[0901.4235] High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion

[0901.4235] High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion.

Forming planets is something of a puzzle – we can’t presently observe them directly in their early days, just the stars they orbit and the dust they’re cocooned in. We have a pretty good idea of how stars form and we can figure out how dust forms, but going from dust to planets has a few obscurities. Or rather from dust to small planet bits about 1-10 km across is obscure. Once there are billions of bite-sized planet bits they quickly accrete into planets according to all the computer simulations. That bit is easy and it forms planets much like the ones we see.

And, in recent years, the path from dust to centimetre size dust bunnies is looking nearly solved too. Zero-gravity experiments with dust on the Space Shuttle formed fractal fluffy aggregates that gradually smush together and get denser. But between a centimetre and a kilometre things get very tricky – the dust balls start seriously feeling the drag of the thin gas around them (the protoplanetary nebula) and this gentle drag is enough to plunge the growing dust-balls into the central star in 100-1000 years. So how do they survive?

Well once they’re about a kilometre across they’re safe, but growing 100,000-fold in size and a quadrillion-fold in mass is tricky. Presumably they butt into each other and ‘stick’, but just how has been doubtful. Now this study appears which answers some of the questions about dust-ball collisions by actually colliding dust-balls at the expected range of collision speeds. Surprisingly they tend to stay together, held together by the cohesion forces of dry dust (the stickiness of space itself we call “van der Waals forces”.) What bits fly off tend to be as big as the bits that collided, and so things seem to only get bigger.

So some of the process is now less obscure, but there’s still a ways to go. They’ll make planetesimals yet!