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The most tantalizing clue found so far came from a meteorite discovered in Antarctica. Named ALH 84001, this hunk of space debris is believed to have been blasted off the surface of Mars about 16 million years ago. The rock survived the perils of space and a fiery trip through the Earth's atmosphere to land in Antarctica some 13,000 years ago.
The meteorite was discovered in 1984, but it was not until 1996 that scientists announced evidence of life in the ancient rock. Though the findings are still controversial, the meteorite contains fossilized remains that could be a primitive form of bacteria. If so, ALH 84001 is the first hard evidence that life of any kind evolved on a planet other than Earth.
Besides Earth and Mars, no other planet in our solar system is believed to have ever had an environment capable of supporting life. Mercury is too hot and bombarded by radiation from the Sun, Pluto is much too cold, and the gas giants have no solid surface. Some researchers have proposed evidence of life high in the atmosphere of Venus, but most believe this world's runaway greenhouse effect makes the planet too hostile to support life.
Nevertheless, at least two moons in the solar system are possible candidates where life may have developed. The first is Jupiter's moon Europa. Although its surface temperature is too cold for life to exist, the moon is covered by a thick crust of ice that may shelter an ocean of liquid water underneath. Europa also shows signs of having a molten core that could generate enough heat to support life within this ocean.
There is precedence for this type of evolution on Earth where many exotic forms of life have been found living near thermal vents deep on the ocean floor. NASA is hoping to launch future missions to Europa to better explore the world. One proposal even includes sending a robotic submarine into the moon's oceans to search for life.
Another intriguing possibility for life is the largest moon of Saturn named Titan. While Titan is also a very cold world, it has a gaseous atmosphere that is believed to be very similar to that of Earth billions of years ago when life began. This atmosphere is heavy in nitrogen, methane, and ammonia, and the surface is covered by regions of water ice. Experiments have shown that such a mixture, when combined with electrical energy provided by lightning, results in the creation of amino acids that are the building blocks of life.
Measurements taken of Titan's atmosphere have already shown evidence of the existence of tholins, an organic substance that produces amino acids when dissolved in water. The Cassini probe is currently studying Saturn, and the craft also carries a lander named Huygens designed to perform a detailed study of Titan's atmosphere and surface. If successful, this probe could tell us much about the prospects for life on this distant moon.
That being said, your question specifically asks about whether people have lived on other worlds, implying that you are asking whether intelligent life has evolved elsewhere in the universe. Clearly, there is no evidence for any advanced civilizations having developed on Mars or any other location besides Earth. As for the rest of the universe, we really don't know since our current technology level makes it very difficult to search for life far from our own solar system. We have only recently been able to detect the presence of planets around distant stars, but we are not yet able to visually inspect these planets or take detailed measurements of them.
The best method we have yet thought of to search for intelligent life has been pursued by the Search for Extraterrestrial Intelligence (SETI) program. SETI uses radio telescopes to "listen" for signals that might be generated by alien civilizations. Our own civilization has generated similar signals from AM and FM radio and television broadcasts for nearly 100 years. These signals continue to propogate across the galaxy at the speed of light and may be detected by a distant species someday. Nevertheless, the galaxy is enormous and we do not know what kind of signals to listen for or what part of the sky they might come from, so this search is quite challenging.
The prospect for discovering another intelligent civilization within our own Milky Way galaxy has been quantified mathematically in a statistical form called the Drake equation. The variables in this equation will likely never be known for sure and are very much subject to opinion, so the approach can only provide a very crude estimation. The greatest usefulness of this method is that it breaks the larger question of life in the universe into smaller pieces that are easier to analyze. The Drake equation takes the following form.
where
Using the range of estimates given above yields answers anywhere from near 0 to 500,000 advanced civilizations currently in our galaxy. Dr. Frank Drake, who first devised the equation that bears his name, estimates the value of N to be around 10,000 based on his opinions of the most plausible numbers for each variable. Yet given the enormous size of the galaxy and how spread apart these civilizations might be, it becomes apparent how difficult it is to detect intelligent life.
Even so, the Drake equation only concerns our Milky Way galaxy, which is just one of billions of galaxies in the
universe. It seems logical that intelligent life must have evolved elsewhere in the vastness of the universe, but
the question is whether such life is close enough that we could ever detect and communicate with an alien
civilization.
- answer by Jeff Scott, 12 September 2004
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