by Frank Crary, CU Boulder
Here is a brief outline of
the events in the early history of the solar system:
1.
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.
2.
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.
3.
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.
4.
The gas cools off enough for the metal, rock and (at
larger distances from the forming star) ice to condense out into tiny
particles. 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).
5.
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.
6.
Once the larger of these particles get big enough to
have a nontrivial gravity, their growth accelerates – Run away growth. 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.
7.
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.
8.
The "planetesimals"
would slowly collide with each other and become more massive. At this point, the solar system is composed only of
solid, protoplanetary bodies and gas giants.
9. 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).