So, once again I was rooting about on my ext. hard-drive looking for something and came across this file on Frank Drake, his equation and the proposed life ... out there. I think we all need to check this out .. digest it .. and realize once again why we come here to discuss this material!
Decker
Source: Astrobiology Magazine
The Drake Equation Revisited: Part I
The Drake Equation Revisited: Part I
Extrasolar Life Summary: This is the first in a series of
presentations given at a public forum sponsored by the NASA
exobiology branch. The forum, held in Palo Alto, CA, on Tuesday,
August 26, 2003, was entitled, "The Drake Equation Revisited,"
and addressed the questions of estimating the probabilities for
finding intelligent life elsewhere in the universe.
The Drake equation:
Drake equation - Wikipedia, the free encyclopedia
was developed as a means of predicting the likelihood of
detecting other intelligent civilizations in our galaxy. At the
NASA forum, Frank Drake:
http://www.seti.org/about_us/leadership/staff/frank_d.html
who formulated the equation 42 years ago, moderated a debate
between paleontologist Peter Ward, co-author of the book Rare
Earth, and astronomer David Grinspoon:
David Grinspoon's World
author of the forthcoming book Lonely Planets: The Natural
Philosophy of Alien Life.
In this installment of the series, Dr. Drake explains the
history and the content of his famous equation. Dr. Drake is the
director for the Center for the Study of Life in the Universe at
the SETI Institute:
SETI Institute
in Mountain View, CA. He is also chairman emeritus of the SETI
Institute board of trustees and professor emeritus of astronomy
and astrophysics at the University of California at Santa Cruz.
Subsequent installments will include the comments made by Drs.
Ward and Grinspoon, and the question-and-answer period that
followed the opening remarks.
Frank Drake: It's a pleasure and honor to be with all of you
exobiologists tonight. When I started in this game there were no
exobiologists. So just seeing you all out there is a lot of
progress.
I want to start out by giving you a little bit of the history
and a brief description of the equation. This all began shortly
after I conducted the first search for extraterrestrial
intelligent radio signals at the National Radio Astronomy
Observatory in Green Bank. That was in 1960. At that very same
time, a very seminal paper was published by Philip Morrison and
Giuseppe Cocconi, pointing out what I had already realized, and
that was that we had the ability to detect reasonable signs of
intelligent technology across the distances which separated the
stars.
This in one way opened the door to detecting life, in this case
specifically intelligent life, beyond Earth. A great new window
of possibilities was opened, which was slowly recognized at the
time and, of course, is widely recognized today. It's expressed
by the great growth in the field of astrobiology.
Shortly after this, the National Academy of Sciences wanted to
convene a small meeting to examine this whole question and
propose where we should go from here. They asked me to do this
and, indeed, just about 42 years ago at this time I sponsored
the first such meeting at Green Bank. I was the entire
scientific and local organizing committee, but it wasn't a hard
task because I invited every person in the world we knew of who
was interested in working in this subject - all twelve of them.
And all twelve of them showed up.
As I planned the meeting, I realized a few day ahead of time we
needed an agenda. And so I wrote down all the things you needed
to know to predict how hard it's going to be to detect
extraterrestrial life. And looking at them it became pretty
evident that if you multiplied all these together, you got a
number, N, which is the number of detectable civilizations in
our galaxy. This, of course, was aimed at the radio search, and
not to search for primordial or primitive life forms.
Well, what is the equation? It encapsulates our understanding of
the evolution of our galaxy and of our solar system. We know our
galaxy is about 14 billion years old and that stars have been
created at almost a constant rate. And since very early on those
newly formed stars have been accompanied by planetary system -
at least in some cases.
So the whole equation is based on a continuous production of new
planetary systems, and then presumably life, intelligent life
and technology-using life. And that tells us, of course, that
the number of detectable civilizations is going to be
proportional to the rate of star formation, which we write as
R*, because the more stars you make, the more civilizations
there will be, eventually. That's an easy one.
We've known for a long time that about 20 new stars are produced
per year in our galaxy, and that has been the case for many
billions of years now. But over time, we've become a little bit
more sophisticated in defining what this factor means. Twenty
stars per year are produced, but we realize not all of them
could produce an intelligent species. Some burn out their
hydrogen cores extremely fast, in literally millions of years -
no time to evolve intelligent species.
If we take all of those away, the fast-burning stars, we're left
with 19 stars per year. Of those, about four are like the sun.
So is the right number for R* four per year? This is still one
of the big questions in astrobiology, and one of the challenging
ones.
What are the other 15? They're all very small red dwarf stars,
stars known as M dwarfs to astronomers. For a long time we
believed that they could not be abodes of intelligent life
because, indeed, they could have planetary systems (although
none has ever been detected yet), but even if there were planets
there, they would be so close to the star that, just as our moon
keeps one face to the Earth, they would keep one face to their
star. And we thought this would create a situation where, on the
dark side of these planets, it would be so cold that the
atmosphere would freeze out. There would be no atmosphere and
therefore no possibility of life arising.
Well, now that belief has been challenged and it has been shown
that with a properly massive atmosphere this freezing out of the
atmosphere will not occur. So perhaps the M stars and their
planets, after all, are abodes of life.
So what is R*? Well maybe it's 4, and maybe it's 19 stars per
year.
If we multiply by the fraction of those stars which have
planets, fp, we get the rate of production of new planetary
systems per year. So what is that? Well, for a long time we have
had nothing but theories to go on. We thought perhaps 50 percent
of the stars had planets. That was based on the fact that half
the stars are binary systems, and the other half must have
something else, something small, a planet.
Of course, one of the great discoveries of the last century,
which only ended about three years ago, was the detection of
other planetary systems. This is one of the greatest discoveries
in the history of science. We now know of over 100 such systems.
Most all of them have what you might call "giant Jupiters" in
them, not planets suitable for life on Earth. But we know this
is the tip of the iceberg, because this is the only kind of
planet we can detect. About 5 percent of the stars have such
planets. What do the other 95 percent have? Perhaps Earth-like
planets, or planets suitable for life. We also ask whether these
giant planets have habitable satellites.
In any case, the primary detector of these stars, Geoff Marcy at
UC Berkeley, estimates that about 50 percent of the stars have
planetary systems.
If we multiply that by the next factor, which is written ne, the
number of planets in the ecosphere, a term we don't use any more
- nowadays we call it the continuously habitable zone - we get
the rate of production of possible life-bearing planets. This is
a complex subject, very much more complex than was first
imagined. Early on it was thought that the planet had to be at
such a distance from the star that liquid water could exist. Not
too close, not too far. You had to be Goldilocks, to give rise
to life.
Now we realize that the nature of the planet can greatly affect
the distance at which it can be from a star and still be
habitable. A prime example is Europa, far out where it's very,
very cold on its surface, and yet there is a potential biosphere
on that object. A deep atmosphere, through the greenhouse
effect, can also make a planet far out from its star
nevertheless habitable. So, again, this is a factor we don't
know very well.
The next factor, fl, is the fraction of potentially habitable
planets that actually give rise to life. That one we seem to
know something about, because the chemists have found a
multitude of chemical pathways to the origins of life. Life
seems inevitable on any planet with suitable characteristics.
And what are those? They seem to be very simple: liquid water,
organic molecules and a source of energy.
The real question is not whether life arises, but how it really
happens. The present consensus is that life does arise in a body
of water, perhaps in Darwin's "warm little pond," or the deep-
sea vents, the froth of ocean waves - these have all been
suggested - or on the molecular templates of clay minerals. We
think that faction is close to one.
Our next fraction, fi, is the one which describes what fraction
of systems of living things give rise to an intelligent species.
This fraction is trying to give the answer to the question: Does
evolution converge or diverge? There is much evidence for
convergence in intelligence, including the growth in brain size
seen in the fossil record, but is an intelligent brain really
contingent on things we're not quite sure about? For instance,
does it require the evolution of a means of sophisticated
communication, one of the possible contingent situations which
may limit the frequency with which intelligence arises? That one
is a big unknown.
The next, fc, is the fraction of intelligent civilizations which
give rise to a technology which we might detect, or which might
communicate - that's what the "c" means. It seems that fc should
be close to one. Once you have enough intelligence in a creature
whose anatomy allows the use of tools, you should get
technology. Technology has, in fact, developed in many places on
Earth in humans independently.
The prime drivers are pretty obvious. The drivers were to
provide food, leading to the development of agriculture and the
tools of agriculture; to provide the ability to live in
otherwise uninhabitable regions, such as artic regions, polar
regions; and, of course, for the development of weapons.
At this point, you multiply this all together and you get the
rate of production of detectable civilizations in the galaxy.
Now, we don't believe, being conservative, that they remain
detectable forever. Perhaps they destroy themselves through
nuclear folly, or through destruction of their environment.
Perhaps they suffer a cosmic catastrophe, like the K/T event
[the asteroid impact that caused dinosaurs to go extinct].
More likely, at least to the optimists such as myself, they come
upon the scene, they are detectable, and then they disappear,
because they become more sophisticated technologically. They've
stopped releasing energy into space. At the present time we are
very detectable, primarily through our television broadcasts.
But we see television going to cable, and especially to the
direct-to-home transmission of television from satellites.
This is terrifying to people like me. The ordinary over-the-air
television transmitter transmits a million watts. It makes a
very detectable signal. The transmitters which transmit
television to those little dishes on people's homes only
transmit 10 watts, far less than a million, and make a signal
which is totally undetectable at interstellar distances.
So, we have to worry: Civilizations may be thriving, with a
tremendous quality of life, and yet be very hard to detect. And
we must account for that by saying, okay, they exist but they
only last some limited amount of time, which we will call L, the
longevity.
L is dominated by those civilizations with very large Ls,
because L is the average lifetime of a civilization. Just as a
numerical example, given 100 civilizations, if 99 are detectable
for only 100 years, and 1 is detectable for a billion years,
then L turns out to be 10 million years. And so L may be larger
than what our intuitive thoughts might be.
So that's the equation. But before we move on, I'll offer a few
comments that are evident but somehow not really seen. One is
that every factor in the equation appears to the first power.
There are no exponentials, no powers, no power laws, no
logarithms. Every one is equally important. And, in the same
vein, the overall error in the result is controlled by the
biggest uncertainties, which are probably fi and L. Thirdly, the
uncertainties grow as we go from the left to the right of the
equation, from the astronomical and chemical factors to the
social ones.
And, finally, we ask whether we need some other factors. I get
letters every week suggesting such. Particularly that we need a
factor for the ignorance of politicians. However, all the other
ones so far proposed are subsumed within the traditional ones.
But the future could well reveal the need for an enlarged
equation.
Decker
Source: Astrobiology Magazine
The Drake Equation Revisited: Part I
The Drake Equation Revisited: Part I
Extrasolar Life Summary: This is the first in a series of
presentations given at a public forum sponsored by the NASA
exobiology branch. The forum, held in Palo Alto, CA, on Tuesday,
August 26, 2003, was entitled, "The Drake Equation Revisited,"
and addressed the questions of estimating the probabilities for
finding intelligent life elsewhere in the universe.
The Drake equation:
Drake equation - Wikipedia, the free encyclopedia
was developed as a means of predicting the likelihood of
detecting other intelligent civilizations in our galaxy. At the
NASA forum, Frank Drake:
http://www.seti.org/about_us/leadership/staff/frank_d.html
who formulated the equation 42 years ago, moderated a debate
between paleontologist Peter Ward, co-author of the book Rare
Earth, and astronomer David Grinspoon:
David Grinspoon's World
author of the forthcoming book Lonely Planets: The Natural
Philosophy of Alien Life.
In this installment of the series, Dr. Drake explains the
history and the content of his famous equation. Dr. Drake is the
director for the Center for the Study of Life in the Universe at
the SETI Institute:
SETI Institute
in Mountain View, CA. He is also chairman emeritus of the SETI
Institute board of trustees and professor emeritus of astronomy
and astrophysics at the University of California at Santa Cruz.
Subsequent installments will include the comments made by Drs.
Ward and Grinspoon, and the question-and-answer period that
followed the opening remarks.
Frank Drake: It's a pleasure and honor to be with all of you
exobiologists tonight. When I started in this game there were no
exobiologists. So just seeing you all out there is a lot of
progress.
I want to start out by giving you a little bit of the history
and a brief description of the equation. This all began shortly
after I conducted the first search for extraterrestrial
intelligent radio signals at the National Radio Astronomy
Observatory in Green Bank. That was in 1960. At that very same
time, a very seminal paper was published by Philip Morrison and
Giuseppe Cocconi, pointing out what I had already realized, and
that was that we had the ability to detect reasonable signs of
intelligent technology across the distances which separated the
stars.
This in one way opened the door to detecting life, in this case
specifically intelligent life, beyond Earth. A great new window
of possibilities was opened, which was slowly recognized at the
time and, of course, is widely recognized today. It's expressed
by the great growth in the field of astrobiology.
Shortly after this, the National Academy of Sciences wanted to
convene a small meeting to examine this whole question and
propose where we should go from here. They asked me to do this
and, indeed, just about 42 years ago at this time I sponsored
the first such meeting at Green Bank. I was the entire
scientific and local organizing committee, but it wasn't a hard
task because I invited every person in the world we knew of who
was interested in working in this subject - all twelve of them.
And all twelve of them showed up.
As I planned the meeting, I realized a few day ahead of time we
needed an agenda. And so I wrote down all the things you needed
to know to predict how hard it's going to be to detect
extraterrestrial life. And looking at them it became pretty
evident that if you multiplied all these together, you got a
number, N, which is the number of detectable civilizations in
our galaxy. This, of course, was aimed at the radio search, and
not to search for primordial or primitive life forms.
Well, what is the equation? It encapsulates our understanding of
the evolution of our galaxy and of our solar system. We know our
galaxy is about 14 billion years old and that stars have been
created at almost a constant rate. And since very early on those
newly formed stars have been accompanied by planetary system -
at least in some cases.
So the whole equation is based on a continuous production of new
planetary systems, and then presumably life, intelligent life
and technology-using life. And that tells us, of course, that
the number of detectable civilizations is going to be
proportional to the rate of star formation, which we write as
R*, because the more stars you make, the more civilizations
there will be, eventually. That's an easy one.
We've known for a long time that about 20 new stars are produced
per year in our galaxy, and that has been the case for many
billions of years now. But over time, we've become a little bit
more sophisticated in defining what this factor means. Twenty
stars per year are produced, but we realize not all of them
could produce an intelligent species. Some burn out their
hydrogen cores extremely fast, in literally millions of years -
no time to evolve intelligent species.
If we take all of those away, the fast-burning stars, we're left
with 19 stars per year. Of those, about four are like the sun.
So is the right number for R* four per year? This is still one
of the big questions in astrobiology, and one of the challenging
ones.
What are the other 15? They're all very small red dwarf stars,
stars known as M dwarfs to astronomers. For a long time we
believed that they could not be abodes of intelligent life
because, indeed, they could have planetary systems (although
none has ever been detected yet), but even if there were planets
there, they would be so close to the star that, just as our moon
keeps one face to the Earth, they would keep one face to their
star. And we thought this would create a situation where, on the
dark side of these planets, it would be so cold that the
atmosphere would freeze out. There would be no atmosphere and
therefore no possibility of life arising.
Well, now that belief has been challenged and it has been shown
that with a properly massive atmosphere this freezing out of the
atmosphere will not occur. So perhaps the M stars and their
planets, after all, are abodes of life.
So what is R*? Well maybe it's 4, and maybe it's 19 stars per
year.
If we multiply by the fraction of those stars which have
planets, fp, we get the rate of production of new planetary
systems per year. So what is that? Well, for a long time we have
had nothing but theories to go on. We thought perhaps 50 percent
of the stars had planets. That was based on the fact that half
the stars are binary systems, and the other half must have
something else, something small, a planet.
Of course, one of the great discoveries of the last century,
which only ended about three years ago, was the detection of
other planetary systems. This is one of the greatest discoveries
in the history of science. We now know of over 100 such systems.
Most all of them have what you might call "giant Jupiters" in
them, not planets suitable for life on Earth. But we know this
is the tip of the iceberg, because this is the only kind of
planet we can detect. About 5 percent of the stars have such
planets. What do the other 95 percent have? Perhaps Earth-like
planets, or planets suitable for life. We also ask whether these
giant planets have habitable satellites.
In any case, the primary detector of these stars, Geoff Marcy at
UC Berkeley, estimates that about 50 percent of the stars have
planetary systems.
If we multiply that by the next factor, which is written ne, the
number of planets in the ecosphere, a term we don't use any more
- nowadays we call it the continuously habitable zone - we get
the rate of production of possible life-bearing planets. This is
a complex subject, very much more complex than was first
imagined. Early on it was thought that the planet had to be at
such a distance from the star that liquid water could exist. Not
too close, not too far. You had to be Goldilocks, to give rise
to life.
Now we realize that the nature of the planet can greatly affect
the distance at which it can be from a star and still be
habitable. A prime example is Europa, far out where it's very,
very cold on its surface, and yet there is a potential biosphere
on that object. A deep atmosphere, through the greenhouse
effect, can also make a planet far out from its star
nevertheless habitable. So, again, this is a factor we don't
know very well.
The next factor, fl, is the fraction of potentially habitable
planets that actually give rise to life. That one we seem to
know something about, because the chemists have found a
multitude of chemical pathways to the origins of life. Life
seems inevitable on any planet with suitable characteristics.
And what are those? They seem to be very simple: liquid water,
organic molecules and a source of energy.
The real question is not whether life arises, but how it really
happens. The present consensus is that life does arise in a body
of water, perhaps in Darwin's "warm little pond," or the deep-
sea vents, the froth of ocean waves - these have all been
suggested - or on the molecular templates of clay minerals. We
think that faction is close to one.
Our next fraction, fi, is the one which describes what fraction
of systems of living things give rise to an intelligent species.
This fraction is trying to give the answer to the question: Does
evolution converge or diverge? There is much evidence for
convergence in intelligence, including the growth in brain size
seen in the fossil record, but is an intelligent brain really
contingent on things we're not quite sure about? For instance,
does it require the evolution of a means of sophisticated
communication, one of the possible contingent situations which
may limit the frequency with which intelligence arises? That one
is a big unknown.
The next, fc, is the fraction of intelligent civilizations which
give rise to a technology which we might detect, or which might
communicate - that's what the "c" means. It seems that fc should
be close to one. Once you have enough intelligence in a creature
whose anatomy allows the use of tools, you should get
technology. Technology has, in fact, developed in many places on
Earth in humans independently.
The prime drivers are pretty obvious. The drivers were to
provide food, leading to the development of agriculture and the
tools of agriculture; to provide the ability to live in
otherwise uninhabitable regions, such as artic regions, polar
regions; and, of course, for the development of weapons.
At this point, you multiply this all together and you get the
rate of production of detectable civilizations in the galaxy.
Now, we don't believe, being conservative, that they remain
detectable forever. Perhaps they destroy themselves through
nuclear folly, or through destruction of their environment.
Perhaps they suffer a cosmic catastrophe, like the K/T event
[the asteroid impact that caused dinosaurs to go extinct].
More likely, at least to the optimists such as myself, they come
upon the scene, they are detectable, and then they disappear,
because they become more sophisticated technologically. They've
stopped releasing energy into space. At the present time we are
very detectable, primarily through our television broadcasts.
But we see television going to cable, and especially to the
direct-to-home transmission of television from satellites.
This is terrifying to people like me. The ordinary over-the-air
television transmitter transmits a million watts. It makes a
very detectable signal. The transmitters which transmit
television to those little dishes on people's homes only
transmit 10 watts, far less than a million, and make a signal
which is totally undetectable at interstellar distances.
So, we have to worry: Civilizations may be thriving, with a
tremendous quality of life, and yet be very hard to detect. And
we must account for that by saying, okay, they exist but they
only last some limited amount of time, which we will call L, the
longevity.
L is dominated by those civilizations with very large Ls,
because L is the average lifetime of a civilization. Just as a
numerical example, given 100 civilizations, if 99 are detectable
for only 100 years, and 1 is detectable for a billion years,
then L turns out to be 10 million years. And so L may be larger
than what our intuitive thoughts might be.
So that's the equation. But before we move on, I'll offer a few
comments that are evident but somehow not really seen. One is
that every factor in the equation appears to the first power.
There are no exponentials, no powers, no power laws, no
logarithms. Every one is equally important. And, in the same
vein, the overall error in the result is controlled by the
biggest uncertainties, which are probably fi and L. Thirdly, the
uncertainties grow as we go from the left to the right of the
equation, from the astronomical and chemical factors to the
social ones.
And, finally, we ask whether we need some other factors. I get
letters every week suggesting such. Particularly that we need a
factor for the ignorance of politicians. However, all the other
ones so far proposed are subsumed within the traditional ones.
But the future could well reveal the need for an enlarged
equation.
