The Drake
Equation
The search
for extraterrestrial intelligence has often been regarded as
nonscientific. After all, there are
thousands of hypotheses about intelligent life in the universe, but no testable
theories to back them up. Nor is there a
single shred of empirical evidence on which to base any of our
speculations. It wasn’t until the 1960s
that the search gained official recognition by the scientific community.
Prior to
this time, any supposition about the prospects of establishing contact with
other worlds was far from achieving scientific respectability. However, in November of 1960, a group of
wellknown scientists held the first conference to discuss just such prospects. Convened under the auspices of the National
Academy of Sciences, the subject of the meeting was so risqué that there were no
announcements made about the conference, nor any official publications
following the meeting. In fact, the
meeting consisted of only ten people.
The attendees called themselves “The Order of the Dolphins”, partially
in jest and partially in celebration of a recent publication by one of the
conference invites, John Lilly, who had just published a controversial work
declaring dolphins as an intelligent species.
One of the
members of this first conference about the search for extraterrestrial
intelligent life was Frank Drake. Drake
was a very young researcher who, just two years prior, had conducted the first,
albeit modest, radio search for intelligent signals from planets surrounding
nearby stars. Even before the
conference, Drake had been thinking about the complexities of predicting
whether or not intelligent civilizations exist beyond our own, and how to
communicate with these civilizations.
Prior to giving a talk at the conference, Drake tried to organize his
thoughts about the conference and to focus them on the topic of intelligent
life in the universe. In his efforts, he
created an organizational tool. He had
no idea that his tool, now famously known as the Drake equation, would become a
cornerstone for SETI theorists for years to come.
Drake
postulated that the number of intelligent, communicating civilizations in our
own Milky Way Galaxy could be reduced to seven simple variables. He expressed them in the following equation
N = R x f_{p} x n_{e} x f_{l} x f_{i} x f_{c} x L
For over forty
years, the Drake equation has been used as the framework for the discussion of
intelligent life in the universe. In this equation, N stands for the number of
planets in the Milky Way Galaxy hosting a civilization that is intelligent
enough to communicate through the distances of space. To get an answer for N,
you simply need to multiply seven different factors together. The letter R is the average rate of star
formation. The factor f_{p}
is the fraction of stars in the galaxy that have planets around them. The factor
n_{e} is
the number of planets around those stars that have conditions favorable for
life to exist. The factor f_{l} is a fraction; it is the fraction of the planets with
favorable conditions for life that actually do develop
life. The factor f_{i}
is also a fraction. It is the fraction
of planets that developed life where that life evolves to be intelligent. The factor
f_{c} is
the fraction of those intelligent species that develop the ability for interstellar
communication. Finally, L is the average lifetime of such civilizations, how
long an intelligent civilization lasts on average.
As you can
see, these factors range from those that are based on our knowledge of
astronomy to those relying on our knowledge of how societies work. Each of these seven factors has a range of
possible values that we can estimate.
However, we should be careful to note that any estimations
we make are based on our knowledge of all the intelligent civilizations we know
about in the galaxy, just one, our own.
It is also important to remember that our estimates of N only predict
the number of intelligent, communicable civilizations within our own galaxy. The distance between our galaxy and others is
so large that even light would have a tough time traveling in an expedient
amount of time.
So, how
would one even go about filling in the Drake equation with actual numbers? Well, it turns out some of the factors we
understand fairly well. For example,
there are about 40 billion stars in our galaxy, and the age of the galaxy is
about 10 billion years. If you divide 40 billion by 10 billion, you arrive at a
number for R of about 4 stars per year.
We could estimate that R would reach values of 10 or greater if we
account for stars that formed in the past but have since exploded or faded from
view. Regardless, the first factor in the Drake equation is the only one that
we know with much accuracy at all.
The next
factor, the fraction of stars with planets, is still uncertain. It is, however, the subject of one of the
most intensive research efforts in the history of astronomy. So far, we have only
detected Neptunesized planets or larger around other stars, but we are
confident of finding Earthsized planets in the near future. Astronomers believe that when stars form, planets
naturally form right along with them.
This would imply that f_{p} = 1, but a more conservative range for this factor might
be 0.1 to 1.
The number
of habitable planets in each system is also highly uncertain. Again, we lean on our assumption of mediocrity
and hope that our solar system is ordinary and typical of other planetary
systems. Our solar system has a habitable zone — a distance from the star where
surface water can exist — that includes Venus, Earth, and Mars. The fact that
two out of three of those planets no longer have surface water tells us that
atmospheric conditions play a role in planet habitability as well. The factor n_{e} must consider the
fact that many lowluminosity stars have tiny habitable zones. In addition,
what if we consider habitable “planets” like Europa or Titan that are outside
the habitable zone? Then for our solar
system, this number could be as high as 5 or as low as 1.
We really
only have one example of how life evolves, life on Earth. Many scientists believe that life started
quickly as soon as there was a suitable site with favorable conditions. If this were the case, then the value for f_{l} could be close to
unity. On the other hand, if life arises
as a random and improbable outcome of chemical evolution, f_{l }could be a really
small number. Right now, we don’t have solid evidence to allow us to make a
decision between these two possibilities. However, if life, past or present, is
discovered on Mars, Europa, or Titan, it would be evidence in support of the
idea that f_{l}
= 1.
The last
three factors of the Drake equation are hopelessly uncertain. We do not know if
intelligence is a natural or necessary consequence of biological evolution. We
have no idea how likely it is that life will develop technology and the ability
to communicate into space. In the absence of any evidence, logical arguments
can’t be made for high and low values of f_{i} and f_{c}. We are equally in the dark as to how long
such a capability will endure. On Earth, L only equals 50 years so far.
Scientists
to this day argue about pessimistic and optimistic estimates for all of the
factors in the Drake Equation. You could
come up with your own, if you wanted to.
In the table below, we have made some "optimistic" and
"pessimistic" estimates for you. The table also calculates N, the
number of potential pen pals in the Galaxy at any particular time, that would
result from the estimates. We combine
all the low and high estimates of each factor purely to illustrate the range of
the possible endresult values of N. In
reality, the various factors may be independent, in which case a low value for
one factor does not necessarily imply a low value for the others. We stress
that these are little more than educated guesses. The estimate of N is as
uncertain as the most uncertain factor. Since several of the factors are
completely unknown, we must conclude that N cannot be determined.
R x f_{p} x n_{e} x f_{l} x f_{i} x f_{c} x L
Factor 
“Optimistic” 
“Pessimistic” 
R 
10 
4 
f_{p} 
1 
0.1 
n_{e} 
6 
1 
f_{l} 
1 
.001 
f_{i} 
1 
.0001 
f_{c} 
1 
.0001 
L 
50,000 
50 
Multiply
them together and…. 


N = 
3,000,000 
.0000000002 