Everyone talks about there being eight planets.
Well that is only counting the ones in our solar system. When I was young the first movie that I saw in theatres was Star Wars. The possibility of planets orbiting all those stars that we see in the night sky was pretty mind blowing. Back then we had no way to tell if there really were planets around other stars. We thought there should be but the technology was not good enough to allow us to detect them.
Now we can. The current count is 832 planets in 664 solar systems. (Note these numbers are changing rapidly (Oct 19th 2012))
So how do we find them?
There are a number of methods. The one to have found the most planets so far is called the Doppler Method. Doppler shift is worth a whole article on its own. Basically we can measure the speed a star is moving away from us. As we saw with binary stars they orbit around their common center of mass.
The same thing happens between a sun and its planets, but with the planets being so much smaller the effect is smaller. We can measure the slight change in the speed away from us caused by a planet wobbling the sun. Showing a repeating cycle of velocity changes allow us to deduce the existence of a planet.
The one to have found the second most planets is called the transit method.
Right now there is a satellite called the Kepler Space Observatory looking at one area of space and carefully measuring the light output for each of the stars in its view. If a planet passes in front of its star we can measure the tiny drop in light caused by the planet blocking that light.
Again we need to see multiple transits to confirm that what we are seeing is a planet.
There are many other methods being used but in the interest of length, I will refer you to the Wikipedia article on them.
Are any of them habitable?
That is a very tough question. The basic definition of habitable is that liquid water could exist on its surface. That defines a ring around the star where the energy from the sun would keep the water liquid. Right now we have six planets that could potentially support life. It also put a limit on size. We have found Jupiter-sized planets in habitable zones. Clearly they wouldn’t be habitable.
We are going to continue finding more and more planets as our technology gets better. We will get a clearer idea of how solar systems and planets form. There are a lot of weird planets out there that will take some time to come up with scientific theories on their formation. We will find more potentially habitable planets and turn new instruments at them to learn more about them. It is only going to get more interesting.