The Milky Way galaxy contains over 400 billion stars (and is only one of billions of other galaxies). Could it be that ours is the only planet where life arose? Perhaps. Or maybe life is common, but too often destroys itself as technology becomes too powerful to handle. But we should also consider that the sheer vastness of space and the great number of stars and planets has given rise to a number of technological civilizations capable of communication.
The Drake Equation gives a means for estimating how many communicating civilizations may be out there. The results can vary widely, depending on the optimism of the numbers you yourself plug in.
The equation computes -
N: the potential number of communicative intelligent civilizations in our galaxy. It is computed using the following equation...
N = R* x fs x fp x ne x fl x fi x fc x L
...using the following factors:
R*: is the rate of formation of stars in the galaxy
fs: is the fraction of stars that are suitable suns for planetary systems
fp: is the fraction of those stars with planets (thought to be around 1/2)
ne: is the number of "earths" per planetary system - planets suitable for liquid water
fl: is the fraction of those planets where life develops
fi: is the fraction of planets with life where intelligence develops
fc: is the fraction of those planets that achieve technology which releases detectable signals into space
L: is the lifetime of such communicative civilizations
Depending on the numbers used, the Drake equation can yield wildly differing results. Possibilities range from a few (relatively) short-lived technological civilizations scattered far apart among the stars, never contacting each other before they disappear, to a more probable (considering the vastness of the cosmos) large number of life-bearing planets. There are over 400 billion stars in our galaxy alone, and it is estimated that approximately 1/2 of all suitable suns have planetary systems of some sort.
It is not known how likely it is for life to develop on a suitable planet. Once life does exist, however, it is quite evident from our own evolutionary history that it is quite adaptable and tenacious. The jury is still out, however, on how long species survive once they have developed intelligence and powerful technology.