Even if we find new planets, we can't get there. Even with an anti-matter drive, you need reaction mass, and it takes a LONG time, so you need a lot of reaction mass. Even a fusion drive won't do it. The ship must be huge, a minor plant almost, and the radiation shielding needed at high fractions of c is very massive. The folks who launch the trip won't get there alive without some kind of suspended animation tech, so you need a complete society on board, with all needed specialties and their associated equipment for a complete technological society on the planet they finally reach, and you can't do the needed fine grain survey to look for the uh-oh's in the environment from stellar distances.
If you get the ship up to say, .9c, roughly 270,000km/sec, and you hit a grain of sand weighing one gram, and you calculate the force using f=mv^2, your ship goes up in a flash of energy, so you need multi-terawatt laser arrays and highly sophistocated detection and aiming tech to sweep the path clear. You can't turn fast enough to miss the junk, and you sure as hell can't slow down fast enough to do any good.
Bussard ramjets are theoretically feasible, which use a funnel of magnetic fields to sweep up interstellar hydrogen into a focus where fusion takes place, but you have to get them up to operating speed (probably a respectable fraction of c) somehow, after which your fuel is all around you - doesn't have to be carried with you. The drawback is the really strong mag fields, which are deadly to anything with a notocord due to the induced currents, and probably deadly to anything period due to the induced currents in the ship itself.
There are other theoretical transport systems. Wormholes where you disappear here and reappear there without actually transiting the intervening space, but the only ones we know about are black holes, which may or may not have an exit, but entering one is deadly. It is theorized that a rapidly spinning VERY massive cylinder can be used, but the calculated mass is more than the biggest star, and the rotation speed is very close to c. Since mass increases with speed, and is infinite at c, c becomes the speed limit in this universe, because it takes infinite energy to accelerate an infinite mass. I recall a scifi story about a fictional starship called the Lazy 8, i.e., the infinity symbol. Subtle humor...
And ponder this: Why hasn't the listen for the aliens project found any? Nobody out there? Nobody had radio long enough to get to us? Or is everybody else hunkered down and being real quiet for a very good reason? The earth-moon system may be unique, the result of a highly unlikely collision during planetary formation in the early solar system, and without the moon, we would not exist.
Believe me, I hate this. I'd like to have aliens around, and go to the stars and see the sights (Beta Lyrae would be most lovely). But the math just doesn't work...
