The Search for Extraterrestrial Life: Scientific Methods - Astronomy Techniques (2014)

Extraterrestrial life, sometimes colloquially referred to as alien life, is hypothetical life that may occur outside Earth and which did not originate on Earth. Such life might range from simple prokaryotes (or comparable life forms) to intelligent beings and even sapient beings, possibly bringing forth civilizations that might be far more advanced than humanity. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as astrobiology.Speculation about the possibility of inhabited "worlds" outside the planet Earth dates back to antiquity. Multiple early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus; Augustine references Epicurus's idea of innumerable worlds "throughout the boundless immensity of space" in The City of God. Pre-modern writers typically assumed that extraterrestrial "worlds" would be inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility that Christ could have visited extraterrestrial worlds to redeem their inhabitants. Nicholas of Cusa wrote in 1440 that the Earth was "a brilliant star" like other celestial objects visible in space, which would appear similar to the Sun from an exterior perspective due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorized that all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun. Descartes wrote that there was no means to prove that the stars were not inhabited by "intelligent creatures," but their existence was a matter of speculation. The writings of these thinkers show that interest in extraterrestrial life existed throughout history, but it is only recently that humans have had any means of investigating it.Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction. Over the years, science fiction has communicated scientific ideas, imagined a wide range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others—citing the tendency of technologically advanced human societies to enslave or wipe out less advanced societies—argue that it may be dangerous to actively call attention to Earth. A "Confidence of Life Detection" scale (CoLD) for reporting evidence of life beyond Earth has been proposed.

The 1967 Outer Space Treaty and the 1979 Moon Agreement define rules of planetary protection against potentially hazardous extraterrestrial life. COSPAR also provides guidelines for planetary protection.A committee of the United Nations Office for Outer Space Affairs had in 1977 discussed for a year strategies for interacting with extraterrestrial life or intelligence. The discussion ended without any conclusions. As of 2010, the UN doesn't have response mechanisms for the case of an extraterrestrial contact.
United StatesIn November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited Earth and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response, "The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race." Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye." The response noted "odds are pretty high" that there is life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved." One of the NASA divisions is the Office of Safety and Mission Assurance (OSMA), also known as the Planetary Protection Office. A part of its mission is to “rigorously preclude backward contamination of Earth by extraterrestrial life.”

https://en.wikipedia.org/wiki/Extraterrestrial_life

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