We have lots of clues that there's probably other life out there. I'd say the chance of Earth being the only planet with life would be a 0.000000000000001% chance. Really, scientists continue to find all kinds of things like this. Why wouldn't they though? Why would all the stuff necessary for life collect at Earth? There's no good reason to think that we're alone except for the fact that we haven't discovered any confirmed alien life yet. It would be nice though. Well.... there's the stuff found in the Martian meteor ALH84001, but many scientists claim it was never life.