Space.com

What's Hot This Week Meet the Solar System's Dwarf Planets by Mike Wall, SPACE.com Senior Writer For three-quarters of a century, schoolkids learned that our solar system has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. But things changed nearly five years ago today. Cosmic Crashes May Give Habitable Planets the Boot by SPACE.com Staff Long-ago collisions between clouds of gas and dust could explain why many alien solar systems have planets with strange, highly tilted orbits - and why habitable worlds may be rare in the universe, a new study suggests. Sponsored by: Vote Now! Best Space Stories of the Week - Aug. 21, 2011 Pluto: A Dwarf Planet Oddity (Infographic) Best Space Photos of the Week - Aug. 20, 2011 Lost Russian Communications Satellite Found in Wrong Orbit 'UFO' Over Chinese Airport Just an Unusual Cloud