At the outskirts of the Solar System, beyond Neptune's orbit, millions of icy chunks circle the Sun — leftover building material from the era of planet formation. Astronomers call them trans-Neptunian objects, and they have a peculiarity: their surfaces are painted in various shades of red. Some are merely reddish, like rust, while others are very red, like dried blood. And when viewed in infrared light, some of these bodies appear dim, while others shine brightly. Why is that?

A new study combined data from the Col-OSSOS survey, which measured the colors of hundreds of such objects, with computer models showing how a young Neptune migrated through the outer disk and scattered icy chunks to the fringes. It turns out that the surface color depends on where the body was born. Those that formed closer to the Sun — at about 30 astronomical units (that's 30 times the Earth-Sun distance)—got neutral or infrared-bright surfaces. Those born farther out, beyond 31 astronomical units, are red and dim. The boundary between these zones was as sharp as a line on a map.

This color boundary is like a geological cross-section, but on the scale of the entire Solar System. It shows that the outer disk, from which the icy worlds later grew, was not uniform. Closer to the Sun, the ices were different — perhaps it was warmer there, and ice sublimated, leaving darker, dimmer surfaces. Farther out, in the cold, ice survived and reddened under cosmic rays. Then Neptune's motion stirred these zones, like a spoon stirring layers in a glass of tea, and now we see bodies with different histories side by side.

By comparing different scenarios of Neptune's migration, scientists found that the data are best explained if the transition from neutral to red surfaces occurred at about 30 astronomical units. This almost exactly matches Neptune's current orbit. So the giant planet not only scattered the icy chunks but also marked the boundary between different surface types.

Today, these icy bodies are like frozen postcards from the past. Their colors tell where they were born and how far they were flung. By studying such details, astronomers are reconstructing the birth of the Solar System — step by step, color by color.