The Background of Little Green Men
Mankind has had a long history of speculations about intelligent life on
other planets. H. G. Wells' wonderful War Of The Worlds novel gave us a
chilling vision of emotionless but intelligent creatures from Mars, intent on
taking over our planet. I still remember listening to a local Denver radio
station many years ago that would play the Orson Welles rendition of the H. G.
Wells classic every Halloween.
But speculations of creatures on other planets far preceded H. G. Wells.
Such ideas were (carefully) speculated about even as early as the 16th
It was probably more of a hope than a belief, but speculations about life
even on our close neighbor Mars existed well into the 20th century. Remember
the canals of Mars ideas? But the Mars Mariner probes seemed to deal an abrupt
emotional blow when they showed a Martian surface that looked more like the
surface of the moon than the surface of our lush planet Earth. That blow killed
off most conjecture about intelligent life living on Mars, but it didn't stop
the dreaming about possible intelligent life forms on planets of other
Speculations Get Serious
By the 1960s, it was apparent to astronomers that our Galaxy was much
bigger than thought only 50 years previous. In fact, it was then known that
countless other galaxies existed beyond the boundaries of our beloved
Milky Way. Some sense of not only the number of stars in our galaxy was
known, as well as estimates of the number of sun-like stars in our
In the early 1960s,
Frank Drake, an
astronomer at the Arecibo radio observatory in Puerto Rico, was interested in searching
for extraterrestrial intelligence. At a meeting with colleagues in 1961, he
presented what has become known as the Drake Equation, which is used to calculate the probable
number of concurrent intelligent species within the Milky Way.
The equation starts with an estimate of the rate of star creation in
our galaxy, then applies a series of multiplicative factors to that to
estimate the probable number of concurrent civilizations in our galaxy.
Concurrent is the operative word here, and you can immediately see that this
might be a small number, given how long it took for Earth to produce
intelligent beings, and how little time, cosmically speaking, we've existed.
The Drake equation has a number of presentations, and herein you'll find one
of the popular formulations. The problem with the Drake equation is that its
solution depends upon a lengthy list of parameters, many of which are poorly
known. Certainly the values of some of them are better known now than when the
equation was first presented, but there's still a lot of wiggle room in the
Pessimistic to moderate parameter estimates entered into the equation
suggest that we may indeed be the only currently existing intelligent life in
our Milky Way, though others may certainly have preceded us or will follow us.
More generous parameter estimates put the possible number of concurrent galaxy
civilizations at as many as 1,000 to 10,000. What's the actual number? Nobody
So while expectations of finding extraterrestrial life in our own
solar system died out, enthusiasm about detecting a radio signal from an
advanced society -- somewhere out among the billions of stars -- continued
to build. In 1967, it seemed for a time that our expectation was finally
The discovery that temporarily caused much excitement came from research
conducted by Jocelyn Bell and
others involved in radio astronomy studies. During their scans of external
radio noise, they made a startling discovery. Using a radio telescope of their
own design, these scientists ran across an incredibly precise beacon of pulsing
radio energy. Such precision was unexpected, and one of the first ideas that
occurred to the researchers was that they'd stumbled upon a signal from another
The researchers referred to the signal as an LGM, for Little Green
Men. But a bit more research showed that while they had indeed discovered
something incredible, it was not a signal from Little Green Men. It was,
in actuality, the first discovered signal from a rapidly rotating
Neutron Star. Good
enough for an eventual Nobel Prize, but not the discovery of another
The Optimism and the Pessimism
Even though the Neutron star discovery fell short of the quest to locate
a radio signal from a remote civilization, hope still exists. On the
optimistic side, there are perhaps 2 to 4 billion stars in our Milky Way
galaxy alone, and countless more in the other billions of galaxies of the
universe. We're not likely to get a message from an external galaxy, but with
billions of stars in our own backyard, it seems that we can hardly be alone.
And while it's true that most of the stars you see with the naked eye are
quite unlike our own sun, most stars in the galaxy are much like our sun. We
just see the biggest and brightest stars, themselves not good hunting ground
for earth-like planets. But with perhaps over 90% of stars being more like
our sun even if we can't see them with the naked eye, that seems to easily
stack the odds in the favor of alien civilizations.
Of course, these stars must have planets or all bets are off. Fortunately, a
recent NASA study has concluded that small planets are common in the galaxy. So
most sun-like stars also likely have planets. And some of those planets are
likely in the life zone, the zone that allows liquid water to
We also know by studies of our own planet that primitive life got started
here very early in the earth's evolution. Perhaps within the first half-billion
years in our 4.5 billion year history. It would almost seem that if you have
liquid water, you have an excellent chance for the formation of life.
But on the pessimistic side, intelligent life on this planet arose only
very recently, with homo erectus evolving only in the last 100,000 years or so.
And our radio communications capability has only existed for 100 years or so.
On the scale of the age of our galaxy, our intelligent capabilities
have existed but for an infinitesimal period of time.
And since it took nearly 4.5 billion years for intelligent life to exist
here, does that indicate that it's a fluke? Or did conditions here in some way,
with our periods of mass extinction, interfere with the development of
intelligent life? Also, how long will our civilization last? We have the
capability of ending it in our hands, and certainly many people fear that we
can't resist using that capability. Perhaps our technical civilization will
eliminate itself after existing only a few hundred years.
If, then, technical civilizations tend to self destruct after only hundreds
of years, what's the likelihood of several technical civilizations on different
stars existing at the same time? Maybe pretty small.
To see how the various parameters come together in the Drake equation, you
own estimates. Play around with it, and see what it takes to get a calculation
greater than zero.
Try Your Own Drake Equation Parameters
The formulation of the Drake Equation used:
N = N* . fp . ne . fl . fi . fc . L/Tg|
|N* ||Estimated number of sun-like stars in galaxy
|fp||Percent of those stars with planets
|ne||Average number of life-potential planets per star
|fl||Percent of those that will harbor life
|fi||Percent of those that will develop intelligent life
|fc||Percent of those that will develop radio communications
|L||Number of years a civilization will exist
|Tg||Number of years a planet will exist
In the following implimentation, Tg is assumed to be
10 billion years, the approximate time a planet exists.