This question is actually more linked to astronomy and astrophysics than to pure physics. I tried posting it on the astronomy page, however it got no answers, so I though this page might help.

Reading about the formation of planetary disks, one of the major problems, it seems like, is the evacuation of angular momentum. Aparently planets can't form with the amount o angular momentum the system has in its early stages. I think I understand where that excess comes from, the collapse of the nebula onto itself and provoking a spin. Then there are many hypothesis on how it's evacuated, which are mostly pretty logical.

Now my question, as a beginner in the study of physics, is this: Why does the angular momentum even need to be evacuated? If the angular momentum is too big, why can't planets still form? Does this have something to do with too much kinetic energy in the system? Thank you!

edit: my main source is this video: youtube.com/watch?v=tEgw0PXwkGE Otherwise the information comes from books, notably L'exporation des planètes by Therese Encrenaz and James Lequeux

link to the astronomy page: https://astronomy.stackexchange.com/questions/10073/angular-momentum-in-planetary-disk-formation


1 Answer 1


Consider a particle in a nice Keplerian orbit around a star, taken to be a circle for simplicity. If it is at a radius $r$ and the star has mass $M$, then its specific angular momentum will be $$ l = vr = \sqrt{GMr} $$ and its total specific energy will be $$ e = -\frac{GM}{2r}. $$ If you want to decrease $r$, you are going to have to decrease $l$ as well as $e$. If you try to place a particle at a certain radius but with too much angular momentum (i.e. too much tangential velocity), it will start to fly off away from the star -- it will be going too fast for gravity to curve its orbit into a circle. This doesn't necessarily mean that that particle will escape to infinity, but it certainly won't be in a circular orbit. Instead it would be in an ellipse, with the point you tried to place it becoming the periastron.

In a disk with lots of particles, collisions will tend to prevent eccentric orbits. If every particle were on an eccentric orbit with a random orientation, particles would be colliding into each other all the time. Collisions redistribute energy and angular momentum so as to make all the orbits match.

Thus sapping the energy out of particles (e.g. by having them occasionally collide, transfer kinetic energy into internal heat, and radiate that heat away into space) isn't sufficient. You need to get rid of angular momentum too, which is what that video is about. This is a very big subject in astrophysics, because disks are everywhere, from forming planetary systems to black holes to galaxies.

Of course, all this just answers the question "Why do we need to get rid of angular momentum in order for material to fall in?" The broader question of "Why does angular momentum matter for planet formation?" is due to the fact that planets need dense environments to form. A protoplanetary disk around a young star is many orders of magnitude denser than the giant molecular cloud that started to collapse.

In short, angular momentum transfer helps a disk collapse to a small size, where densities are high enough for planets to form. Even planets themselves might start to form from small accretion disks within the main disk, and these only work if they can get rid of angular momentum.

See also this related question on star formation.


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