In order for life to have evolved on a planet conditions have to be perfect. The planet has to be in a narrow band the right distance away from a sun. This is called the ‘Goldilocks Zone’ (not too hot and not too cold). Not only that but it has to be the right sort of star; one that will be stable and not give out devastating radiation.
That narrows the possibilities down substantially.
Fortunately the number of stars out there with planets in our own galaxy is trillions. When you narrow it down there are billions of suns with the right attributes and planets that exist in the ‘Goldilocks Zone’.
Given the early emergence of life on this planet shortly after it had cooled (it took one and a half billion years to cool and is now four and a half billion years old) it is extremely likely that there is life on hundreds of thousands of planets. The limiting factor here is the formation and incorporation of DNA (DeoxyriboNucleic Acid) to create the replication, information, organisation, mutation, change and stability necessary for life to reproduce and undergo evolution.
Time is the factor here. Given enough time (a billion years or so) anything that can happen does.
The consensus is that our galaxy alone is probably teeming with life. If it wasn’t for the vast, as yet insurmountable, distances involved we would already had discovered it. As it is we are unlikely to in the foreseeable future because distances between stars are too large. Even light takes hundreds and thousands of years to get here.
There are possibilities that we will discover extra-terrestrial life in our own solar system. It could be on Mars or the moon Europa. Time will tell. What we can be sure of is that this life is not likely to be as complex as us.
This life will probably be in the form of prokaryotic slime (bacterial scum).
We are not alone! There is life out there!
The bigger question is does this life ever evolve to create intelligence?