What IS a planet, anyway?

Astronomers are trying anew to define exactly what should qualify as a planet and how many of them inhabit our solar system. The discovery of a large "borderline" object past Pluto (officially known as 2003 UB313, but better known as Xena) has brought new attention to the fact that Pluto is pretty borderline itself. The topic will be the headliner at the 12-day International Astonomical Union conference now beginning in Prague.