What Do We Know About Exoplanets?
There are a total of 353 known exoplanets as of the writing of this article. Considering how difficult it is to detect them, that's a pretty big number!
As you might imagine, most of the exoplanets we've detected so far are large gas giants, often many times the size of even Jupiter, and borderline brown dwarfs.
However, this doesn't mean that the majority of exoplanets are, in fact, gas giants. It's really just that they're easier to detect. We won't really know what the typical make up is of other solar systems, including the number and types of exoplanets, until either our detection techniques and technology improves. The guess is that rocky bodies vastly outnumber gas giants, based on our own Solar System and a bit of logic, though this has yet to be definitively proven.
Nor do we really know what percentage of stars have exoplanets. Since we can't even detect the majority of potential size ranges for exoplanets, we can't make a proper guess. The current estimate is 10%, but this is subject to a huge range of variation amngst astronomers. Pick an opinion, any opinion.
People often speculate on what sort of star systems would have more exoplanets on it. The general conclusion is that the fewer stars there are in the system, the more likely that there will be exoplanets, as any orbiting material would have otherwise long since been sucked up by such dynamic gravitational system. Similarly, more stable stars, that is, main sequence stars that haven't already gone on to the red giant stage, are more likely to have not destroyed any exoplanets by that point.
Exceptions do exist, however. There are exoplanets that are known to orbit pulsars, for instance, and speculation on how this managed to happen has been floating in astronomy journals ever since their detection.
Want to see currently known exoplanets laid out? Check out the Exoplanet Atlas with NASA's PlanetQuest search for exoplanets.