This line in particular caught my eye:
If it struck, it would have stuck. And we wouldn't be here today. Planets, even dwarf planets, have too much mass and gravity to carom off each other like billiard balls. They would have collapsed into each other, releasing a lot of energy in the process, making one big mushy ball of molten goo.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort