David Kipping

And the higher densities will self-gravitate.

So gravity wants to make, it's like a greedy algorithm, wants to make everything get denser and denser and denser, super greedy.

It's relentless, gravity.

It never stops.

And that's why eventually we end up with black holes, right?

Because it just refuses to lose black holes.

Gravity always wants to win the game.

So eventually these clouds collapse, and the thing that stops them from collapsing into a black hole is that you start getting fusion in the center, right?

Because the temperatures get so hot as you compress this gas that you basically make a star in the center.

And the stuff that's left over on the outside, that disk of material, because the star kind of blasts out of its poles and kind of pummels all the gas from north and south, you end up with a disk of material, the centrifugal forces, like spinning a pizza ball, which kind of force it into a disk.

And then from that disk, you start to coalesce again, just some areas are slightly denser, some areas are slightly less dense, and gravity again takes over and starts to collapse things together.

So we have this story, but there's lots of parts of the story that we don't understand.

So we know how to go, for instance, from pebbles, if you start off with pebbles.

And imagine them kind of bouncing around.

We can imagine sticking them into boulders.

We kind of understand how that could happen.

But we don't quite understand how to do some of the steps like go all the way from dust, which presumably at one point was just dust.

How do you go from dust all the way up to pebbles, all the way up to these boulders, all the way up to planetesimals?

That whole story we don't understand.

We've got bits of it where we think we understand it, but the whole thing we don't.