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.
as written by the authors arXiv:2601.08938
We investigate whether photoevaporation alone can open and sustain gaps in protoplanetary discs by coupling the evolving disc structure with the photoevaporative flow in two dimensional radiation hydrodynamical simulations. Our results show that once a density depression forms, the local mass-loss rate decreases sharply, suppressing further gap deepening. Viscous inflow and radial mass transport along the disc surface act to partially refill the depleted region, preventing complete clearing. The resulting configuration is a persistent, partially depleted zone whose evolution is largely insensitive to the initial disc morphology. This behaviour challenges the standard paradigm that photoevaporation efficiently carves clean inner cavities and directly produces transition discs. However, the pressure maximum at the outer edge of the depression may still trap dust grains, giving rise to transition disc like observational signatures. We also present a first-order prescription to approximate this behaviour in one dimensional disc evolution models, suitable for use in planet formation and population synthesis studies. Although the prescription improves upon static mass-loss treatments, it remains approximate, underscoring the need for further multidimensional simulations and parameter-space exploration to derive robust recipes for global disc and planet population models.
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.