Around a young star spins a flat pancake of gas and dust — the future building blocks of planets. The star heats it with ultraviolet radiation, and gas from the disk's surface escapes into space, like water from a puddle under the sun. If evaporation is brisk, a gap appears in the disk — a ring-shaped clearing devoid of matter.
Previously, it was thought that such a gap quickly expands and clears out the disk's center entirely. A new computer simulation showed otherwise. As soon as the density in the gap drops, evaporation there nearly shuts off: there's nothing left to escape. Meanwhile, gas from the edges slowly creeps back in, like sand filling a hole on the beach. The result is not a clean hole but a semi-empty zone that persists for a long time and barely depends on the disk's initial state.
This dusty rim might look like a transitional disk through a telescope — the very stage when planets are almost assembled. But in reality, there may be no planets inside. The authors provide a simple formula to account for this "lazy" evaporation in large-scale disk evolution models, but they honestly warn: the formula is approximate, and more detailed calculations are needed.
Sunlight that evaporates puddles doesn't make neat holes in them. But a young star's ultraviolet tries to, and stops halfway.
A gap in a protoplanetary disk can look like the work of a newborn planet, but in reality it was carved by the star's radiation, and there's no planet there.