Interesting article...

http://www.usc.edu/isd/publications/...-anderson.html

[...]

NW: Do you believe in the existence of extraterrestrial life forms?

DA: I'm totally agnostic about it. If there's evidence of it, then I'll
believe in it. There's a model called Drake's equation that estimates
the probability of extraterrestrial intelligence in our galaxy. This
involves multiplying the number of stars in the Milky Way by the
probability of a star having planets, of a planet having the chemistry
needed for life, and so on.

The problem of course is that none of these factors is known exactly
except for the number of stars in the Milky Way which is 400 billion.

[ this is the exact number of start in the milky way? ]

We're starting to get improved estimates for some of these factors as
people have discovered planets around stars recently, so we know that
that number is at least not zero. It seems to be pretty common that
stars have planets. People have looked at the way in which the chemicals
of life can develop in the appropriate environment. When you look at all
this evidence, it's likely that there's a lot of life in the Milky Way.

Now that doesn't mean that we're ever going to hear from them. There's
the unfortunate fact that things are very, very far apart. Even the
stars that are closest to us are still so far away that signals of the
kind that we're emitting would be probably lost against the background
of noise that naturally occurs in the galaxy. Unless somebody is
directing a communications beam towards us or has some very clever
scheme for harnessing the energy of a star to produce a powerful
transmission beam, or unless we're able to build much more sensitive
detection means than what we have now, we might never be able to hear
from any of these other civilizations.

For those reasons I think the chances of SETI@home finding anything are
very small.

I still think it's worth doing because of the incredible payoff if we
succeed. As our technology evolves over the next 50 to 100 years and
we're able to put a giant radio telescope in orbit beyond Jupiter and
have millions of times more computing power than we have now, our
chances of hearing signals from other civilizations get greater.