> Hi Eris,
> I don't think we have enough information to scale
> "likelihood." There is no probability to
> "Possible" ranges from "almost impossible" to
> "almost certain."
Perhaps you dont think we have enough information, yet it is quite clear that many other actual scientists in this field would disagree with that view. I would also disagree with that view.
The probability of a civilization developing on a potentially habitable alien planet would have to be less than one in 10 billion trillion — or one part in 10 to the 22nd power — for humanity to be the first technologically advanced species the cosmos has ever known, according to the study.
"To me, this implies that other intelligent, technology-producing species very likely have evolved before us," said lead author Adam Frank, a professor of physics and astronomy at the University of Rochester in New York.
"Think of it this way: Before our result, you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet was, say, one in a trillion," Frank said in a statement. "But even that guess — one chance in a trillion — implies that what has happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history."
The odds of an alien civilization coexisting with humans is often calculated by the Drake equation. It was first proposed by Frank Drake in 1961. Simply take the rate at which stars form in our galaxy and multiply it by the fraction of stars with planets, the average number of planets per star that could support life, the fraction of those that actually develop life, the fraction of life bearing planets that develop civilization, the fraction of civilizations that have detectible signals, and finally the length of time a civilization might last. Crunch the numbers and you have the number of civilizations in our galaxy capable of communicating with us.
When Drake first proposed the equation, the values for each term were largely unknown, but we now have good estimates for many of them. We know that most stars have planets, and the odds of a potentially habitable planet is actually quite high, possibly as high as 100 billion in our galaxy alone.
Astronomers estimate that there are 100 billion galaxies in the universe. If you want to extrapolate those numbers, that means there are around 50,000,000,000,000,000,000 (50 quintillion) potentially habitable planets in the universe.
As Arthur C. Clarke, physicist and author of 2001: A Space Odyssey wrote, "The idea that we are the only intelligent creatures in a cosmos of a hundred billion galaxies is so preposterous that there are very few astronomers today who would take it seriously. It is safest to assume therefore, that they are out there and to consider the manner in which this may impinge upon human society."
Frank Drake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy -- and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:
N = RfpneflfifcL
In this equation, N is the number of detectable civilizations in our galaxy. The other variables are described below:
R is the rate of star formation in the galaxy
fp is the fraction of stars that form planets
ne is the number of planets hospitable to life (i.e., Earth-like planets)
fl is the fraction of these planets on which life actually emerges
fi is the fraction of these planets on which intelligent life arises
fc is the fraction of these planets with intelligent beings capable of interstellar communication
L is the length of time such a civilization remains detectable
The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year [source: Cain]. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.
Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets -- those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that ne equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:
N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000
which gives us a value of 12,000 civilizations in our galaxy.
Drake's original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way [source: Garber]. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy [source: Lemarchand]. That's a lot of ETs!
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