What is a Protoplanetary Disk?
A protoplanetary disk is the flattened, rotating disc of gas and dust that surrounds a newly formed star. Everything a planetary system will ever contain is in there — and the Solar System looked exactly like this about 4.6 billion years ago.
The disc exists because of a conservation law. The fragment of cloud that collapses to make a star is always rotating a little, and as it shrinks that rotation has to speed up, the same way a spinning skater accelerates by pulling their arms in. Material falling along the rotation axis reaches the centre unimpeded; material falling in the equatorial plane cannot, because it is moving too fast sideways. So the infall flattens, and the star ends up fed by a disc rather than by a sphere.
The obvious problem with that story is that the disc has too much angular momentum to fall in at all. Nature solves it violently: magnetic field lines threading the inner disc fling a fraction of the material back out along the poles at hundreds of kilometres per second, carrying away the excess rotation and letting the rest spiral inward. The twin jets are not a side effect of the star forming. They are the mechanism that permits it.
What ended the debate about how fast planets form was a picture. In 2014 ALMA resolved the disc around HL Tauri — a star well under a million years old — and found it cut by clean concentric gaps, each swept out by a body already massive enough to clear its own orbit. Planets, it turned out, are not built slowly after the star settles down; they are built while the star is still forming. The model here carries those gaps, the shepherding protoplanets sitting in them, the spiral density waves they launch, and the Keplerian shear that makes the inner disc lap the outer one.