Defining Planets 
Astronomers long ago ran out of enough fingers and toes to count all of the planets they've discovered in other star systems. The count is in the hundreds, with new planets joining the list every month.
One problem, though, is that there's no formal definition of what is and isn't a planet. So far, it's pretty much "you know it when you see it."
Astronomers do have a definition for planets in our own solar system: A body that's massive enough to pull itself into a spherical shape and to clear out its orbit around the Sun.
But that definition doesn't apply to the worlds orbiting other stars, which leaves some wiggle room.
Many of these objects are much bigger and heavier than Jupiter, the giant of the solar system. Some of these may not be planets at all. Instead, they may be brown dwarfs -- objects that are more massive than planets, but not massive enough to shine as stars.
The dividing line between planets and brown dwarfs is somewhere around a dozen times the mass of Jupiter. And the masses for some of the extrasolar planets are still uncertain -- astronomers have a minimum mass, but not an exact mass. Without an exact number, it's impossible to say if some of these objects are planets or brown dwarfs. Even with that uncertainty, though, it is certain that we're gonna need a lot more fingers and toes to count all of the worlds orbiting other stars.
We'll talk about one of these planetary systems tomorrow.
Script by Damond Benningfield, Copyright 2010