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Extraterrestrial Life

Responding to Public Questions and Misconceptions

David Morrison

June 1, 2010

Questions from the public regarding extraterrestrial life.

Question: I would like to ask you about the SETI program and the radio signals we are transmitting every day...How exactly are humans or aliens supposed to translate the received signals?

SETI (the search for extraterrestrial intelligence) does not transmit, it receives. Sensitive radio telescopes and optical telescopes that look for ultra-short pulses are involved in the searches. We do not transmit for several reasons. We are a very young and immature technical civilization. With current technology, we could detect the “leakage” radio transmission from a planet like Earth only to the distance of Alpha Centauri, the nearest star. Therefore we are looking for powerful radio beacons that might be operated by more advanced civilizations. Also, as you note, we would not know what to transmit that could be understood (or would even interest) an advanced technical civilization. Thus it is a much better strategy for us to listen quietly. For more about SETI look it up in Wikipedia or check out the SETI Institute.

Question: Stephen Hawking said in a special on the Discovery Channel that aliens exist and are dangerous. Could you clarify it and perhaps calm my nerves.

Hawking's show was primarily about the science of astrobiology. He explained the likelihood of life beyond Earth and the value of searching for it, including missions to Mars and Europa. In the last few minutes of the show, he speculated about intelligent alien life and explained the value of doing a SETI radio search. He also briefly discussed his concerns about intentionally transmitting, including the possibility that there might be some advanced life forms that could be motivated to visit our solar system in search of resources. I have no problem with any of these statements. Since we have no evidence that aliens (intelligent or otherwise) exist, all such comments are speculations. While I don’t share Hawking’s concern, I note that these are the sort of reasons that are frequently given for why it is better for us to receive but not to transmit any interstellar radio beacons of our own.

Question: Does the Drake equation accurately predict alien civilizations?

The Drake Equation is a useful tool for organizing the unknowns that need to be filled in to estimate the number of communicating civilizations that might co-exist in our Galaxy. It was introduced in 1961 by Frank Drake as a list of discussion topics for a meeting about life in the universe held at the National Radio Astronomy Observatory. Like any equation, the quality of the answer depends on the quality of the input (the so-called "garbage-in, garbage-out" phenomenon). Several factors in the Drake equation are completely unknown, such as the probability that intelligent aliens will develop radio technology, or the lifetime of the transmitting stage of such a technical civilization. Therefore the equation makes no actual prediction of the number of alien technical civilizations.

Question: Is there any consideration given to, instead of simply looking for life on other planets, actually sending it there? Is it possible to send out probes teeming with bacteria to crash land on planets which could be habitable? Bacteria that could terraform planets for us perhaps?

A lot of serious consideration has been given to the issues of planetary contamination, planetary protection, and terraforming. There are three persuasive arguments against intentionally contaminating another planet with microbes from our planet. (1) We don’t know of any planet on which terrestrial microbes could grow (or equivalently we don’t know of any microbes that could grow on the other planets in our solar system). (2) It would be unethical to introduce terrestrial life-forms on another planet that may have its own indigenous life. (3) It would be foolish for scientists to contaminate anther planet and thus render it impossible for subsequent missions to distinguish transplanted terrestrial life from possible indigenous life.

David Morrison

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David Morrison is a long-time NASA senior scientist and Committee for Skeptical Inquiry fellow. He now divides his time between the SETI Institute and the NASA Lunar Science Institute. He hosts the "Ask an Astrobiologist" column at NASA's website.