Tiny, icy worlds at the edge of the solar system are helping scientists rewind the story of how planets like Earth first came together.
These distant, city‑size bodies orbit far beyond Neptune in space and are leftover building materials from the solar system's early days. They have spent billions of years in a deep freeze, preserving clues to how the first solid worlds formed.
Until now, astronomers could only study larger members of this population, from dwarf planets like Pluto and its moon Charon down to mid‑size objects such as Arrokoth. The truly tiny ones, only a few miles wide, stayed out of reach.
In two new studies, scientists used NASA's James Webb Space Telescope to find dozens of these tiny worlds, then followed up with the Hubble Space Telescope to measure the light from their frozen surfaces.
Together, the results give astronomers the largest census of these distant objects ever made, showing how many of these tiny worlds exist and what their surfaces are made of. The observations turn a scattered set of icy rocks into a detailed look at how the building blocks of planets formed and how little they have changed. The scientific papers both appear in The Astronomical Journal.
In Webb's deep images, astronomers picked out 27 new Trans‑Neptunian Objects — the small icy bodies that orbit beyond Neptune — in a small patch of sky. Some are only about six miles wide, making this the most sensitive study of such worlds to date. From their brightness, the team inferred how common each size is.
Researchers expected that over time, crashes between these tiny bodies would change their appearance and grind them into shards. Instead, the new data show only a slight uptick in the number of small objects. That means the Kuiper Belt — the broad ring of debris beyond Neptune — doesn't consist mostly of dust and crumbs. Instead, most of its mass is tied up in objects about 120 miles wide.
That pattern backs a "born big" view of planet construction. In the early solar system, clouds of pebble‑size material in the disk of gas and dust likely clumped and collapsed under their own gravity to form large, solid bodies in a relatively quick phase, rather than slowly growing from dust through countless scrapes with other bodies.
Equally striking, two major families of these distant worlds share almost the same size pattern down to the smallest objects Webb could detect. One group follows calm, nearly circular orbits and probably formed more or less where it now resides. The other group moves on more tilted and stretched paths and likely began closer to the sun before giant planets pushed it outward.
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This similarity hints that the process that turned dust, rock, and ice into "planetesimals" — the first solid pieces big enough to build planets — may work the same way under many different conditions. That challenges earlier thinking, which had predicted many more small chunks than Webb and Hubble actually saw: Either collisions aren't drastically reshaping these objects, or they aren't happening as often as scientists expected.
The color measurements add another key clue. The smallest members of the calmer outer‑belt family tend to have a similar reddish color to the larger bodies in that group. Earlier work links those hues to surfaces rich in complex organic compounds and certain ices that slowly darken and redden under cosmic radiation over billions of years.
"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," said Anastasia Morgan, a Northern Arizona University doctoral candidate who led the color and composition study, in a statement. "It's really fascinating to see that the smallest objects are somehow 'remembering' and preserving the history of how they were made."
Astronomers also watched how these tiny worlds brighten and dim as they spin. Most show only modest changes in brightness over a rotation, which could mean that extreme football‑like shapes or close pairs, known as contact binaries, are rarer than some models suggested.
