A young star is surrounded by a flat, rotating disk of gas and dust — the construction site for future planets. The disk lives its own life: gas flows toward the star, dust clumps together, and from outside, harsh ultraviolet light from neighboring hot stars presses down on it all, like wind blowing leaves off a path.
Astronomers ran computer models to understand which disks survive such bombardment. The key question: where does the angular momentum go: that very "rotational inertia" that keeps the disk from falling entirely onto the star? One mechanism is friction within the gas, as if the disk's layers rubbed against each other, transferring momentum outward. Another is magnetic winds: magnetic field lines threading the disk carry momentum away, like a spinning figure skater throwing out their arms.
It turns out that disks with magnetic winds expand more slowly and therefore suffer less from external radiation. But under weak or moderate irradiation, they do not live longer than disks with ordinary friction. However, under very strong ultraviolet, the disk's lifetime stops depending on how exactly it loses momentum — only the power of the "bombardment" matters.
For dust, the picture is even harsher. Solid particles drift toward the star faster than gas, and the ultraviolet wind sweeps them away especially efficiently. For planets to form under such conditions, the disk must have "pockets"—clumps or rings where dust lingers, like sand behind a rock in a stream.
So young planetary systems are not quiet nurseries but an arena of struggle between internal friction, magnetic winds, and the harsh radiation of neighbors. Those that manage to hold onto their dust long enough for planets to assemble are the survivors.
Ultraviolet radiation from massive neighboring stars can evaporate the outer parts of a protoplanetary disk within a few million years — faster than a giant planet can form.
In Stanisław Lem's novel "Solaris," the planet's ocean behaves like a giant self-organizing system — somewhat similar to how a gas-and-dust disk, under external influence, restructures itself to preserve material for future planets.