Well the first question we have to ask is...
how do we spot / find other exoplanets which are very far away and which we can not see?... easy ,( well sort of), you have to find a star which fluctuates position, basically you are looking for a star with a wobble. this genrally happens when there are large masses at play. Thus far 115 out of the odd 177 planets have been gas giants all with some large degree of Orbital eccentricity, eg planet HD41004Bb has an orbit of 1.328 days that is pretty fliping fast! there are a whole lotta theories on how stellar orbital systems are fromed, but what does turn up in the stats is that rocky earth like planets are rare, and gas giants the norm. go to
http://exoplanets.org to find out more.