The basic question is: does life exist beyond earth.
Scientists, who are called astrobiologists, are tying to find that out right now. Most astrobiologists are trying to figure out if there’s microbial life on Mars, or in the ocean under the frozen surface of Jupiter’s moon Europa, or in the liquid hydrocarbon lakes that we’ve found on Saturn’s moon Titan.
One group of astrobiologists, works on SETI. SETI is the search for extraterrestrial intelligence. And SETI researchers are trying to detect some evidence that intelligent creatures elsewhere have used technology to build a transmitter of some sort. But how likely is it that they will manage to find a signal?
“We’re now finding exoplanets almost everywhere.”
There are certainly no guarantees when it comes to SETI, but something called the Drake Equation, named after Frank Drake, can help us organize our thinking about what might be required for successful detection.
If you’ve dealt with equations before, then you probably expect that there will be a solution to the equation; a right answer. The Drake Equation, however, is different because there are so many unknowns. It has no ‘right’ answer.
As we learn more about our universe, and our place within it, some of the unknowns get better known, and we can estimate an answer a bit better. But there won’t be an answer to the Drake Equation until SETI succeeds, or something else proves that humans are the only intelligent species in our portion of the cosmos.
In the meantime, it is really useful to consider the unknowns. The drake Equation attempts to estimate the number of technological civilizations in the galaxy (we call that N) with whom we could make contact. And it’s usually written as:
All those factors multiplied together help to estimate the number of technological civilizations that we might be able to detect right now. R* is the rate at which stars have been born in the Milky Way galaxy over the last few billion years, so it’s a number that is stars per year. Our galaxy is 10 Billion years old, and early in its history stars formed at a different rate. All of the f factors are fractions; each one must be equal to, or less than one. F sub p is the fraction of stars that have planets; n sub e is the average number of habitable planets in any planetary system, f sub l is the fraction of planets on which life actually begins, and f sub i is the fraction of all those life forms that develop intelligence. F sub c is the fraction of intelligent life that develops a civilization that decides to use some sort of transmitting technology, and finally L, the longevity factor. On average, how many years do those transmitters operate?
Astronomers are now almost able to tell us what the product of the first three terms is. We’re now finding exoplanets almost everywhere. The fractions dealing with life and intelligence and technological civilizations, are ones that a great many experts ponder--- but nobody knows for sure.
So far, we only know of one place in the universe where life exists, and that’s right here on Earth. In the next few decades, as we explore Mars, and Europa, and Titan, the discovery of any kind of life there will mean that life is abundant in the Milky Way. Because if life originated twice within this one solar system it means it was easy. And given similar conditions elsewhere, life will happen. So the number 2 is a very important number here.
Scientists, including SETI researchers, often tend to make very crude estimates, and acknowledge that there are very large uncertainties in these estimates, in order to make progress. We think we know that R* and n sub e are both numbers that are closer to 10, than to 1. And all the f factors are less than 1, some of them, maybe, much less than 1. But of all these unknowns, the biggest unknown is L. So perhaps the most useful version of the Drake Equations is to say that N is approximately equal to L. The information in this equation is very clear: unless L (longevity) is large, N (number of technological civilizations in the galaxy) will be small.
But, you can also turn that around: if SETI succeeds in detecting a signal in the near future, after examining only a small portion of the stars in the Milky Way, then we learn that L, on average, must be large, otherwise we couldn’t have succeeded so easily. A physicist named Philip Morrison summarized this by saying that "SETI is the archaeology of the future." By this he meant that, because the speed of light is finite, any signals detected from distant technologies will be telling us about their past by the time they reach us. But because L must be large for a successful detection, we also learn about our future, particularly that we can have a long future. We’ve developed technologies that can send signals into space and humans to the moon, but we’ve also developed technologies that can destroy the planet, that can wage war with weapons and biological terrorism. In the future, will our technology help stabilize our planet and our population, leading to a very long lifetime for us? Or, will we destroy our world and its inhabitants after a brief appearance on the cosmic stage? Remember, this is an equation, but there’s no right answer— not yet.
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Astronomer Jill Tarter
is the holder of the Bernard M. Oliver Chair for SETI. She is one of the few researchers to have devoted her career to hunting for signs of sentient beings elsewhere, and there are few aspects of this field that have not been affected by her work.
Currently, Jill heads up the Institute’s efforts to build and operate the Allen Telescope Array, a massive new instrument that will eventually comprise 350 antennas, each 6 meters in diameter. This telescope will be able to enormously increase the speed, and the spectral search range, of the Institute’s hunt for signals. A subset of the full array will begin operations in the Fall of 2007.
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