At least, from time to time they do.  You’d think there was enough extra space in an infinite universe for this not to happen, but it does.  And when there’s a large enough size difference, the big planets pulverize the tiny ones.

It must have been one bodacious crunch — akin to the moon and Mercury racing to occupy the same spot at more than 22,300 miles an hour.

The story of a collision between nascent planets is written in a disk of dust around a star 100 light-years away, according to an international team of astronomers formally reporting the results in an upcoming issue of the Astrophysical Journal. A plain-English version appears here.

The dust surrounds a 12-million-year-old star labeled HD172555.

Dust.  The little planet couldn’t get out of the way of the big dude and became dust.  I hope some behemoth isn’t bearing down on our planet right now.

Actually, the article goes on to say planets in our solar system bear evidence of past colisions:

Mercury’s current surface is thought to represent lower layers of what originally was a heftier planet. Venus spins backward compared with other planets. Mars is more heavily cratered in its southern hemisphere than in its northern hemisphere. Uranus got knocked on its side — spinning almost as if it had an east pole and west pole, rather than a north pole and south pole.

Poor Uranus.  Not only is it saddled with a most unfortunate name, but it’s all lopsided too because some big mean planet knocked it over.

All these cosmic collisions make my little life seem remarkably calm and uneventful, even on days when I seem to spin backward.