Can a planet collide with another planet?

On Sun, 19 Dec 1999, you wrote:

> can a planet colide with another planet? Example can Mars shake out of
> its position in the solar system and head for a collision impact with
> Jupiter or any of its 16 kown satalites?

Within our solar system, it seems very unlikely that any planet will break
out of its orbit and collide with another planet. There are two good
pieces of evidence that this is true. First, the solar system has existed
for nearly 5 billion years. Planets show evidence that there were lots of
collisions up to about 3.5 billion years ago, and very few after that.
So, for the past 3.5 billion years, the planets have been pretty much in
their current places. Second, computer simulations show that the planets'
orbits are very stable over periods of billions of years.

However, asteroids and comets have orbits that are unstable, and over the
lifetime of the solar system, they will bounce around and perhaps collide
with the planets. But the planets themselves will not collide.

Thanks for asking!

Sincerely,
Kurtis Williams

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