In 1961, Dr. Frank Drake, an American astronomer, and a founder of SETI (search for extraterrestrial intelligence), formulated an equation known as the Drake equation, to calculate the number of intelligent civilizations in our Milky Way galaxy. By multiplying together a series of terms relating to the probability of extraterrestrial life (the rate of star formation in the universe, the fraction of stars with planets, the fraction of planets with conditions suitable for life, etc.), he calculated that the existence of intelligent life on other planets is extremely likely. However, the Drake equation had several serious drawbacks. First, the equation had at least four utterly unknown terms in it, namely 1) the fraction of planets with life, 2) the odds life becomes intelligent, 3) the odds intelligent life becomes detectable, and 4) the detectable lifetime of civilizations. It suffered from a highly questionable premise, namely that advanced alien civilizations arise and die out in their own solar system. Therefore, scientists like Dr. Carl Sagan could optimistically predict over one million advanced alien civilizations in 1966, while other less-optimistic scientists predicted we were alone. All used the same equation, but with different assumptions for the unknowns. As you can imagine, instead of resolving the paradox, it fueled it. In fairness though, the Drake Equation was not proposed as a hypothesis. It was not intended to be proved or disproved. Its main purpose was to fire our imaginations to the possibility that extraterrestrial life may exist in our galaxy.
If we are not alone in the universe, it would be reasonable to assume some extraterrestrial civilizations would more advanced that ours. If intelligent life exists, imagine if they evolved one million years earlier than we did. From a cosmological perspective, one million years is a blink of an eye. Imagine what our capabilities will be a thousand years in the future, assuming humankind exists one thousand years in the future. It is entirely reasonable to assume intelligent life may have gotten an earlier start in the universe, and be scientifically more advanced. This brings us to the Fermi paradox, which poses a deceptively simple question: if the probability of advanced aliens is so high, why haven’t we detected them or been contacted by them? The paradox has to do with the high probability of existence, in this case advanced aliens, and the lack of evidence. Ancient alien theories and Roswell conspiracy theorists notwithstanding, there is no widely accepted scientific proof that aliens have visited the Earth or tried to contact us.
In 1950, employee Enrico Fermi was walking to lunch with his colleagues at Los Alamos National Laboratory. The topic of UFOs came up because of numerous sightings and reports sensationalized by the media. Although the conversation started on a light note, it soon became serious. Fermi and his colleagues began to discuss the possibility of faster-than-light travel, which from Einstein’s special theory of relativity, is impossible. However, if advanced aliens were going to visit the Earth, they would likely need to travel faster than light given the vast distances between interstellar destinations. Although Fermi’s colleagues considered faster-than-light travel a long shot, Fermi believed that science would discover a way to make objects travel faster than light within a decade. He was wrong about that, but his main point was a question. In the middle of lunch, he jumped up and asked, “Where is everybody?” His point, if the universe contains advanced extraterrestrial life, where is the evidence? Fermi began to calculate the potential existence of advanced aliens. His rough calculations indicated that the Earth would have been visited numerous times, from ancient times to the present. This became known as the Fermi Paradox, namely the probability that advanced aliens exist does not square with the lack of evidence.
However, recent discoveries of distant planets that could theoretically harbor life, though, have raised hopes that we might detect extraterrestrials, as our technology to detect them improves and if we just keep looking. Current, scientists estimate there are about 20 billion Earth-like planets in just our galaxy, the Milky Way. When we use the term “Earth-like,” we mean the planet resembles the Earth in three crucial ways:
1) It has to be in an orbit around a star that enables the planet to retain liquid water on one or more portions of its surface. Cosmologists call this type of orbit the “habitable zone.” Liquid water, as opposed to ice or vapor, is crucial to all life on Earth. There might be other forms of life significantly different from what we experience on Earth. However, for our definition of an Earth-like planet, we are confining ourselves to the type of life that we experience on Earth.
2) Its surface temperature must not be too hot or too cold. If it is too hot, the water boils off. If it is too cold, the water turns to ice.
3) Lastly, the planet must be large enough for its gravity to hold an atmosphere. Otherwise, the water will eventually evaporate into space.
In December 2011, NASA’s Kepler (i.e., the Kepler spacecraft) astronomers announced the discovery of the first Earth-like planet, now called “Kepler 22b.” It is about 2.4 times wider than the Earth, and circles a star that is similar to our sun. They estimate Kepler 22b’s average surface temperature to be about 72ºF (degrees Fahrenheit). It is 600 light years from Earth, which cosmologically speaking makes it a near neighbor. The most crucial aspect that makes the planet Earth-like is that it is in the habitable zone.
Today, NASA has confirmed 1,004 planets found, including two that are most Earth-like. The issue now is to determine how to investigate if any of the planets, especially the Earth-like planets, contain life.
The original eq’n has been updated. I remember Carl Sagan mentioned the equation and so did Arthur Clarke and Isaac Asimov.