I am working on the whole Big Bang Theory.
Please give me a few million years. Thanks.
The Origin of the Solar System
by Frank Crary, CU Boulder
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
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
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
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
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.)
Text by Frank Crary, converted to html by Bill Arnett; last updated: 1998 Mar 17