And assuming other planets is a simple application of the principle of mediocrity, that we should not assume there's anything peculiar about our immediate environment just because we're in it. To assume that out of the hundreds of billions of stars just in our own galaxy the only planet was this one was absurd statistically as well as an appalling display of hubris.
Ironically, it was an assumption based on our solar system that kept Butler and Marcy from making the first exoplanet discovery around a main sequence star: they were looking for nice neat nearly-circular ellipses and an orderly set of orbits at "sensible" differences. Mayor and Quiroz were willing to look at any sort of orbit, and they got the prize; Butler and Marcy in fact had data on 51 Pegasi and were able to confirm their rivals' discovery within a week, and everyone was shocked that a Jupiter-class planet could orbit its star in just 4 days in just an eccentric orbit.
This, by the bye, is one of the things I love about science, because certainly Butler and Marcy had every reason to hope the finding was in error so they could be first. But that's not what the data said, and the data is the only thing that counts, and so they ended up providing the essential confirmation of the discovery that they didn't get to make.
FWIW, the first exoplanet discovered was in 1989, but it wasn't confirmed as a planet until 2012. It was originally thought to be a brown dwarf. Its existence was confirmed in 1991, but its status as a sub-stellar object wasn't settled until 21 years later.