When the young Earth was still a molten, restless world, something about the size of Mars slammed into it. The debris from that impact eventually gathered into the Moon, at least according to the leading theory of lunar formation. For decades that story rested on computer models and the chemistry of Moon rocks. Now the James Webb Space Telescope has found something remarkable: other young star systems where the same kind of violence appears to be happening right now, written in clouds of warm dust.

Astronomers pointed Webb at 21 young stars surrounded by unusually large amounts of warm dust, a rare setup found around only about one in a hundred young stars. These so-called extreme debris disks sit close to their stars, in exactly the zone where rocky planets like Earth and Mars form. Webb's infrared vision could read the dust in detail that earlier telescopes could not, revealing smaller-than-usual dust grains, very high concentrations of warm material, and brightness that flickers irregularly over time.

That flickering matters. A calm, settled disk of dust shines steadily. A disk freshly churned by a giant collision keeps changing as debris spreads, clumps and grinds itself down. Combined with the mineral makeup of the dust, the pattern points to recent, energetic impacts rather than a slow, gentle rain of small rocks. In other words, Webb is catching planetary construction sites in the act, not long after the crash.