Figuring the Odds
Any practical search for extraterrestrial intelligence involves looking for some manifestation of technology. Many scientists consider searching for radio signals to be the most promising method. The American astronomer Frank Drake devised the equation illustrated at right to stimulate discussion about the probability of finding intelligent life outside Earth.
The equation identifies factors thought to play a role in the development of technological civilizations. It has no single solution. Indeed, at a 1961 conference, values suggested for these factors gave the number of communicating civilizations within our galaxy as anywhere from 100 to 100 million. Nonetheless, the Drake equation has become an effective tool for organizing our knowledge as well as our ignorance. |
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| Communicating Civilizations (N)
The number of civilizations in the Milky Way galaxy whose radio emissions are detectable. |
 Starbirths (R*)
The average rate of star formation per year in the Milky Way. | Planet Formation (fp)
The fraction of stars with planetary systems. | Life-supporting Environments (ne)
The number of planets or moons per planetary system that contain liquid water, assuming liquid water is necessary for life. | Life (fl)
The fraction of habitable worlds on which life actually emerges. | Intelligence (fi)
The fraction of such planets on which intelligence arises. | Communication (fc)
The fraction of alien civilizations both willing and able to communicate, presumably by radio. | Life Span (L)
The average number of years a technological civilization lasts after developing radio. |
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As you undoubtedly recall, one bowl of porridge was too hot, another was too cold, but the third was just right. Temperatures vary on the surfaces of planets as well. Thermometer readings on other worlds depend mostly on their distances from their parent stars and the compositions of their atmospheres. Planets very close to their stars fail the Goldilocks test, as do those orbiting in the deep freeze far away from their suns. For life as we know it, the "just right" range encompasses the temperatures at which liquid water can exist. At ordinary atmospheric pressures, that range is 32 to 212 degrees Fahrenheit.
This emphasis on liquid water is not arbitrary. Hydrogen and oxygen--the ingredients of the water molecule--are the first and third most common elements in the universe. Liquid water provides a medium within which chemical reactions can occur stably and quickly. That's not the case for the other forms of matter that water usually assumes, ice and vapor (or steam). The crucial chemical reactions involve substances that contain carbon, the fourth most common element. (The second most common, helium, is inert and plays no role in living things.) Carbon-based chemistry is the most versatile chemistry in the universe. The element's chemical structure allows it to bond readily and strongly with itself and with many other elements. As a result, "organic" carbon-bearing compounds form the basis of all life with which we are familiar. It's encouraging that we see organic compounds everywhere we look. They exist in interstellar gas, dust grains, comets, meteorites, and on some moons in our solar system. Space is chock-full of the raw materials of life.
We must be careful not to rule out other possible building blocks of life. Perhaps silicon-based life covers a distant planet, powered by electricity rather than blood or sap. Indeed, our biggest challenge in the search for extraterrestrial life may be to shed our preconceived notions of what forms life must assume. But for now we know of no basic recipe for living things other than liquid water and carbon-based molecules.
With that in mind, one look around Earth is enough to see that we are safely within the Sun's "just right" range for life--a region that we more properly call the habitable zone. Rain falls, the oceans ebb and flow, and life teems within any (continued) |
