First is that something like 70% of all stars in our galaxy are red dwarfs. What life might evolve on them would be very different from life on our planet. Keep in mind we have a relatively large moon that affects things like tides. And that those tides were no doubt crucial in the evolution of life on this planet.
So far, no exo-planets we've yet found have our kind of large moon. Okay, so we really have a very long way to go in figuring out if any of those exo-planets are even remotely close to our planet, but we're working on it.
It's also important to know about interstellar distances. If you could actually travel at the speed of light, or some fraction just below it. it would take you about four and a half years to get to the closest star. Which may or may not have a planet capable of Earth-like life.
Another thing to know: cosmic radiation between stars is a real problem. Even travelling at the kinds of speeds we're currently capable of makes the radiation a problem. And the faster you travel, the more and faster that radiation will affect you. Ooops. It is possible that actual travel between stars is truly impossible. Impossible for us, impossible for any other species. Which leads to the possibility that we may have never, ever, been visited by aliens. Think about it.
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