Asteroids and the formation of planets

[Yaymy]

I was watching How the Universe Works, and they mentioned that planets were formed by dust particles that came together to form rocks, and that those rocks collided with one another and became larger and larger eventually forming planets. My question is why did the asteroids in the asteroid belt never collide to form planets as well? Does the answer to this question also relate to why some planets have rings? Why haven't the rings collided together to form a rocky/icy moon?

 

[Shenstar]

Probably to do with the temperature of early solar system is different to the temperature now?

I know hot rocks and metal like magma flows like a liquid. And maybe this is more likely to merge to form round bodies.

 

[Vagn]

I was watching How the Universe Works, and they mentioned that planets were formed by dust particles that came together to form rocks, and that those rocks collided with one another and became larger and larger eventually forming planets. My question is why did the asteroids in the asteroid belt never collide to form planets as well? Does the answer to this question also relate to why some planets have rings? Why haven't the rings collided together to form a rocky/icy moon?

It's thought that Jupiter is responsible for the asteroid belt. Once the planetesimals reached a fairly large mass Jupiter began to accelerate them quite significantly such that when they impacted with each other they smashed each other apart rather than combine. We have got evidence for this in the form of http://en.wikipedia.org/wiki/Asteroid_families" , where a group of asteroids shares similar orbits and are relatively close to each other.

For your second question, there are several possible reasons.
Rings can be formed when a ring of dust is within the planet's Roche limit, so that it can't coalesce into a moon. Another method is that small rings can be formed by impacts with already formed moons, so that the moon shares its orbit with some dust from the impact. Rings can also form if a moon passes through the Roche limit of the central planet, which can cause the moon to split apart and form a ring.
http://en.wikipedia.org/wiki/Planetary_ring

The rings are maintained through several processes also.
There's some points in the ring systems where there are orbital resonances with the larger moons of the central planet, which will throw any dust in a resonant orbit out of the system. The rings can also be maintained by the presence of shepherd moons on either side of a ring, or the presence of a moonlet inside a gap, which causes a narrow ring of dust to form in the same orbit as the moonlet.

 

[tiny-tim]

Welcome to PF!

Hey Yaymy! Welcome to PF!

The asteroid belt is too close to Jupiter (the largest planet), and anything orbiting there is likely to be disrupted, either by being ejected to a more distant orbit, or by being broken up.

See http://en.wikipedia.org/wiki/Asteroid_belt#Origin" for details, including …

The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about a million years of formation, leaving behind less than 0.1% of the original mass.[26] Since their formation, the size distribution of the asteroid belt has remained relatively stable: there has been no significant increase or decrease in the typical dimensions of the main-belt asteroids​

… why did the asteroids in the asteroid belt never collide to form planets as well? Does the answer to this question also relate to why some planets have rings? Why haven't the rings collided together to form a rocky/icy moon?

To stick together, the particles must have a reasonably high relative speed (but not too high) …

planet formation needs quite a lot of chaos! …

the matter in the rings is all going at virtually the same velocity, and any collisions will be far too gentle.