archive: SETI The Drake Equation (Sagan: A Carl Sagan Web site)

SETI The Drake Equation (Sagan: A Carl Sagan Web site)

Larry Klaes ( )
Tue, 18 May 1999 08:22:45 -0400

At 04:37 PM 05/17/1999 CST6CDT,4,1, Greg Loeb wrote:

>I just read this in a July 98 posting by Larry to the list, and I'm
>hoping you guys can remark on this.
>It seems to me that, because of our very limited knowledge of the
>different fractions that are included in the famous Drake Equation, that
>it is meaningless to come up with an 'estimate' of ETI civilizations in our
>galaxy. Although it may help us think about the problem more clearly, aren't
>the error bars so high that we can come up with any number we like? I think
>Sagan himself also estimated there was only 1 (at least in the Cosmos

Sagan's optimistic estimate of intelligent life in the
Milky Way galaxy under the parameters of The Drake Equation
were one million technical civilizations. On the other end,
he saw only ten civilizations. Frank Drake himself thought
there could be perhaps 10,000 civilizations. On average,
there could be an ETI society roughly once every 200 light

The Drake Equation is not "meaningless", despite the fact
that we cannot plug in all the numbers exactly - though we
are getting better about how many exoplanets there are in
the galaxy, seeing as none were known when the equation was
created in 1961. We have to start somewhere to make some
kind of reasonable estimate of life in the Universe, and
this is certainly better than just making a wild guess.

>This brings up another point. In many of Larry's forwarded mail from some
>kind of SETI organization, I see the quote "We know we are not alone!" at the
>end of each mail. What do they mean by know? Where's the evidence?

That organization is called The SETI League and their
goal is to involve amateur astronomers in the search
for alien intelligences with their own radio and optical

Executive Director Dr. Paul Shuch uses the phrase above
not to imply that he has some kind of Area 51-type knowledge
of extraterrestrial life, but that he and those who participate
in The SETI League are convinced that other evolved beings are
out there in space and that they will find them in time by
diligent exploration of the skies.

Check them out on their Web site at this URL:

By the way, I am the League's Northeastern US Coordinator.

>I don't think we need to jump to conclusions in order to want to search
for ETI.
>Isn't being hopeful (and acknowledging it is feasible) enough?

Yes, it is always a good start, but we cannot just sit around
and hope they will show up. We have to search for them.

>It has a nice graphic representation of the famous Drake Equation.
>His inputs come up with 6 million possible ETI civilizations in the
>Milky Way galaxy. Sagan estimated there were 1 million using
>the Drake Equation in the early 1970s.
>With 400 billion stars in a galaxy 100,000 light years across, a few
>million can still be small potatoes, as it were, which would explain
>one reason as to why we have yet to find ETI. Plus just because
>they may be intelligent does not mean they would use technology
>or devices we could detect at interstellar distances, etc.
>However, the hope in SETI is that with so many chances for life to
>develop, at least a few of them may be able to commuicate with us
>across the stars. If we do not search, we will likely never find out.
>The Web site URL:

The Web site URL has since changed to this one:

Here are some other Web pages on The Drake Equation
for further enlightenment: