Hot Air links to this video of a computer simulation of an asteroid collision with the earth. It's best watched full screen with the captions on. The asteroid in the sim is about 300 miles in diameter (about as wide as Pennsylvania is long):
The video notes that this is believed to have happened about six times in the earth's history. One such collision, which struck more of a glancing blow to the earth, is believed to have ejected enough molten rock from the earth's crust to have formed the moon. Another impact, smaller than the one depicted, is believed to have wiped out the big dinosaurs.
Such collisions would be even more frequent were it not for the fact that the earth orbits the sun in the same plane as the large outer planets, Saturn, Jupiter, Neptune, and Uranus. These, as well as our large moon, act as cosmic vacuum sweepers whose powerful gravity sucks debris into themselves preventing it from striking the earth. This is one of the many facts about life on earth that often gets overlooked in discussions of the possibility of finding life in other solar systems. Any planet which would give rise to life has to meet an extraordinary number of conditions, among which is that it has to be shielded from impacts such as this one by larger planets in the same solar system.
Few, if any, other planets in the galaxy, or even in the entire universe, meet all the criteria necessary to sustain life which is why scientists Peter Ward and Donald Brownlee titled their book on the subject Rare Earth.
Until just a decade or so ago, scientists pretty much accepted the principle of mediocrity which held that the earth was an unexceptional planet of an unexceptional star situated in an unexceptional galaxy. It was believed that such characteristics must be common in the universe and that therefore life, too, must be common. With the dawning realization, however, that the earth is anything but ordinary and that living things require hundreds if not thousands of specific conditions unlikely to be found together in any one place anywhere else in the cosmos, scientists have of late had to change their thinking, and the principle of mediocrity is being silently laid to rest.
The earth is an extraordinary place. One might almost think, if he didn't know better, that it was all intentionally set up that way.RLC