Beyond the planet Neptune, in the dark and cold distance, drifts a multitude of icy chunks, like islands in an ocean. These chunks have remained nearly unchanged for four billion years and hold memories of how planets were born. But spotting the small ones has been extremely difficult: they shine fainter than a candle seen from the Moon. Now the James Webb Space Telescope has managed to see even those the size of a city, about ten kilometers across.

The telescope imaged the same patch of sky three times, with five-day gaps. During that time, a distant chunk shifts against the background stars, like a slow raft among stationary buoys. To catch such faint light, scientists aligned the images according to each candidate's motion, then used a computer program trained on artificial examples to discard false signals. In this way, they found 27 genuine icy islands.

This discovery helps us understand how planets come into being. Previously, it was thought that the number of icy chunks should drop off at small sizes, but Webb has shown that they continue to exist even down to ten kilometers. Now scientists know that the bulk of the Kuiper Belt's mass is concentrated in bodies around 200 kilometers across, not in small debris. The census of icy islands is just beginning, and many more discoveries lie ahead.