Hubble and Webb Team Up to Spy Tiny Frozen Worlds beyond Neptune

For the first time, astronomers combined Hubble and Webb space telescopes to hunt down some of the smallest, faintest objects ever spotted in the outer Solar System.

Two teams of astronomers led by University of Victoria and Northern Arizona University PhD candidates used Hubble’s visible-light vision alongside Webb’s infrared eye to examine 27 newly found trans-Neptunian objects (TNOs).

These tiny bodies are icy relics left over from the Solar System’s construction phase, before planetesimals ever merged into full planets.

The smallest was just 5 km (3 miles) wide, and one was so dim it’s comparable to spotting fireflies on the Moon from Earth.

These objects matched the colors of their much larger TNO cousins, suggesting collisions haven’t reshaped their surfaces as much as scientists expected.

The astronomers also found fewer small TNOs than predicted, and discovered that both ‘hot’ and ‘cold’ populations of TNOs — sorted by their orbital paths — share strikingly similar size distributions, hinting that planetesimal formation doesn’t much care about the conditions of the disk it happens in.

“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,” said Marielle Eduardo, a Ph.D. candidate at the University of Victoria.

“The process seems to be insensitive to disk conditions, producing similar planetesimal sizes whether the disk is hot or cold, and dense or fluffy.”

“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,” added Anastasia Morgan, a Ph.D. candidate at Northern Arizona University.

“So it’s really fascinating to see that the smallest objects are somehow ‘remembering’ and preserving the history of how they were made.”

“Dynamically ‘hot’ TNOs retain a signature of where they were born, even though they’ve been orbitally scrambled since then,” said Dr. David Trilling, an astronomer at Northern Arizona University.

“Both the ‘hot’ and ‘cold’ populations seem to keep the same colors as when they were formed, with little change since the birth of the Solar System.”

The results appear in two complementary papers in the Astronomical Journal.

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Anastasia N. Morgan et al. 2026. Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects. AJ 172, 188; doi: 10.3847/1538-3881/ae9084

Marielle R. Eduardo et al. 2026. The Luminosity Function of Ultrafaint Trans-Neptunian Objects Detected by JWST. AJ 172, 187; doi: 10.3847/1538-3881/ae907f