Eventually, you can improve your craft so that it can construct colonies on planets lacking an atmosphere (by using domed cities). More impressively, you can buy a "Genesis" upgrade that can turn a dead planet into one with an atmosphere and full ecosystem; just like in one of those late-Shatner-era Star Trek films.
You can colonize other dead planets and move your species around a bit. But why stop there? Why not zoom out further and view all the stars in your star system, and then the entire galaxy? Not enough? Soar out further and see a universe full of galaxies. Each of the hundreds of thousands of stars that exist when you're fully zoomed out can be explored. You can go anywhere and each solar system is different - some with life, some without.
You search for life by hovering your radio wave sensor over a star system - if you hear noise, something is there. You can then go there and explore, see the different type of creature and collect its data. Then either leave again or kill everyone, colonizing the planet for yourself.
The point, though, is that there are hundreds of thousands of star systems, many with life on them. Given the unique way different creatures can be generated in this game, you'll never see the same thing twice. This is not speculation. We've seen this running and, soon, so will you. The only question is - how is Will Wright ever going to justify a sequel?