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The minimum-mass solar nebula estimates a primordial gas-and-solid protoplanetary disk by spreading planetary material into annuli and restoring a solar mixture. A common idealized surface density is proportional to $r^{-3/2}$. This reconstruction is a model of minimum material requirements, not a unique history of the Solar System. Extrapolating it far beyond the planetary region need not give a physically consistent disk.

Radiation heats gas sufficiently to drive an escaping thermal outflow. It can remove material from protoplanetary disks, planetary atmospheres and irradiated clouds. In a heated Keplerian disk, the photoevaporative gravitational radius compares the sound speed with the orbital speed. The mass-loss rate depends on the heating, density and wind geometry rather than radiation pressure alone.

When a protoplanetary disk's inward viscous supply approaches its wind-loss rate, photoevaporation can prevent replenishment of the inner disk. That region drains on a viscous timescale, leaving a gap or hole. A negative density in a formal steady state calculation signals the breakdown of that approximation, not a physical continuation. An exposed outer edge can subsequently disperse rapidly.