How do planets form in a protoplanetary disk?

Dust grains collide and stick into pebbles, pebbles into planetesimals kilometres across, and the largest of those sweep up everything nearby. Once a body is massive enough it clears an annulus of the disc, which is visible as a gap.

Why do protoplanetary disks have gaps?

A gap is the signature of a planet. Once a forming planet becomes massive enough, its gravity ejects or accretes the material in its orbital path, leaving a dark ring in the disc. ALMA has now imaged these around many very young stars.

How long does a protoplanetary disk last?

A few million years. The gas is dispersed by the star's radiation and winds, which sets a hard deadline: any giant planet that will exist has to gather its atmosphere before then.

What is a Protoplanetary Disk?

Q: What is a protoplanetary disk?

It is the disc of gas and dust orbiting a newly formed star, out of which planets, moons and asteroids assemble. Every planetary system, including the Solar System, formed from one.

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.