometers across. Their colors and size distribution suggest these tiny worlds may preserve surprisingly old information about how the solar system’s planet-building blocks formed.
RESEARCH · OPEN QUESTION
The smallest fossils are still moving
Trans-Neptunian objects, or TNOs, are icy bodies that orbit beyond Neptune. Many are leftovers from the era when dust and pebbles were assembling into planetesimals—the building blocks of planets. A 2026 Hubble–Webb survey detected 27 new, extremely faint TNOs, including an object only about 5 kilometers across. [1]
These are not fossils in the biological sense. They are physical survivors whose surfaces and orbits can retain information about the solar system’s early disk.
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JOIN THE SIGNAL →Hot and cold populations have different histories
Astronomers distinguish dynamically “cold” TNOs, which remain on relatively calm orbits, from dynamically “hot” populations whose paths were stirred as the giant planets migrated. The hot objects are thought to have formed closer to the Sun before being pushed outward. [1]
That gives researchers two populations with different formation and migration histories. If later collisions had strongly altered their smallest members, the tiny objects might look different from larger relatives.
The surprise was in their colors
The teams measured visible and infrared colors as a proxy for surface composition. The small objects followed the same color relationships seen in larger members of their respective populations. In other words, shrinking the object size did not erase the chemical family resemblance researchers expected collisions to disrupt. [1]
That is the “memory” in NASA’s headline: the surfaces appear to preserve a signature linked to where the objects formed, even after billions of years of orbital evolution.
There were fewer tiny objects than expected
Webb also helped measure the size distribution. Researchers found fewer very small TNOs than some formation models predicted, while the hot and cold populations showed unexpectedly similar overall size distributions. [1]
This matters because the number of objects at each size records how planetesimals formed and how often they were later broken apart. A shortage of the smallest bodies may mean fragmentation was less efficient, collisions were less common, or early formation produced a characteristic size distribution that survives today.
Why Hubble and Webb needed each other
Hubble measures visible light while Webb extends the view into infrared wavelengths. Using both telescopes on the same faint targets lets researchers constrain colors and surface properties that either observatory alone would sample less completely. [1]
The result is a rare experiment on objects too small and distant for detailed images: astronomers cannot see craters or landscapes, but they can still read population-level clues from light, color, size, and orbit.
What is still unknown
The observations do not yet explain exactly why the smallest TNOs retain their population signatures or why there are fewer of them than expected. More surveys are needed to determine whether this pattern holds across a larger part of the outer solar system.
For Dark Nova, these bodies are a reminder that some of the solar system’s deepest history may be stored not in the largest worlds, but in the tiny survivors that never became worlds at all.
Sources and further reading
- NASA Science — Hubble, Webb Find Far-out Solar System Objects “Remember” Past (September 8, 2026)
- NASA Science — Trans-Neptunian Object Illustration and mission context (September 8, 2026)
Sources checked September 29, 2026. Written with AI assistance using the linked primary or agency sources. Reported findings are separated from Dark Nova editorial explanation. Send a correction or source.