In the most remote corners of our solar system, far beyond the orbit of Neptune, tiny icy bodies—trans-Neptunian objects—quietly circle. Most of them are hundreds of millions of times fainter than the faintest stars visible to the naked eye. And now, for the first time, astronomers have been able to peer into this darkness using two space telescopes at once—Hubble and James Webb—and discovered 27 new, incredibly faint objects. One of them shines so weakly that it resembles a swarm of fireflies on the Moon, seen from Earth. The smallest has a diameter of only about five kilometers.
These bodies are frozen 'building blocks' of planets. Once, a disk of dust and pebbles around the young Sun began to clump into city-sized planetesimals. Closer to the center of the system, they continued to grow and turned into planets. But beyond Neptune, this process stopped. There, almost untouched remnants of that era still fly—icy witnesses to the birth of worlds.
Scientists divided the objects into two groups. The 'cold' ones still move in nearly circular orbits in the plane of the solar system—where they were born. The 'hot' ones once formed closer in, between Uranus and Neptune, and then the giants, during their migration, ejected them outward onto elongated, inclined trajectories. It was expected that small bodies in both groups would have collided many times, and their surfaces would have changed. Colors—a kind of fingerprint of composition—should have differed from those of their larger neighbors.
But observations showed otherwise. Even the smallest objects retained the same colors and ratios as their larger counterparts. They seem to 'remember' what they were made of and where. Collisions either occurred less frequently than thought, or the surfaces somehow retained their primordial composition. The size distribution in both populations also turned out to be nearly identical—although the objects formed under different disk conditions. And there were fewer tiny bodies than some models predicted.
Hubble captured visible light, Webb—infrared. Only together could they detect these barely visible points and understand what they are. This is the deepest survey of trans-Neptunian objects to date.
All this happens in a vast, nearly empty outskirts of the system, where icy fragments have circled for billions of years, holding within them an exact imprint of how planets assembled from the chaos of dust and ice—one of which gave rise to life. The universe is arranged so that even its faintest, most distant, and coldest corners continue to tell a story that began almost four and a half billion years ago. And we are just beginning to learn to read it.
