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View Full Version : How do planets accrete?



Tom Mazanec
2014-Feb-06, 05:31 PM
Planets are supposed to grow by small pebbles colliding and sticking together to form cobbles, boulders, mountains...
But they travel at astronomical speeds...
If you hit a brick with a bullet, it does not get bigger, it gets shattered.
So how do planets accrete?

PetersCreek
2014-Feb-06, 06:28 PM
As I understand it, the individual particles in the protoplanetary disk were travelling in very similar orbits, so bullet-like impacts probably weren't the norm. Additionally, the geological processes required to turn accumulated dust into solid pebbles, cobbles, or boulders weren't present during the very early stages. So I think collisions between clumps of dust were bound to be more elastic without much of the shattering you mention.

George
2014-Feb-06, 08:39 PM
Planets are supposed to grow by small pebbles colliding and sticking together to form cobbles, boulders, mountains...
But they travel at astronomical speeds...
If you hit a brick with a bullet, it does not get bigger, it gets shattered.
So how do planets accrete? The impact energies are indeed a challenge to explain things. When they are tiny, they can build-up in size, but when they get larger, perhaps a 1/2 meter or so, then the solutions become wanting. This is known as the "1 meter" barrier or problem, I think.

Stellar or planetary accretion is extremely complex since the variable have variables, so to speak. Turbulence, magnetic fields and ionization, cluster tugging, composition, etc. are daunting issues.