Worlds in Near Collision

At www.nytimes.com/2011/01/16/weekinreview/16chang.html?ref=kennethchang there is a story sent in by member Gary Gilligan. It begins with the theme of plate tectonics and earthquakes and proceeds to the solar system and planetary astronomy. Apparently, it is now thought the solar system is not a constant but has changed – over time. It is being seriously suggested the larger planets were once much closer to each other than nowadays but the orbits of Uranus and Neptune became unstable and they were flung outwards.