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Are We Alone in the Milky Way?

by Louise Good


The Frontiers of Astronomy Community Event held on November 17 on the Manoa campus tackled the question of whether we are likely to have company (other intelligent life) in our Milky Way Galaxy. Entitled "Who Wants to Be a One-In-A-Millionaire? The Odds of Intelligent Life & Civilization in the Galaxy," it used the format of the popular game show Who Wants To Be A Millionaire?

The UH NASA Astrobiology Institute (UHNAI) presented the program, and Steven Desch of Arizona State University, a visitor to UHNAI, served as host. Biologist Stephen Freeland, astronomer Karen Meech, and planetary scientist Jeff Taylor served as an expert panel for contestants, chosen from the audience, to consult as a lifeline. Contestants also consulted the members of the audience equipped with "clickers" that enabled them to vote for a selection of answers.

The program consisted of a discussion of the Drake equation, which was first proposed in 1961 by radio astronomer Frank Drake to determine the number of civilizations in our Galaxy from which we might receive radio transmissions. Desch emphasized that there are no right or wrong answers, since even scientists have insufficient data to come to a clear conclusion.

The components of the equation, which are all multiplied together, are (a) the number of stars born in our Galaxy each year that might have planetary systems suitable for life, (b) the fraction of those stars that do have planets, (c) the number of planets in each planetary system that are habitable, (d) the fraction of habitable planets that actually have life, (e) the fraction of life-bearing planets that have intelligent life, (f) the fraction of those with intelligent life that can send detectable radio signals into space, and (g) the average number of years such communicating civilizations last.

While there are 100 billion stars in the Galaxy, only a few are likely to support intelligent life. Some are too hot and short-lived, while others are too small to have a planet likely to support intelligent life. Only mid-sized stars like our Sun are likely to do so. How many of these are born each year? The consensus of the scientific community announced by Desch after the contestant, local expert panel, and audience had had their say is (a) two, (b) only half of these stars will actually have planets, and in those planetary systems, (c) only one planet is likely to be habitable.

But what does habitable mean? So far, we know of only one habitable planet—Earth. Generally, habitable means that a planet has liquid water, and in our solar system, only Earth has water on its surface. However, there are organisms living near volcanic vents deep beneath the sea, inside rocks, and in extremely cold conditions. Because primitive life formed within the first billion years of Earth's existence and can exist even under extreme conditions, scientists guess that (d) close to 100 percent of habitable planets will have life.

But it is a long, long road (over 3 billion years) from primitive life to intelligent life, so chances are that (e) only one percent of planets with life will develop intelligent life, and (f) only one percent of those with intelligent life will be able to communicate across interstellar space. Although 70 percent of its surface is covered with water, Earth may be one of the drier planets with intelligent life. Some planets with intelligent life may be totally covered with water, and dolphins, while quite bright, are not physically able to build anything.

Finally, how long will a civilization capable of communicating across space last? If you are a pessimist, you say 100 years, given that nuclear weapons and environmental degradation seem to go hand in hand with such technological capabilities. Optimists may say 10,000 years. Desch proposed to use (g) 1,000 years as this value.

So what is the answer to this equation? The values that Desch suggested the scientific community might pick equal 0.1. This means that there is one intelligent communicating civilization at a time that lasts for 1,000 years out of each 10,000. The total based on answers from the night's contestants was even smaller, 0.002. In either case, we are essentially alone.