For the planetary chauvinists in the space movement, the prospects are not bright. In our Solar System only two planets (Mars and Venus) are suitable for human colonization, though colonizing Venus would require terraforming that planet (which would take at least some 200 years). On the other hand suitable planets in other planetary systems are minimally several light years away from us, and any journey to those exo-planets will take a period comparable to terraforming Venus. And in case we arrive at a yet uninhabitable planet, we would need to terraform it anyway.
Well, this analysis assumes that our understanding of our Solar System is complete. Unfortunately, our understanding Solar System is far from complete. As far as we know, there are four terrestrial and four giant planets in our Solar System. (Regardless whether you consider Pluto as a planet or not – we do not chose a side in that controversy -, Pluto is unsuitable for colonization.) The giant planets are unsuitable for colonization, and can’t be terraformed and Mercury is too close to the Sun.
Nevertheless there are still people looking for new planets within our Solar System. The question is whether such planets might exist? Depends on what you are looking for. If you consider Pluto and similar objects as planets, then you could expect the discovery of many planets during the next few decades. However, if you are looking for something more substantial, it will be much harder.
The point is that massive objects are exerting gravity upon each other: the orbits of objects in our Solar System are the result of complex interaction between the several planets and the Sun. Scientists consider the Solar System as a chaotic system, i.e. a small change in the configuration of the Solar System, will have enormous consequences for the orbits of all Solar System objects.
At this moment planetary scientists are able to explain the orbits of the planets at the hand of all known planets. (The so-called anomaly of the orbit of Uranus has been solved, after the mass of Neptune was determined more accurately.) Consequently the gravitational effect of any trans-Neptunian planet on the known planets should be negligible. Either because such planet has a small mass (as in case of Pluto) or because such planet is located at large distance from the known planets.
But if you are a planetary chauvinist looking for planet suitable for colonization, you want this distance to be as small as possible. Hence what is the minimal distance at which a planet might be located?
Many astronomers have proposed trans-Neptunian planets in the last two decades. Computer simulations performed by Rodney Gomes showed that a Mars-sized planet could exist at 53 AU, and a Neptune-sized planet at 1500 AU. An Earth sized planet could exist in between. Calculations done by Lorenzo Iorio show that a planet twice the mass of Earth, should have a minimal distance from the Sun of 496 to 570 AU. An Earth-sized planet might consequently exist somewhere between 53 and 500 AU from the Sun.
(Recent empirical evidence seems to rule out Saturn-sized planets up to 10,000 AU from the Sun, and Jupiter-sized ones up to 26,000 AU.)
Even in the optimistic case that we find a Earth-sized planet within 500 AU from the Sun, the planetary chauvinist has to face an important issue. At such a distance away from the Sun, the planet is most likely to be covered with ice of frozen water, nitrogen and similar substances. In other to terraform such planet, one would need to heat it up.
At first glans Solar power seems to be unsuitable for this task. However, by using a vast amount of space mirrors it might be possible to concentrate enough Solar power to heat up a planet. The important question is, of course, how much we want to spend on Solar mirrors to make a far-away planet inhabitable for humans.