planet - Why, in the Solar System, all the mass seems to be concentrated at the centre?

I feel that HDE gives a good start of an answer, but stops short of the important part. We have seen in HDE's answer the formation of a star at the center of the collapsing molecular cloud. When the star begins to fuse lighter elements, the protoplanetary disk has several forces acting on it:

• The momentum of the particles in the disk.


• The gravity of the star in the center and other particles of the protoplanetary disk on the other side.


• The gravity of the particles of the protoplanetary disk opposite the sun (outward).


• The radiation pressure of the new star.

Interestingly, the second and third forces pretty much cancel out when the disk is still uniform in distribution. But once large clumps of matter accumulate those large masses (protoplanets) gravitationally pull at the other matter in inconsistent and sometimes violent ways, especially when multiple protoplanets align (once per orbit of the inner body).

Thus, the area of the protoplanetary disk is being 'swept up' of all the mass: some is pulled into the sun by gravity and loss of momentum due to collisions, some is pushed out of the solar system by radiation, and whatever matter was not disturbed by one of those processes is then subject to being disturbed by the gravity of the protoplanets themselves. The protoplanets will, over time, either absorb that matter or gravitationally slingshot that matter out of the system or too it's very edges.

In short, the protoplanetary disk within a few tens of AU of the star in the center is a chaotic mess of a place. Not much matter can form a stable orbit there.