The article linked below suggests that a gas-giant planet, like Jupiter, in a specific spatial relationship to an asteroid belt at about the distance of the ice-line may be necessary for the existence of life on terrestrial planets in extra-solar planetary systems. Few Jovian planets have been observed in such orbits.
I contend that this is not a convincing reason for assuming that life-bearing worlds may be relatively rare, as the article suggests. Using our own system as an example, Jupiter would have to be observed for several decades (several orbital cycles) from another star system in order to confirm its existence. Shorter surveys would find mostly close-in, 'hot Jupiters'. This is, in the main, just what has been observed. It is still probably too early to assume that Jovian planets in orbits similar to that of Jupiter are are rare, or even unusual. http://www.space.com/18326-asteroid-belt-evolution-alien-life.html