## Extraterrestrial life

• Extraterrestrial life, also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that does not originate from Earth. These as-yet-hypothetical life forms may range from simple single-celled organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist, there is no unambiguous evidence for its existence so far. The science of extraterrestrial life is known as exobiology.

The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."

Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Over the years, science fiction works have increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try to get in contact with life in outer space, whereas others argue that it might be dangerous to actively call attention to Earth.

Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth. This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.

${\displaystyle N=R_{\ast }\cdot f_{p}\cdot n_{e}\cdot f_{\ell }\cdot f_{i}\cdot f_{c}\cdot L}$
N = the number of Milky Way galaxy civilizations already capable of communicating across interplanetary space
R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life
fl = the fraction of planets that actually support life
fi = the fraction of planets with life that evolves to become intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
L = the length of time over which such civilizations broadcast detectable signals into space
Searches for extraterrestrial life
Subjects
Hypotheses
• Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN .
• Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN .
• Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN .
• Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN .
• Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant. Cambridge.
• Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN .
• Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN .
• Dick, Steven J.; Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN .
• Fasan, Ernst (1970). Relations with alien intelligences – the scientific basis of metalaw. Berlin: Berlin Verlag.
• Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN .
• Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN .
• Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book.
• Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Berlin: Springer. ISBN .
• Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN .
• Roth, Christopher F. (2005). Debbora Battaglia, ed. Ufology as Anthropology: Race, Extraterrestrials, and the Occult. E.T. Culture: Anthropology in Outerspaces. Durham, NC: Duke University Press.
• Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House.
• Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN .
• Ward, Peter D. (2005). Life as we do not know it-the NASA search for (and synthesis of) alien life. New York: Viking. ISBN .
• Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse: Syracuse University Press. ISBN .
Wikipedia