Here is a brief outline of the current theory of the events in
the early history of the solar system:
- A cloud of interstellar gas and/or dust (the "solar
nebula") is disturbed and collapses under its own
gravity. The disturbance could be, for example, the shock
wave from a nearby supernova.
- As the cloud collapses, it heats up and compresses in the
center. It heats enough for the dust to vaporize. The
initial collapse is supposed to take less than 100,000
years.
- The center compresses enough to become a protostar and
the rest of the gas orbits/flows around it. Most of that
gas flows inward and adds to the mass of the forming
star, but the gas is rotating. The centrifugal force from
that prevents some of the gas from reaching the forming
star. Instead, it forms an "accretion disk"
around the star. The disk radiates away its energy and
cools off.
- First brake point. Depending on the details, the gas
orbiting star/protostar may be unstable and start to
compress under its own gravity. That produces a double
star. If it doesn't ...
- The gas cools off enough for the metal, rock and (far
enough from the forming star) ice to condense out into
tiny particles. (i.e. some of the gas turns back into
dust). The metals condense almost as soon as the
accretion disk forms (4.55-4.56 billion years ago
according to isotope measurements of certain meteors);
the rock condenses a bit later (between 4.4 and 4.55
billion years ago).
- The dust particles collide with each other and form into
larger particles. This goes on until the particles get to
the size of boulders or small asteroids.
- Run away growth. Once the larger of these particles get
big enough to have a nontrivial gravity, their growth
accelerates. Their gravity (even if it's very small)
gives them an edge over smaller particles; it pulls in
more, smaller particles, and very quickly, the large
objects have accumulated all of the solid matter close to
their own orbit. How big they get depends on their
distance from the star and the density and composition of
the protoplanetary nebula. In the solar system, the
theories say that this is large asteroid to lunar size in the inner solar
system, and one to fifteen times the Earth's
size in the outer solar system. There would have been a
big jump in size somewhere between the current orbits of Mars and Jupiter:
the energy from the Sun would have
kept ice a vapor at closer distances, so the solid,
accretable matter would become much more common beyond a
critical distance from the Sun. The accretion of these
"planetesimals" is believed to take a few
hundred thousand to about twenty million years, with the
outermost taking the longest to form.
- Two things and the second brake point. How big were those
protoplanets and how quickly did they form? At about this
time, about 1 million years after the nebula cooled, the
star would generate a very strong solar wind, which would
sweep away all of the gas left in the protoplanetary
nebula. If a protoplanet was large enough, soon enough,
its gravity would pull in the nebular gas, and it would
become a gas giant. If not, it would remain a rocky or
icy body.
- At this point, the solar system is composed only of
solid, protoplanetary bodies and gas giants. The
"planetesimals" would slowly collide with each
other and become more massive.
- Eventually, after ten to a hundred million years, you end
up with ten or so planets, in stable orbits, and that's a
solar system. These planets and their surfaces may be
heavily modified by the last, big collision they
experience (e.g. the largely metal composition of Mercury or the Moon).
Note: this was the theory of planetary formation as it
stood before the discovery of extrasolar
planets. The discoveries don't match what the theory
predicted. That could be an observational bias (odd solar systems
may be easier to detect from Earth) or problems with the theory
(probably with subtle points, not the basic outline.)