Tiny trans-Neptunian objects 'remember' how they formed, Hubble and Webb show

Europe's ESA is a partner in both space telescopes behind a new look at the solar system's most distant bodies: Hubble and Webb jointly observed 27 icy trans-Neptunian objects and found the smallest still carry the colours they formed with 4.6 billion years ago. Unexpectedly, collisions have not altered their surfaces, and there are fewer small objects than planet-formation models predict. The findings, published in The Astronomical Journal, offer the best view yet of the planetesimal stage that built the planets.

Bottom line — With 27 objects surveyed, Hubble and Webb data now constrain how planetesimals formed — fewer small ones than models allow.

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• Small TNOs look identical to their larger siblings despite billions of years of collisions, said colour-study lead Anastasia Morgan of Northern Arizona University — they are 'remembering' how they were made.
• Dynamically 'hot' TNOs formed between Uranus and Neptune, then were pushed outward by migrating gas giants — yet kept a colour signature of their birthplace, said co-author David Trilling.
• Hot and cold populations share the same size distribution despite forming in different regions; the process 'seems to be insensitive to disk conditions,' said Marielle Eduardo of the University of Victoria.
• There are fewer tiny TNOs than planet-formation models expected, NASA said — a puzzle the teams are still trying to unravel.
• The smallest object spotted is about 5 km across — five times smaller than the most sensitive ground-based telescopes can detect, the agency said.
• One of the 27 objects is so faint that seeing it from Earth would be like watching a swarm of fireflies on the Moon, NASA added.
• Cosmos Week cautions the result needs follow-up from other observatories — the VLT, future Extremely Large Telescopes, radio arrays — a process usually measured in years.

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