Hubble and Webb find that far-out solar system objects ‘remember’ the past

For the first time, scientists used the combined power of NASA’s Hubble and James Webb Space Telescopes to study some of the most far-flung bodies in our solar system, trans-Neptunian objects (TNOs). Some are the smallest and faintest ever directly seen. The researchers unexpectedly found fewer small TNOs than expected and that the colors of these bodies followed the same relationships as those of their larger family members.

These objects are typically small, faint, icy bodies orbiting the sun beyond the orbit of Neptune. Most are more than 100 million times dimmer than objects visible to the unaided eye. In two complementary papers published Tuesday in The Astronomical Journal, teams analyzed the color, composition and size distribution of 27 newly discovered tiny, dim TNOs.

This class of small bodies offers the best view into an early stage of planet-building, when a disk of dust and pebbles in orbit around the sun coalesced into city-sized “planetesimals”—the solid building blocks that clump together to form planets—but had not yet merged into full-size worlds. Beyond Neptune, this second stage never happened, leaving behind a frozen population of planetesimals.

In the deepest TNO survey to date, teams led by Ph.D. candidates from the University of Victoria in Canada and Northern Arizona University in Flagstaff, under the guidance of the National Research Council of Canada, examined a patch of sky simultaneously with Hubble, observing the TNOs’ visible light, and Webb, observing their infrared light. The researchers measured the objects’ colors, which are like fingerprints of surface composition, as well as their sizes, and determined their orbits.

Two populations, different histories

In the coordinated observations, the teams studied two different types of TNOs. The first, dynamically “cold” TNOs, are on their original, relatively circular orbits around the sun in the plane of the solar system. The second type, dynamically “hot” TNOs, formed between the current locations of Uranus and Neptune but were pushed outward to their current locations when the outer gas giants migrated early in the solar system’s history. Today, they reside in highly elliptical orbits and move in and out of the plane of our solar system.

Before these observations, astronomers thought that small TNOs from both the hot and cold populations would have undergone many collisions, changing their surfaces compared with larger TNOs. But that’s not what the observations showed. Instead, the small bodies look like their larger counterparts. This implies that collisions are not changing the surfaces significantly—perhaps because there are fewer collisions than expected or because the TNOs somehow retain their primordial, pre-collision compositions. The teams are still trying to unravel this mystery.

“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 Northern Arizona University Ph.D. candidate Anastasia Morgan, who led the study of color and composition.

“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.

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.

Unexpected shortage of tiny bodies

The Webb data also allowed researchers to measure the number of objects of each size. They found that the overall size distributions for both populations were surprisingly similar.

“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 University of Victoria Ph.D. candidate Marielle Eduardo, who led the study on size distribution.

Researchers also found fewer of these very small bodies than expected based on some planet formation models. Webb discovered 27 new, remarkably dim TNOs, including one so faint it is equivalent to standing on Earth and seeing a small swarm of fireflies on the moon. The smallest one they observed has a diameter of about 3 miles (5 kilometers), which is about five times smaller than what is possible to detect with the most sensitive ground-based telescopes.

This project would not have been possible without Hubble and Webb working together to detect and characterize these TNOs. With Hubble’s sensitivity in visible light and Webb’s in infrared, the space telescopes provide more insights than either can on its own.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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