How many planets are there?

There are eight planets in our native solar system. We used to consider there to be nine, but then scientists decided that Pluto doesn't fit the requirements to be a planet. I don't really know what that is, but that's what they decided. Mercury is the closest planet to the sun, and doesn't really have an atmosphere. Venus is subject to a runaway greenhouse effect. Earth is where we live. Mars is red, and slightly smaller than Earth. We've recently found evidence of water there. The asteroid belt comes next, which isn't really a planet. Following the asteroid belt are the gas giants Jupiter, Saturn, and Uranus. They have rings. Neptune comes last. Other solar systems seem to have planets too - in fact this appears to be far more common than we would have thought.