In fact we do not know the answer to this question, for a simple reason: we have available only one Earth to examine, so far. As far as we know regarding carbon-based life, we can roughly determine where planets would be that there could be life as we know it. This will help our great looking, will help us not waste the efforts of searching in all corners of the universe.
Condition number one
The first condition refers to mass of the star. The giant ones . . glowing beauty, but have a life too short (less than a billion years) to allow for life to arise and evolve.
At the other end, stars too young (with masses one tenth the mass of the Sun) just does not light up and so will not release into space enough energy for the emergence of life. We can say that our Sun is very close to a star capable of sustaining life on average. This is why our future research will go to the stars similar to ours.
Condition number two
We will still talk about mass, but this time on planet's mass. This parameter is related to life, a very important parameter. It's about atmospheric composition. A very massive planet would not let escape into space, light gases such as hydrogen. But it will combine with nitrogen, resulting in ammonia, or carbon, resulting methane, we would have an atmosphere similar to that of Jupiter, an atmosphere unfavorable to life.
The same thing about small planets. Could not harbor life. They will let hydrogen escape, which puts us problems if too massive planets, but in addition will escape into space oxygen and water vapor.
Thin atmospheres of small planets have another drawback: they can't protect against ultraviolet radiation and meteorites. Again, we can say that our blue planet is perfect for appearance and evolution of life.
Condition number three
Distance from the star, is in turn, a critical parameter for life. As we know, life is based on liquid water. We therefore have restricted temperature range within which life is possible, hence the planet that is, required to be within a certain distance from its star. In our case, if Earth's orbit would change only slightly, then life would not exist.
Condition number four
The latter condition is related to the composition and structure of the planet. The composition of the planet in terms of content of chemical elements we'll not dwell, in this area is quite a large variations allowed, provided that there exist those ingredients necessary to life. But there is another important aspect: the internal structure of the planet. Fortunately, the Earth possesses a outer molten core, which allows generating a magnetic field intense enough to protect us from harmful radiation emitted by the sun.
Here, we reviewed briefly as possible, the main conditions necessary for the appearance of life. You already found that, as we understand it now, life is something very fragile. Several conditions must be met simultaneously, which entitles us to say that life could be a rare phenomenon. This may seem to a skeptical view, an argument in favor of our loneliness in the universe. But there is another approach that can be summed up in one simple sentence. Whenever there are necessary conditions for the emergence of life, it appears necessary!
by "environment clean generations"
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