Why do galaxies form disks while planets are spherical?

[Hippasos]

Hi!

Can someone illuminate why there are gravitational tendency in larger scale to form disks (galaxies, solar systems, accretion disks) whereas all smaller scale bodies (planets etc.) are spherical?
What about the spherical halos in the galaxies, is just "unorganized matter" which will "settle" in time and be part of the galaxy disk? How about the universe then? If it is not disk like or spherical, does it mean that universe is not rotating as a whole? Or is it because the universe is not in its final shape yet if there is one final shape ever to be?

Thanks!

 

[mgb_phys]

Gravity isn't very strong.
To form a star or a planet you must have a lot of material pretty close together for gravity to pull it into a ball, the stars in the galactic halo are rather a long way apart and so the gravitational force on them isn't very strong.

 

[cadnr]

It comes down to the balance between centrifugal and gravitational forces. As mgb says, gravity is quite weak, but it goes as 1/r^2 so it only starts to become strong enough to make spheres at very high densities or small sizes.

 

[russ_watters]

Galaxies and solar systems are disks because they form from a single rotating cloud of matter, and an object above or below the plane of rotation would not be as likely to have a stable orbit.