how many?
"Stuff your eyes with wonder, live as if you'd drop dead in ten seconds. See the world. It's more fantastic than any dream made or paid for in factories." -Ray Bradbury
It wasn't all that long ago -- back when I was a boy -- that the only planets we knew of were the ones in our own Solar System. The rocky planets, our four gas giants, and the moons, asteroids, comets, and kuiper belt objects (which was only Pluto and Charon at the time) were all that we knew of.
Image credit: NASA's Solar System Exploration, http://solarsystem.nasa.gov/planets/index.cfm.
But these were just the worlds…
"Arithmetic! Algebra! Geometry! Grandiose trinity! Luminous triangle! Whoever has not known you is without sense!" -Comte de Lautréamont
When you think about it, it's amazing that our physical Universe makes sense at all. The fact that we can observe what's happening, determine the laws that govern it, and predict what will happen under the same or similar circumstances is the most remarkable power that science has. If that's what you're doing in any aspect of your life, congratulations, you are a scientist. But that doesn't tell us, fundamentally, what the Universe is like at its most basic…