For thousands of years people have wondered if there are other worlds and other living beings in the universe. In the 1990s, we discovered the first planets around other stars—alien worlds as big as our Jupiter. Since then, we’ve found many such extra-solar planets. In the years ahead, improved methods will allow us to detect planets like Earth—alien Earths . Whether they harbor life is a harder question that may take generations to answer.

How many extra-solar planets have we found so far? Visit NASA's PlanetQuest site for the latest news. You can even take a 3D tour of all the planets that have been discovered.