Are We Alone?

It 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 planets?

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.

The Drake Equation

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.

Key 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.

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 next.

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 life.

"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 habitable period."

"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.

Typical Planetary System

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.

Life 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 planets.

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.

A Partial Bibliography

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

Copyright Lee Krystek 2004. All Rights Reserved.