At the time of the Manhattan Project, Enrico Fermi posed a famous question: if extraterrestrials exist, where are they, and why have we not yet met them?
Later, this was subverted: if extraterrestrials exist, where are their von Neumann machines? These are self-replicating machine which exist to copy themselves.
One of the answers is that any species smart enough to make von Neumann machines would realize the consequences of doing so and avoid making them.
Another answer may be that life as we know it is a von Neumann machine, programmed to spread itself across space, a notion that would appeal to Fred Hoyle.
The von Neumann machines have a modern parallel in the 'grey goo' of the technophobes who fear and fail to understand nanotechnology and see it running wild.
Many stars have planets: this can be shown in a variety of ways, even when the planets themselves remain invisible, by looking for wobbles in the star.
In 1989 Wolszczan and Frail reported the first detected extra-solar planet orbiting a pulsar, when a large planet was detected, orbiting the pulsar PSR 1257+12.
In 1995 Mayor and Queloz reported the first confirmed extra-solar planet orbiting an ordinary star, , orbiting the star 51 Pegasi.
The science of looking for life elsewhere is called exobiology. The most likely near-Earth sites to find traces of life right now are Mars and Europa.
Planets which are large enough, and so have enough gravity, will have and retain an atmosphere, but that does not necessarily mean that it will be breathable.
There is less evidence available to indicate if other stars have planets like Earth, located in the gap between hot and cold, where life is likely to be found.
According to the Zoo Hypothesis, intelligent life has detected us on Earth, and is carefully observing us from a distance, just as we would regard zoo animals.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsseti.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.