This is the stuff of every sci-fi writer's dreams, except that it's true...
Astronomers have discovered over 700 exoplanets (extrasolar planets), of which 120+ are in the habitable zone - not too far or too near their sun for life. Of these, most are gas planets the size of Jupiter or bigger, and won't support life ("as we know it"). One reason for this is that bigger planets are easier to detect, so there's no reason not to think smaller Earth-like planets also exist in those solar systems.
Last month NASA announced the discovery of the first Earth-like planet orbiting a Sun-like star. The planet is 2.4 times the radius of Earth and similar in temperature, with a 290-day year, while the star (Kepler-22) is G-class, the same as our Sun, and 600 light-years away.
Little is yet known about the composition of this exoplanet, so we can only speculate that it might be similar to Earth's - mostly ocean with a rocky core - in which case, it might harbor life.
There are probably tens of billions of exoplanets in our galaxy alone. Multiply that by the 100 billion galaxies in the universe and you have - well, squillions of planets! It's nice to imagine (hope) a few of them have life that we may one day explore, and even nicer if that life is sentient. Preferably friendly.
And even if the planets themselves don't support life, their moons might. Our moon might be a big ol' dusty pock-marked rock but they're not all like that.