In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll just be getting started with today’s topic: protoplanetary disk!
Young stellar systems are notoriously nasty places. It starts off okay, with a nebula beginning to condense down to form a star. As the nebula shrinks, however, conservation of momentum kicks in and it rotates faster, eventually flattening into a disk: the protoplanetary disk.
Initially, that disk is just a loose collection of gas with a little sprinkling of dust. In the core of the disk you have a star just beginning to form – it’s heating up from all the activity but it hasn’t ignited nuclear fusion yet.
As millions of years go by, the protoplanetary disk begins to fragment. At first little dust grains stick to each other. Then they find other clumps. Then those clumps get bigger. In the outer regions of the protoplanetary disk, those clumps can get very big. That’s because the dust also comes along with ices, which can only survive far from the heat of the forming star.
Astronomers aren’t exactly sure how planetary formation proceeds, but it certainly involves a lot of smashing (if the protoplanetary disk has enough mass to form planets at all). Small clumps of dust gravitate towards other small clumps, which then feed on the gas surrounding them.
Thousands of protoplanets form out of the disk, and eventually – after enough collisions and ejections – they settle into a family of planets. In the inner protoplanetary disk, rocky planets emerge, while in the outer disk, the gas and ice giants dominate. The leftover bits either scatter into interstellar space or settle into belts of debris, like the asteroid belt or the Kuiper belt.
After enough time, the gas leaves too. Either it gets blown away by the radiation of the new star, or it settles onto one of the gas giants. When all the dust settles, the protoplanetary disk has finally evolved into a planetary system.