Hubble and Webb Team Up to Probe Solar System's Faintest Icy Relics

NASA's Hubble and James Webb space telescopes have jointly studied some of the most distant and faintest bodies in our solar system for the first time, directly observing 27 newly discovered Trans-Neptunian Objects. Researchers found fewer small TNOs than expected and discovered that these tiny bodies share the same color relationships as their larger family members, suggesting they preserve their primordial compositions.

Trans-Neptunian Objects are small, faint, icy bodies orbiting the Sun beyond Neptune, most of them more than 100 million times dimmer than objects visible to the unaided eye. These objects offer the best view into an early stage of planet-building, when a disk of dust and pebbles around the Sun coalesced into city-sized planetesimals but had not yet merged into full-sized worlds. Beyond Neptune, that second stage never happened, leaving behind a frozen population of planetesimals.

Teams led by PhD candidates from the University of Victoria in Canada, guided by the National Research Council of Canada, and from Northern Arizona University in Flagstaff conducted the deepest TNO survey to date. They examined a patch of sky simultaneously with Hubble, observing visible light, and Webb, observing infrared light, measuring the objects' colors, sizes, and orbits. The two complementary papers were published Tuesday in The Astronomical Journal.

The teams studied two types of TNOs. Dynamically cold TNOs remain on their original, relatively circular orbits in the plane of the solar system. Dynamically hot TNOs formed between the current locations of Uranus and Neptune but were pushed outward when the outer gas giants migrated early in the solar system's history, and now reside in highly elliptical orbits. Astronomers had thought small TNOs from both populations would have undergone many collisions that changed their surfaces compared to larger TNOs, but the observations showed the small bodies look like their larger counterparts.

Webb's smallest observed object has a diameter of about 3 miles, five times smaller than what the most sensitive ground-based telescopes can detect. One object was so faint it is equivalent to seeing a small swarm of fireflies on the Moon from Earth. The teams also found the overall size distributions for both hot and cold populations were surprisingly similar.

"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 PhD candidate Anastasia Morgan, who led the study of color and composition. Co-author David Trilling of Northern Arizona University said the hot TNOs "retain a signature of where they were born, even though they've been orbitally scrambled since then."

The findings imply that collisions are not significantly changing TNO surfaces, either because there are fewer collisions than expected or because the objects retain their pre-collision compositions. University of Victoria PhD candidate Marielle Eduardo, who led the size distribution study, said the planetesimal formation process appears insensitive to disk conditions, producing similar sizes whether the disk was hot or cold, dense or fluffy.

The teams are still working to unravel why the smallest bodies preserve their primordial colors and why fewer very small TNOs were found than some planet formation models predicted.