Maybe different gravity, different air pressure, different chemical makeups of land, sea, or air led to totally different forms of life... maybe they are all plant based or reptile based or insect based... maybe even energy based.
The real question you have to ask is why do the alien species travel the stars... is it conquest.... is it just raw materials to keep their civilisation growing... is it curiosity... keeping in mind that while Star Trek suggests it is curiosity... it normally either turns into conquest or trying to find resources that are useful.
Lets face it... it is probably mining companies that will reach out the most... but I think most in the mining industry have a rather amusing view of space... I saw a programme on space colonisation where these guys were talking about asteroids and the mineral wealth they contain... what they don't mention that the asteroid fields you see in movies like Star Wars are bullshit... if you had to dodge and weave like that through a real asteroid field there would be so much mass there that it would relatively rapidly collapse and form major bodies like moons or planets.
If our asteroid belt looked like the one in Star Wars there would be more rocky material in our solar system than there is matter in the sun. The orbital distance of the asteroid belt is huge... if it was full of matter lots more planets would have been formed.
The reality is that you could be standing on an asteroid in our asteroid belt and not see another asteroid at all, or if you did it would be a tiny tiny dot.
The fact is that there are a few big asteroids that might be worth a visit but the idea of zipping from one to another mining valuable metals and stones is bullshit the distances would be enormous.
Most likely one big rock will be mined completely.... anything valuable will be launched on a trajectory passed earth that could be intercepted and collected... any rubbish material could be ground up into powder and launched in the opposite direction you wanted to move and therefore used as propellent to get to the next target asteroid.
At best the alien life will be bacteria in this solar system, though some moons with oceans are potentially interesting subjects to talk about.