There is only one data point. And that is us. All else is speculation. However, using probability, we can make some predictions. Thusly was born the "Drake Equation".
What is your opinion of the arguments made in "Rare Earth". The Drake equation only gives the upside, but doesn't seem to take into account the conditions that would preclude higher forms of life. For instance, the radiation conditions in the core of galaxies and in the arms of spiral galaxies are too high to allow the survival of higher life forms. That leaves the area between the arms for habitable zones in spiral galaxies, and stars only stay in between the spiral arms for extended periods of time if they are 1/2 the radius of the galaxy from the center of the galaxy. These factors alone remove several orders of magnitude from the number of possible stars that could sustain higher forms of life.
What about all the other lifeless planets and moons we can observe in our own neighborhood?
First, most scientists accept that "binary" star systems (or - any arrangement where 2 or more stars rotate around each other) are not likely to have any conditions necessary to support life. The constantly changing gravitational stresses would probably prevent the formation of any planet that could support life, and if a planet existed - the constant upheavals, earthquakets, etc. would be catastrophic. [Oh, By The Way ... 70% of the stars are not "by themselves" ... and are in binary or higher configurations.]
Again, most scientists agree that the center of a galaxy, where 20 - 30% of the stars are in much higher density, the radiation levels are very high, and would not be conducive to life.
And most scientists agree that stars on the very outer reaches of a galaxy probably lack the "super nova" remnants that provide the heavy elements that are found on the Earth (elements with an atomic number greater 56 (Fe - iron). What would happen if a small earth sized planet formed but there was nothing heavier than iron??
So the first 3 suppositions remove possibly 95% of the stars from being considered as possibly supporting life.
The latest analysis considers the way planets form. The analysis suggests that perhaps a more normal way for planets to form is not the way our planets formed around our Sun. Suppose that 80% of the time, the method of planet formation is what we see elsewhere (large gas giants, closer to a star). .. and that method would not be conducive to creating a small planet that would be habitable to life ... This means that once again, we are removing additional stars from consideration where life might form.
Mike
Most of them are, however, much closer to their sun than Jupiter is to ours: they are known as 'hot Jupiters'.
Can you say "selection effect?"
Obviously, large, close planets gravitationally interact more with the parent star than do large, distant planets or small planets at any distance. Thus, our extrasolar planet-detecting systems will preferentially detect the type of planets that the study then cites as the basis on which to conclude that earth-like planets must be rare.