If some awful disaster happened, like another star came crashing through the solar system, the planets might be knocked out of orbit. However, this is very unlikely because of how far apart the stars are.

The planets tug on each other with gravity, though... the question is, can these little tugs reinforce each other and be enough if you wait long enough? In the short term, no. In the long term, no. In the REALLY long term... (I think) no one has figured that out yet.

Question: If you nail down the sun at the center of the solar system and have only one planet going around, it's pretty easy to calculate the orbit of the planet forever and ever. If I know where the planet is right now, I can tell you exactly where it will be billions of years from now. But if you add just one more planet, the problem suddenly gets much, much harder. If I make a tiny little error in where the planet is right now, billions of years later I'll be totally wrong. Why?

There is a very, very small chance that a planet could go out of orbit.How likely do you think this is? Remember that the planets have been in their orbits for a long time, billions of years.

They could, but not for at least a few million years. You see since we have 9 planets, a sun, and a bunch of other stuff in the solar system, our solar system will behave chaotically. This just means that it is very hard to predict what things will be like in the future... just like when you try to predict the weather!! Fortunately, things happen much slower with the movement of the planets than with the weather. In fact it is interesting to notice how well tuned the Universe around us is. There are many things that seem to have been carefully balanced to allow us to exist. Why do you think this is?