| MadSci Network: Earth Sciences |
Somehow I'm not entirely surprised that there isn't much information availible on accretion theory. I encountered the same problem myself. It's unfortunate, because it is also very cool stuff.
In a sentence, accretion theory is the theory of how planets formed from collisions between particles in the early solar system.
To explain in more depth, the solar system began (we believe) as a great big cloud, thousands of time its present size. As the cloud of gas (which was mostly hydrogen, with some helium, oxygen and other gasses thrown in, plus some other, heavier stuff like silicon and iron) collapsed under its own weight, the center started getting dense and heating up. This would become the sun. More important to us here, the cloud flattened into a disk, sort of like a pizza dough tossed into the air while spinning (and for the same reason). It was in this disk that the planets formed.
Basically, the planets formed when small solid bit of matter started colliding with each other. Sometimes they bounced off of each other like billiard balls. But other times, they stuck together. This meant that they particles got bigger (two particles making a single bigger particle). Over millions of years, the particles grew in size, kind of like a snowball rolling down a snowing mountain side, picking up more and more stuff as they got bigger. Of course, when they got really big, their gravity started helping them grow, which only made them grow faster.
The biggest of these guys, now called planetesimals, would become the planets, like Earth and Jupiter, and comets and asteroids. The process of collisions building planets is called accretion.
To expand on this a bit (if you've heard enough, you don't need to read this part; I will include some magazine articles you might take a look at at the end, though): the only things that can stick like this are solid bits of matter. In the inner solar system, where Earth and the other rocky planets are now, this meant only metals and rocks. But out past where the asteroid belt is now, the disk was cold enough for ice to form. If you've ever had a snowball fight, you know water ice makes good stuff for this sort of sticking, so the planets out there had a lot more stuff to build with. As a result, they got a lot bigger (Jupiter is over 300 times as big as Earth). In fact, they got so large that they got to grab up gas from the disk in addition to solids, so now they are big, gassy planets. This is why we have different kinds of planets in our solar system.
While no one has ever seen this happen in real life (it takes millions of years, anyhow), we have run many computer programs which seem to show that this does build planets like our own. We also see some disks which may be forming planets around other stars. These are two of the best pieces of evidence we have that this theory works (meteorites also help, but I don't really want to explain that here).
To summarize: accretion theory is basically a way of saying that the planets formed by stuff in the disk sticking together. The process let the grow even bigger (like the snowball rolling down the hill), and eventually become our loveable planets.
OK, the magazine articles I promised. These might be a bit more than you want, but you could take a look.
A nice article on where we now stand regarding studying the disks around other stars that are believed to give rise to planets.
An excellent article on planet formation. A great place for the non-scientist (or even the scientist!) to start. The article is rather long, but also very thorough
Try the links in the MadSci Library for more information on Earth Sciences.