Extraterrestrial life

alienextraterrestrialaliensextraterrestrialsalien raceextraterrestrial beingsalien lifespace alienslifespace alien
Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth.wikipedia
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Drake equation

Green Bank equationSagan equationDrake's Equation
The Drake equation speculates about the existence of intelligent life elsewhere in the universe. In 1961, University of California, Santa Cruz, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI).
The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.

Astrobiology

astrobiologistexobiologyastrobiological
The science of extraterrestrial life in all its forms is known as astrobiology.
Astrobiology considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it.

Carl Sagan

Sagan, CarlSaganDr. Carl Sagan
According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as notable personalities such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth.
His best known scientific contribution is research on extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation.

Science fiction

sci-fiscience-fictionSci Fi
The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly in works of science fiction.
Written in the 2nd century CE by the satirist Lucian, A True Story contains many themes and tropes characteristic of modern science fiction, including travel to other worlds, extraterrestrial lifeforms, interplanetary warfare, and artificial life.

Life

livinglife on Earthbiota
Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth.
Though currently only known on Earth, life need not be restricted to it, and many scientists speculate in the existence of extraterrestrial life.

Planetary habitability

habitabilityhabitablehabitable planet
Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life's flourishing.

Exoplanet

extrasolar planetexoplanetsplanet
Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life.

Stephen Hawking

HawkingProfessor Stephen HawkingStephen William Hawking
According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as notable personalities such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth.
On 20 July 2015, Hawking helped launch Breakthrough Initiatives, an effort to search for extraterrestrial life.

Europa (moon)

EuropaEuropanLife on Europa
Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface.
The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably harbor extraterrestrial life.

Callisto (moon)

CallistoAtmosphere of Callistofourth Galilean moon of Jupiter
Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa, Ganymede, and Callisto.
The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life.

Taphonomy

taphonomictaphonomistpreservation bias
The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
It has been suggested that biominerals could be important indicators of extraterrestrial life and thus could play an important role in the search for past or present life on the planet Mars.

Circumstellar habitable zone

habitable zonehabitable zonesGoldilocks zone
Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars. Since the 1950s, astronomers have proposed that "habitable zones" around stars are the most likely places for life to exist.
Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

Europa Clipper

Europa Multiple-Flyby MissionEuropa exploration programEuropa Mission
The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2025.
Europa has been identified as one of the locations in the Solar System that could possibly harbor microbial extraterrestrial life.

Extremophile

extremophilesextremophilicpolyextremophile
Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe: extraterrestrial life and life on Earth.

Search for extraterrestrial intelligence

SETISearch for Extra-Terrestrial Intelligencesearch for extraterrestrial life
This encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds. In 1961, University of California, Santa Cruz, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI).
The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets.

Panspermia

lithopanspermiatranspermiaPseudo-panspermia
Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia.

Francis Crick

CrickFrancis Harry Compton CrickFrancis H.C. Crick
The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth.
Near the end of the article, Crick briefly mentioned the search for life on other planets, but he held little hope that extraterrestrial life would be found by the year 2000.

Biosignature

biosignaturesbiomarkerssigns of life
The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed.
Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life.

Fossil

fossilsfossil recordfossilized
The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
It has been suggested that biominerals could be important indicators of extraterrestrial life and thus could play an important role in the search for past or present life on the planet Mars.

Life on Titan

Titana form of lifeorganisms there
Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there if present, could be consuming hydrogen, acetylene and ethane, and producing methane.
Because of the extreme cold and also because of lack of carbon dioxide (CO 2 ) in the atmosphere, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for extraterrestrial life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth.

Fermi paradox

Fermi's paradoxdark forestTranscension Hypothesis
The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for such civilizations is known as the Fermi paradox.
The Fermi paradox, named after Italian-American physicist Enrico Fermi, is the apparent contradiction between the lack of evidence for extraterrestrial civilizations elsewhere in the Milky Way galaxy and various high estimates for their probability (such as those that result from optimistic parameters for the Drake equation).

Interstellar communication

communicationinterstellarradio or laser signals
Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.
The SETI project has for the past several decades been conducting a search for signals being transmitted by extraterrestrial life located outside the Solar System, primarily in the radio frequencies of the electromagnetic spectrum.

Microorganism

microorganismsmicrobemicrobes
Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms. Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe.
They extend terrestrial life into much of the Earth's hydrosphere, crust and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in biotechnology, and their very existence under such extreme conditions increases the potential for extraterrestrial life.

Galilean moons

Galilean satellitesGalilean moonmoons of Jupiter
Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa, Ganymede, and Callisto.
The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life.

Frank Drake

Drake, Frank DonaldFrank Donald Drake
In 1961, University of California, Santa Cruz, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI).
He reports that he considered the possibility of life existing on other planets as an eight-year-old, but never discussed the idea with his family or teachers due to the prevalent religious ideology.