Are We Alone?
Equation was designed to calculate the number of technological
civilization in our galaxy (NASA/JPL)
was once assumed that Earth was just a typical planet in a run-of-the-mill
solar system. Some evidence is beginning to appear that suggests
this isn't true. If so, what does this mean about life on other
Sci-fi movies since the 1950's have shown invaders
from other planets coming to attack Earth. Captain Kirk of Star
Trek made a career out of seeking out new life forms and new
civilizations. "Contactees" swear they have been kidnapped
by aliens visiting our planet. In the public mind, the galaxy
seems to be swarming with life, especially intelligent life. How
much do we really know scientifically about the chance that life
exists on other planets, however? And if life exists apart from
Earth, is any of it intelligent?
The first man to try and put this question on a
scientific basis was Frank Drake. Drake was an astronomer who
wondered whether it would be possible to discover other intelligent
life by using radio waves. Drake knew since the invention of radio
in the early 20th century, mankind had been accidentally announcing
its existence to the rest of the galaxy by signals from its radio
and TV stations. He reasoned if others could detect Earth from
its radio waves, it must be possible for Earth to detect other
intelligent civilizations by listening for their electronic emissions.
equation multiplies the following factors together to
get an estimate of the number of civilizations currently
in the galaxy that might be capable of producing radio
of stars in galaxy.
of sun-like stars.
number of planets per star.
of planets suitable for life.
of planets where life actually develops.
of planets where intelligent civilizations arise.
of civilization with ability to communicate.
Drake and other scientists at the time were interested in searching
the skies for other intelligent life. Spending the money to do
this, however, would only make sense if there was a reasonable
chance that there was a civilization nearby that was technologically-advanced
enough to use radio waves, but how could you figure out how many
civilizations might be broadcasting at any particular time? In
1961, Drake came up with an equation he thought might give him
an answer. First, Drake decided to limit the area of his inquiry
to our own galaxy: The distances between galaxies is so great
that even at the speed of light it would take many thousands,
perhaps even millions, of years for radio signals to travel the
distance between, and when they arrived, they would be extremely
The equation Drake produced multiplied a number
of variables together (for example, the number of stars in the
galaxy times the fraction of stars that are "sun-like")
to get an answer. The results, though, were highly dependent on
the value of the variables entered. Some were well-known and accepted
values, like the number of stars in the galaxy, while others weren't
much more than wild guesses (like the number of years a technological
civilization might exist before going extinct).
Various scientists have used the equation over time
and have gotten a variety of answers. The most optimistic result
is that there are several billion civilizations in our galaxy.
The most pessimistic estimate is about 100. Many scientists feel
comfortable with a figure of million radio-using civilizations.
The more optimistic figures have been used to justify
SETI (Search for Extraterrestrial Intelligence) projects where
receivers search the sky for signs of artificial, extraterrestrial
radio sources. Even with as many as a million radio-capable civilizations
in our galaxy at this moment, however, it might be hard to detect
one from Earth. Because those million civilizations are spread
over a galaxy that contains 300 billion stars, it is unlikely
that any are very close to our position.
One characteristic of the Drake formula is that
it multiples all the values together to get the estimate. This
means if any of the variables approaches zero (a value can never
be zero itself because we know that at least one radio using civilization
exists, ours) it can drive the results, the number of radio-capable
civilizations in the galaxy, to a very low value. Perhaps even
a value of one.
Factor: the Moon
One of the key values in the equation is the fraction
of planets suitable for life where life actually develops. In
the past this was often set to a value of one. This means that
scientists expect that for every planet where the conditions are
suitable for the development of life, life is very likely to develop.
This may not be the case, however. Many scientists considered
Earth to be a typical "rocky" planet. Nothing significantly
special about it. After all, with all the millions of planets
in the galaxy, what are the chances that ours was somehow unique?
There is, however, something very unusual about
Earth that may profoundly affect the establishment of life. It
is the moon. No other planet in our solar system has a natural
satellite that is nearly as big in relationship to itself.
Most moons are thought to have either formed out
of leftover material from the creation of their mother planet
or were asteroids captured by the mother planet's gravity. Not
so our moon. The leading theory suggests that sometime early in
Earth's history, a large body, perhaps near the size of Mars,
was orbiting the sun in a highly-elliptical orbit. The path of
the body, or planetesimal, took it across earth's nearly circular
orbit. They avoided each other for many millions of years, but
at some point they collided.
Moon may make Earth unique (NASA/JPL)
A collision like this was not that unusual in the
early solar system. What was extremely unusual about the impact
was the angle at which the planetesimal struck. It nearly bounced
off. This strange angle was to be a key factor in what happened
The violence of that impact can hardly be imagined.
An explosive force equal to a billion, trillion, tons of TNT would
have been unleashed. Five billion cubic miles of the earth's outer
primordial crust were blown off the surface. Because of the angle
of the impact, however, the material neither fell back to earth
nor was blown into interplanetary space. Instead, it went into
orbit, forming an enormous ring around our planet not unlike those
we now see around Saturn. Over time the bits of rock in that ring,
under the influence of gravity, coalesced into the moon. The moon
was originally ten times closer to Earth than we see it. Over
time the moon moved outward to its current position.
Having such a large moon has profoundly affected
our planet,. most obviously through extremely strong tidal forces.
The gravity of the moon drags the waters on the surface of earth
towards it to give us our low and high tides. The tides, in turn,
affect our planet. Originally (when the earth formed) it probably
rotated once every six hours. Tides have slowed this down to one
rotation every 24 hours. If there were no moon, the earth's ocean
would still have tides caused by the gravity of the sun, but these
would be much weaker. It is estimated that a moonless earth with
weaker tides would rotate every eight hours.
Drake's equation assumes that life evolves under
the right conditions, but do these conditions require tremendous
tides that mix the seas into a chemical brew? Perhaps. Almost
all scientists who accept that if life on earth came into being
though evolution, agree the process would have taken much longer
without the strong tides caused by the moon. Some theorize it
might not have happened at all.
A study by Andew Watson, of the United Kingdom's
University of East Anglia in Norwich, seems to support this view.
Watson's study, based on a mathematical probability model shows
that on Earth intelligent life took a long time to evolve. So
long that we have only appeared near the end of what will be our
planet's bio-friendly window of time. In about a billion years
the sun will brighten so much that the Earth will be too hot for
"If we had evolved early," states Watson,
"then even with a sample of one, we'd suspect that evolution
from simple to complex and intelligent life was quite likely to
occur. By contrast, we now believe that we evolved late in the
"This has implications for our understanding
of the likelyhood of complex life and intelligence arising on
any given planet," he adds. "It suggests that our evolution
is rather unlikely -- in fact, the timing of events is consistent
with it being very rare indeed."
The moon also has a huge impact on another key factor
in Drake's formula: The fraction of planets that develop life
that go on to have technological civilizations. Earth's current
topography, which includes mountains and continents, is the result
of a process known as plate tectonics. Plates composed of crust
"float" on earth's semi-molton mantle. As they move,
collide and pull apart, they create our continents, mountain ranges
and oceans. Some scientists think that if most of the earth's
primordial crust hadn't been blown into orbit creating the moon,
the process of plate tectonics might not ever had gotten started.
No other planet in our solar system, not Venus nor Mars (which
are most like our own world) has plate tectonics. Without this
process, it is likely there would be no continents and any mountains
would have been ground down to below sea-level eons ago. Our planet
would be covered by a huge sea that might have a few volcanic
islands. Perhaps life would emerge on this alternate earth, but
without dry land, fire, and the developments it spurred would
not be possible. In this scenario it seems unlikely that a technological
civilization probably would have emerged.
At the time that Drake came up with his equation,
scientists knew nothing about planetary systems other than our
own. No extra-solar planets had ever been found. In the last ten
years, however, astronomers have used a number of clever techniques
to detect distant planets circling far away stars. At this point
in time, well over a hundred planetary systems have been found.
None of them seem to resemble ours, however. Most have large,
giant, gas planets in tight orbits around the central star. This
is unlike our system which has a number of "rocky" planets
close to the star and gas giants further out. Also, the giant
planets seem to have a much more elliptical orbit than the planets
in our system, which might generate extreme temperature changes
on those worlds.
So far, our astronomical instruments are not precise
enough to detect anything as small as a terrestrial-type planet
circling a distant star system, so perhaps they are out there
somewhere. However, the differences we do see may indicate that
most star systems formed differently than ours did. If so, this
might well indicate that the number of earth-like planets per
star system is much smaller than once thought. This will also
affect the results of Drake's equation.
in our Galaxy
So are we alone? Evidence seems to be mounting that
instead of being typical, the earth and our solar system are very
unusual. However, this does not preclude life developing somewhere
else under completely different circumstances. Systems containing
only gas giant planets may develop life on the moons of those
In our own solar system Europa, a moon of Jupiter,
is considered a possible haven for primitive life as well as Titan,
a moon of Saturn. Both Europa and Titan are probably too far from
the warmth of the sun to develop any kind of advanced life, but
what about moons circling gas giant planets close to their stars
in other solar systems? Would the mother planet of the stars provide
the necessary tidal forces to accelerate life?
Perhaps some kind of life can also exist in the
clouds of those gas giant planets. Picture a colony of huge jelly-fish-like
animals floating though the alien sky. Even stranger possiblities
exist. The late Robert Forward, a scientist and science-fiction
writer, suggested in his novel Dragon's Egg life might
even be able to exist on the surface of a neutron star, though
such life would be radically different than our own.
As we venture out beyond our solar system, we should
be prepared to find life, but it may be far different than the
little gray men with large eyes we so often picture in our minds.
Far different and much, much rarer.
What if the Moon Didn't Exist? by Neil
F. Comins, Harper Collins, 1993.
The Moon and Plate Tectonics: Why we are Alone
by Nick Hoffman, www.spacedaily.com/news/life-01x2.html
The Drake Equation: Estimating the Number of
Advanced Civilizations in the Milky Way, www.angelfire.com/indie/anna_jones1/drake.html
Our Solar System, Earth May Be Rarities,
Los Angeles Times, August 7, 2004.
Intelligence: A Rare Cosmic Commodity by
John D. Ruly AstroBiology Net http://www.astrobio.net/exclusive/2682/intelligence-a-rare-cosmic-commodity
Lee Krystek 2004. All Rights Reserved.