There is actually a lot more out there than we know! For a long time, scientists knew of 9 planets within our solar system. This number included a planet called Pluto, which was the farthest from the sun. As telescopes got better, astronomers discovered that there are many thousands of rocky chunks out by Pluto — and that Pluto is basically one of the largest. So why call that a planet and not the rest? As a result, astronomers decided to call Pluto — and the rest of those bodies — “plutoids”. So now there are officially 8 planets in our solar system, and a great many plutoids.

But beyond that — in the last 20 years, astronomers have begun to discover that there are planets around other stars, which are outside of our solar system. These are called “exoplanets” (exo means “outside”, so they’re planets outside our solar system). Since 1988, a few thousand new exoplanets have been discovered orbiting other stars! These are very far away, and are very hard to see.

More are being discovered every year, and astronomers are getting closer and closer to finding some that are even “earth-like” — which have the right size, distance from the star (so they’re not too hot and not too cold) and may even have water. If (or when) such earth-like planets are discovered, then the conditions would be right for life as we know it, and it would be very exciting question would be whether something may be living there...

Yes, lots.

Planets that are large or close in to the solar system are noticeable, so I think I can say confidently that we've found all of them. The Kuiper objects - Pluto, Eris, and many others - are far from the sun, faint, and small, which makes them difficult to detect. There could easily be many more Pluto/Eris-sized Kuiper objects far out enough that we haven't seen them yet. And then, there are other stars, and they have planets, too.