What defines a 'planet' ?

The familiar solar system that you learned about in school — nine well-behaved planets, from Mercury to Pluto, circling sedately in tidy paths around the sun — isn't what it used to be. Astronomers recently have discovered a flock of at least eight other planet-like objects in sometimes wildly eccentric orbits. Four new "ice dwarfs," plus two more probable moons around Pluto, were announced in the last six months.The latest mini-world, temporarily nicknamed "Buffy" and mo... (Read more..)