They don't have a clue if these planets could support life. They only know the planets are there the "wobble" effect in the star created by the planet's gravitation effect. Thus, the general mass of the planet and it's distance from the star is all they know. Scientists have shown that there are at least 23 finely tuned conditions of the earth which are absolutely necessary to support life. This includes things like the exact composition of the atmosphere, the very narrow temperature range, gravitational forces on the surface, tilt of the axis, etc. etc. The probability of even 15 of these conditions all being present on any given planet are more than 10 to the -256 power against it. Such a large improbability is generally considered an impossibility.
Kepler works by measuring the dimming of the star as a planet passes in front of it.