SUMMARY OF KEY IDEAS
Focus Question 15-6
What negative consequence could arise from an alien civilization discovering a Pioneer spacecraft?
The chemical building blocks of life exist throughout the Milky Way Galaxy.
Organic molecules and water have been discovered in interstellar clouds, in some meteorites, in comets, and in newly forming star and planet systems.
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Astronomers are using radio telescopes to search for signals from other self-
aware life in the Galaxy. This effort is called the search for extraterrestrial intelligence, or SETI. SETI is primarily done at frequencies where radio waves pass most easily through the interstellar medium. So far, these searches have not detected any life outside Earth. The Drake equation is used to estimate the number of technologically advanced civilizations in the Galaxy whose radio transmissions we might discover. Estimates of this number vary from one to millions.
Everyday radio and television transmissions from Earth, along with intentional broadcasts into space, may be detected by other life-
forms.
WHAT DID YOU THINK?
Why is water so important to the formation of life? Water allows many interactions between atoms and molecules dissolved in it.
What element is uniquely suited to be the foundation of life as we know it, and why? Carbon is the only atom that can bond flexibly (but not too flexibly) with three or more other atoms.
How do astronomers search for extraterrestrial intelligence? They search for radio signals from other advanced civilizations.
Have astronomers located any extraterrestrial civilizations? No. Extraterrestrial civilizations have not yet been discovered.
If advanced alien civilizations exist, is there any way they might know of our existence? Yes. They might detect our intentional radio broadcasts into space, our everyday radio and television broadcasts, or, possibly, one of our spacecraft traveling in interstellar space.