For the first time, astronomers pointed NASA’s Hubble and James Webb Space Telescopes at the same patch of sky at once to study Trans-Neptunian Objects, the small icy bodies that orbit the sun beyond Neptune. The joint observations turned up some of the smallest and faintest of these objects ever directly seen, several times fainter than what ground-based telescopes can detect. Rather than looking battered and altered like larger, more collision-prone bodies, the tiniest objects in the survey matched the colors and compositions of their much bigger relatives, a result the research teams say suggests these frozen leftovers have barely changed since the solar system’s earliest days.
Pairing Hubble’s Visible Light with Webb’s Infrared for a Deeper Look
Trans-Neptunian Objects, or TNOs, are typically small, faint bodies of ice and rock circling the sun far beyond Neptune’s orbit, and most are more than 100 million times dimmer than anything visible to the naked eye. Two research teams, led by PhD candidates from the University of Victoria in Canada, working under guidance from the National Research Council of Canada, and Northern Arizona University in Flagstaff, examined the same region of sky simultaneously with both observatories: Hubble capturing visible light and Webb capturing infrared. Neither telescope could have produced this result alone, since Hubble’s strength lies in visible-light sensitivity while Webb’s lies in infrared, and combining the two let researchers measure each object’s color, composition, size, and orbit in a single coordinated survey. The findings, described in a NASA release detailing the results, were published as two companion papers Tuesday in The Astronomical Journal, and the researchers describe the effort as the deepest TNO survey conducted to date.
Sorting the Solar System’s “Cold” and “Hot” Leftover Populations
The teams focused on two distinct TNO populations. Dynamically “cold” TNOs sit on roughly circular orbits close to the plane of the solar system, largely unchanged since they formed in place. Dynamically “hot” TNOs, by contrast, originally formed between the current orbits of Uranus and Neptune before being flung outward into steep, elliptical orbits when the giant planets migrated early in the solar system’s history. Comparing the two groups side by side gave researchers a way to test whether TNOs from different origins and orbital histories still carry the same physical fingerprints.
Tiny Objects That Still “Remember” How They Were Made
Before this survey, astronomers expected that repeated collisions over billions of years would have reshaped the surfaces of the smallest TNOs, leaving them looking different from their larger, less battered siblings. The data showed the opposite: small and large TNOs, in both the hot and cold populations, share the same color relationships. “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 PhD candidate Anastasia Morgan, who led the study of TNO color and composition. Co-author David Trilling, also of Northern Arizona University, said the hot population in particular “retain a signature of where they were born, even though they’ve been orbitally scrambled since then.”
Webb Uncovers 27 of the Dimmest Objects Ever Directly Detected
Webb’s infrared sensitivity allowed the teams to spot 27 newly discovered TNOs far fainter than anything catalogued before, including one so dim that the researchers compared spotting it to standing on Earth and seeing a small swarm of fireflies on the moon. The smallest object measured about 3 miles across, roughly five times smaller than what the most sensitive ground-based telescopes can pick out. Despite that sensitivity, the teams found fewer of these very small bodies than some planet-formation models predicted, a gap researchers are still working to explain.
A Matching Size Distribution Despite Very Different Birthplaces
Webb’s data also let researchers count how many objects of each size exist within the hot and cold populations, and the two size distributions turned out to be remarkably similar despite forming under very different conditions. “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 University of Victoria PhD candidate Marielle Eduardo, who led the study on TNO size distribution. Eduardo noted the process appears insensitive to whether the original disk of material was hot or cold, or dense or diffuse.
A Frozen Snapshot of Planet-Building’s Missing Middle Step
TNOs matter to planetary scientists because they represent a stage of planet formation that never finished beyond Neptune: a disk of dust and pebbles coalescing into city-sized planetesimals, the solid building blocks that elsewhere merged into full-sized planets. Inside Neptune’s orbit, that process continued until planets formed; beyond it, the leftover planetesimals were essentially left in a deep freeze, never colliding and merging often enough to grow further or lose their original character. Because the smallest, faintest TNOs preserve their original surfaces rather than being reshaped by collisions, researchers say they now have a clearer, largely unaltered window into that unfinished stage of solar system construction. The Hubble Space Telescope has operated for more than three decades as a joint project between NASA and the European Space Agency, while Webb is led by NASA with ESA and the Canadian Space Agency as partners, and the research teams say this coordinated observing approach opens the door to studying even fainter, more distant populations of small bodies in the years ahead, work that would not have been possible with either observatory operating alone.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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