What secrets are hidden in the most remote corners of our solar system? Recent research has unveiled surprising insights into the formation and evolution of our cosmic neighborhood, challenging long-held beliefs.
Two groundbreaking studies, including one led by Anastasia Morgan, a Ph.D. candidate at Northern Arizona University, have focused on Trans-Neptunian Objects (TNOs) to unravel the mysteries of our solar system’s past. These studies are noteworthy as they are the first to utilize the combined capabilities of NASA’s Hubble and Webb telescopes.
Contrary to expectations, Morgan’s research revealed minimal variation in TNOs’ colors, shapes, and materials, suggesting less movement and collision activity than previously thought. “For these objects to have a lot of the same color and shape, it’s telling us that they have probably sat there and not been very collisionally active,” Morgan noted. “The models say they should have been altered by collisions, but that’s just not what we’re seeing.”
Alongside a complementary study by University of Victoria Ph.D. candidate Marielle Eduardo, Morgan’s findings were published on September 8 in The Astronomical Journal.
Exploring the Nature of TNOs
TNOs are icy, dim objects that orbit the Sun beyond Neptune. These distant bodies are incredibly faint—more than 100 million times dimmer than the faintest objects visible to the naked eye. Morgan’s study focused on some of the smallest and faintest TNOs ever observed, comparing their detection to spotting fireflies from Earth on the Moon without a telescope.
Morgan explained that TNOs provide a glimpse into an early stage of planet formation when a disk of dust and pebbles began forming planetesimals, which are the building blocks of planets. Beyond Neptune, this process halted, leaving behind a population of frozen planetesimals.
“TNOs hold some of the history of the formation and evolution of the outer solar system,” Morgan explained. “They tell us what was here at the beginning, and more broadly, they give us clues into how other planetary systems are formed.”
Using Hubble and Webb telescopes, Morgan and Eduardo’s teams examined TNOs’ visible and infrared light. Morgan analyzed their colors, while Eduardo assessed their size distribution. Surprisingly, the small TNOs appeared similar to their larger counterparts, questioning the expected collision-driven surface changes.
“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,” Morgan explained. “So, it’s really fascinating to see that the smallest objects are somehow ‘remembering’ and preserving the history of how they were made.”
David Trilling, a study co-author and professor at NAU, added, “They retain a signature of where they were born, even though their orbits have been scrambled since then.”
Eduardo discovered fewer very small TNOs than anticipated by previous models. The Webb telescope identified 27 new, exceptionally dim TNOs, with the smallest measuring five to 10 kilometers in diameter—five times smaller than what ground-based telescopes can detect.
Challenging Our Understanding
These findings challenge existing knowledge about the outer solar system. “We thought we’d see lots of variation in the color and size of these TNOs, and we didn’t,” Trilling stated. “That means there’s something not quite right about our understanding of how the solar system got to where it is now. Is it an evolutionary misunderstanding? Did we not get the upheaval right? The previous science doesn’t seem to capture how we got where we are today.”
Morgan emphasized the implications for future research: “The models tell us TNOs collided with other bodies a lot, but our observations tell us the history of the outer solar system may have been less collisional,” she said. “When the data are inconsistent with the existing models, it means we need to refine and re-test the models. It also means we need to analyze more data, and we will; there’s terabytes and terabytes of it out there.”
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