Extraterrestrial life is defined as life that does not originate from Earth. It is unknown whether any such life exists or ever
existed in the past, although many scientists think it likely that on Mars, for
instance, life either exists or has existed. A less direct
argument for the existence of extraterrestrial life relies on the vast size of
universe. According to this argument, endorsed by Carl Sagan and Stephen Hawking, it would be improbable for
life not to exist somewhere other than Earth.
Suggested locations on which life might have developed, or which might
continue to host life today, include the planets Venus and Mars; moons of Jupiter and Saturn such as Europa,
Enceladus and Titan; and extrasolar planets
such as Gliese 581 c and
d, recently discovered to
be near Earth mass and apparently located in their star's habitable zone, with the potential to have
Several theories have been proposed about the possible basis of alien life
from a biochemical, evolutionary or morphological
viewpoint. Alien life, such as bacteria, has been theorized by scientists such as Carl
Sagan to exist in the Solar
System and quite possibly throughout the universe. No samples have been
found, although there is some controversy about possible traces of life in
Martian material, of which the most famous are on the Allan Hills 84001 meteorite.
All life on Earth requires carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus as well as numerous other elements in
smaller amounts, notably minerals; it also requires water as the solvent in which biochemical reactions take place.
Sufficient quantities of carbon and the other major life-forming elements, along
with water, may enable the formation of living organisms on other planets with a
chemical make-up and average temperature similar to that of Earth. Because Earth
and other planets are made up of "stardust", i.e. relatively abundant chemical elements
formed from stars which have ended their lives as supernovae, it is very probable that other planets
may have been formed by elements of a similar composition to the Earth's. The
combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can also
provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants). Plants derive energy through the conversion of light
energy into chemical energy via photosynthesis. Life, as currently recognized,
requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon
oxides) states. It also appears to require nitrogen as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins,
and phosphorus oxidized to phosphates in genetic material and in
energy transfer. Adequate water as a solvent supplies adequate oxygen as
constituents of biochemical substances.
Pure water is useful because it has a neutral pH due to its continued dissociation between hydroxide and hydronium ions. As a result, it can dissolve both positive metallic ions and
ions with equal ability. Furthermore, the fact that organic molecules can be
either hydrophobic (repelled by water) or hydrophilic
(soluble in water) creates the ability of organic compounds to orient themselves
to form water-enclosing membranes. The fact that solid water (ice) is less dense than liquid water also means
that ice floats, thereby preventing Earth's oceans from slowly freezing. Without
this quality, the oceans could have frozen solid during the Snowball Earth episodes.
Additionally, the hydrogen
bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released. This
helps to moderate the climate, cooling the tropics and warming the poles,
helping to maintain the thermodynamic stability needed for life.
Carbon is fundamental to terrestrial life for its immense flexibility in
creating covalent chemical
bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the
storage of solar energy in sugars, such as glucose. The oxidation of glucose releases biochemical energy
needed to fuel all other biochemical reactions.
The ability to form organic
acids (–COOH) and amine bases (–NH2) gives rise to the possibility of
dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA (the information-storing molecule of inheritance), but
also ATP (the principal energy "currency" of
Due to their relative abundance and usefulness in sustaining life, many have
hypothesized that lifeforms elsewhere in the universe would also utilize these
basic materials. However, other elements and solvents could also provide a basis
for life. Silicon is most often deemed to be the probable
alternative to carbon. Silicon lifeforms are proposed to have a
crystalline morphology, and are theorized to be able
to exist in high temperatures, such as on planets which are very close to their
star. Life forms based in ammonia
(rather than water) have also been suggested, though this solution appears less
optimal than water.
When looked at from a chemical perspective, life is fundamentally a
self-replicating reaction, but one which could arise under a great many
conditions and with various possible ingredients, though carbon-oxygen within
the liquid temperature range of water seems most conducive. Suggestions have
even been made that self-replicating reactions of some sort could occur within
the plasma of a
star, though it would be highly unconventional.
Several pre-conceived ideas about the characteristics of life outside of
Earth have been questioned. For example, NASA scientists believe that the color of
photosynthesizing pigments on extrasolar planets might not be green.
In addition to the biochemical basis of extraterrestrial life, many have also
considered evolution and morphology. Science fiction has
often depicted extraterrestrial life with humanoid and/or reptilian forms. Aliens have often been depicted as
having light green or grey skin, with a large head, as well as four limbs—i.e.
fundamentally humanoid. Other subjects, such as felines and insects, etc., have also occurred in fictional
representations of aliens.
A division has
been suggested between universal and parochial characteristics.
Universals are features which are thought to have evolved independently more
than once on Earth (and thus, presumably, are not too difficult to develop) and
are so intrinsically useful that species will inevitably tend towards them.
These include flight, sight, photosynthesis and limbs, all of which are thought to have evolved
several times here on Earth. There is a huge variety of eyes, for example, and many of these have radically
different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation.
Parochials, however, are essentially arbitrary evolutionary forms. These often
have little inherent utility (or at least have a function which can be equally
served by dissimilar morphology) and probably will not be replicated.
Intelligent aliens could communicate through gestures, as deaf humans do, or by
sounds created from structures unrelated to breathing, which happens on Earth
when, for instance, cicadas vibrate
their wings, or crickets rub their legs.
Attempting to define parochial features challenges many taken-for-granted
notions about morphological necessity. Skeletons, which are essential to large terrestrial
organisms according to the experts of the field of gravitational
biology, are almost assured to be replicated elsewhere in one form or
another. Many also conjecture as to some type of egg-laying amongst
extraterrestrial creatures, but mammalian mammary glands might be a singular case.
The assumption of radical diversity amongst putative extraterrestrials is by
no means settled. While many exobiologists do stress that the enormously
heterogeneous nature of life on Earth foregrounds an even greater variety in
space, others point out that convergent evolution may dictate
substantial similarities between Earth and extraterrestrial life. These two
schools of thought are called "divergionism" and "convergionism"
The first important Western thinkers to argue systematically for a
universe full of other planets and, therefore, possible extraterrestrial life
were the ancient Greek
writer Thales and his student Anaximander in the 7th and 6th centuries B.C. The atomists of Greece like
Epicurus took up the idea, arguing
that an infinite universe ought to have an infinity of populated worlds. Ancient
Greek cosmology worked against the idea of extraterrestrial life in one critical
respect, however: the geocentric universe. Championed by Aristotle and codified by Ptolemy, it favored the Earth and
Earth-life (Aristotle denied that there could be a plurality of worlds) and
seemingly rendered extraterrestrial life philosophically untenable. Lucian of Samosata, in his novels, described
inhabitants of the Moon and other celestial
bodies as humanoids, but significantly different from humans.
Authors of Jewish
sources also considered extraterrestrial life. The Talmud states that there are at least 18,000 other
worlds, but provides little elaboration on the nature of those worlds, or on
whether they are physical or spiritual. Based on this, however, the 18th century
exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist, and
that some may well possess intelligence. It adds that humans should not expect
creatures from another world to resemble earthly life any more than sea
creatures resemble land animals.
According to Hindu scriptures, there are innumerable universes
to facilitate the fulfillment of the separated desires of innumerable living
entities. However, the purpose of such creations is to bring back the deluded
souls to correct understanding about the purpose of life. Aside from the
innumerable universes which are material, there is also the unlimited spiritual
world, where the purified living entities live with perfect conception about
life and ultimate reality. The spiritually aspiring saints and devotees, as well
as thoughtful men of the material world, have been getting guidance and help
from these purified living entities of the spiritual world from time immemorial.
However, the relevance of such descriptions has to be evaluated in the context
of a correct understanding of geography and science at those times.
Within Islam, the statement of the Qur'an "All praise belongs to God, Lord of all the worlds" indicates multiple
universal bodies, and maybe even multiple universes, which may indicate
extraterrestrial and even extradimensional life. Surat Al-Jinn also mentioned a statement from a Jinn regarding the current status and ability
of his group in the heavens.
According to Ahmadiyya Islam a
more direct reference from the Quran is presented by Mirza Tahir Ahmad
as a proof that life on other planets may exist according to the Quran. In his
Rationality, Knowledge & Truth, he quotes verse 42:29 "And among His
Signs is the creation of the heavens and the earth, and of whatever living
creatures (da'bbah) He has spread forth in both..."; according to this
verse there is life in heavens. According to the same verse "And He has the
power to gather them together (jam-'i-him) when He will so please";
indicates the bringing together the life on Earth and the life elsewhere in the
universe. The verse does not specify the time or the place of this meeting but
rather states that this event will most certainly come to pass whenever God so
desires. It should be pointed out that the Arabic term Jam-i-him used to express
the gathering event can imply either a physical encounter or a contact through
In Shia Islam the 6th
al-Sadiq has been quoted as saying that there are living beings on other
planets. He has also stated that they may be more intelligent or advanced than
When Christianity spread
throughout the West, the Ptolemaic system became very widely accepted, and
although the Church never issued any formal pronouncement on the question of
alien life, at least tacitly,
the idea was aberrant. In 1277, the Bishop of Paris, Étienne Tempier, did overturn Aristotle on
one point: God could have created more than one world (given His
omnipotence). Taking a further step, and arguing that aliens actually existed,
remained rare. Notably, Cardinal Nicholas of Kues speculated about aliens on
the moon and sun.
There was a dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it
became clear that the Earth was merely one planet amongst countless bodies in
the universe, the extraterrestrial idea moved towards the scientific mainstream.
God's omnipotence, it could be argued, not only allowed for other worlds and
other life; on some level, it necessitated them. The best known early-modern
proponent of such ideas was the Italian philosopher Giordano Bruno, who argued in the 16th century
for an infinite universe in which every star is surrounded by its own planetary system.
Bruno's thoughts about God and the universe, of which many contradicted
essential dogmas of the Catholic Faith, led to his eventual condemnation as a
heretic by a tribunal of the Roman Inquisition of the Roman Catholic
Church. Contemporary civil authorities enforced the penal statute of Emperor
Frederick II Inconsutilem
Tunicam of 12 February 1220 mandating the burning of heretics, resulting in
his being among the last of heretics to be burned alive at the stake in Rome in
the year 1600.
In the early 17th century the Czech astronomer Anton Maria Schyrleus of Rheita
mused that "if Jupiter has (...) inhabitants (...) they must be larger and more
beautiful than the inhabitants of the Earth, in proportion to the
[characteristics] of the two spheres". Dominican monk Tommaso
Campanella wrote about a Solarian alien race in his Civitas Solis. The
Catholic Church has not made a formal ruling on the existence of
extraterrestrials. However, writing in the Vatican newspaper, the astronomer,
Gabriel Funes, director of the Vatican Observatory near Rome, said in 2008
that intelligent beings created by God could exist in outer space.
Such comparisons also appeared in poetry of the era. In "The Creation: a
Philosophical Poem in Seven Books" (1712), Sir Richard Blackmore observed: "We may pronounce
each orb sustains a race / Of living things adapted to the place". The didactic
poet Henry More took up the
classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay
Upon the Infinity of Worlds" (1647). With the new relative viewpoint that the
Copernican revolution had wrought, he suggested "our world's sunne / Becomes a
starre elsewhere". Fontanelle's "Conversations on
the Plurality of Worlds" (translated into English in 1686) offered similar
excursions on the possibility of extraterrestrial life, expanding, rather than
denying, the creative sphere of a Maker.
The possibility of extraterrestrials remained a widespread speculation as
scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th-19th century
astronomers convinced that the Solar System, and perhaps others, would be
well-populated by alien life. Other luminaries of the period who championed
"cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment, even the Sun and Moon were
considered candidates for extraterrestrial inhabitants.
The trend to assume that celestial bodies were populated almost by default
was tempered as actual probes visited potential alien abodes in the Solar System
beginning in the second half of the 20th century. At the same time, the
beginning Space Age was
accompanied by a surge of UFO reports, particularly in the United States, during the 1950s. The term UFO
itself was coined in 1952 in the context of the enormous popularity of the
concept of "flying saucers" in the wake of the Kenneth Arnold UFO sighting in
The moon was decisively ruled out as a possibility, while Venus and Mars,
long the two main candidates for extraterrestrials, showed no obvious evidence
of current life. The other large moons of our system which have been visited
appear, to our knowledge, similarly lifeless, though the interesting geothermic
forces observed (Io's volcanism,
Europa's ocean, Titan's thick atmosphere and
hydrocarbon lakes) have underscored how broad the range of potentially habitable
environments may be.
Although the hypothesis of a deliberate cosmic silence of advanced
extraterrestrials is also a possibility,
the failure of the SETI program to announce
an intelligent radio signal after four decades of effort has at least partially
dimmed the prevailing optimism of the beginning of the space age.
Notwithstanding, the unproven belief in extraterrestrial beings is voiced (not
as a hypothesis) in pseudoscience, conspiracy theories in popular folklore like
51' and legends. Critics
view the search
for extraterrestrials as unscientific, despite the fact that the SETI program is not the result of a continuous, dedicated
search, but instead utilizes what resources and manpower it can, when it can.
Furthermore, the SETI program only searches a limited range of frequencies at
any one time.
In the words of SETI's Frank
Drake, "All we know for sure is that the sky is not littered with powerful
microwave transmitters". Drake has also
noted that it is entirely possible that advanced technology results in
communication being carried out in some way other than conventional radio
transmission. At the same time, the data returned by space probes, and giant
strides in detection methods, have allowed science to begin delineating habitability
criteria on other worlds, and to confirm that at least other planets are
plentiful, though aliens remain a question mark. The Wow! signal, from SETI, remains a speculative
In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald
Brownlee published a book entitled Rare Earth: Why Complex Life is
Uncommon in the Universe. In it, they
discussed the Rare Earth hypothesis, in which they
claim that Earth-like life is rare in the
universe, while microbial life is common. Ward and Brownlee are open
to the idea of evolution on other planets which is not based on essential
Earth-like characteristics (such as DNA and carbon).
The possible existence of primitive (microbial) life outside of Earth is much
less controversial to mainstream scientists, although, at present, no direct
evidence of such life has been found. Indirect evidence has been offered for the
current existence of primitive life on Mars. However, the conclusions that should
be drawn from such evidence remain in debate.
The scientific search for extraterrestrial life is being carried out both
directly and indirectly.
Scientists are directly searching for evidence of unicellular life within the
Solar System, carrying out
studies on the surface of Mars and examining meteors which have fallen to Earth. A mission is also
proposed to Europa, one
of Jupiter's moons with a possible
liquid water layer under its surface, which might contain life.
There is some limited evidence that microbial life might possibly exist (or
have existed) on Mars. An experiment on
the Viking Mars lander
reported gas emissions from heated Martian soil that some argue are consistent
with the presence of microbes. However, the lack of corroborating evidence from
other experiments on the Viking indicates that a non-biological reaction is a
more likely hypothesis. Independently, in 1996, structures resembling nanobacteria were reportedly
discovered in a meteorite, ALH84001, thought to be formed of rock ejected from
Mars. This report is also controversial, and scientific debate
In February 2005, NASA scientists
reported that they had found strong evidence of present life on Mars. The two
Stoker and Larry
Lemke of NASA's Ames Research Center, based their claims
on methane signatures found in Mars' atmosphere resembling the methane
production of some forms of primitive life on Earth, as well as on their own
study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists' claims,
and Stoker herself backed off from her initial assertions.
Though such findings are still very much in debate, support among scientists
for the belief in the existence of life on Mars seems to be growing. In an
informal survey conducted at the conference at which the European Space
Agency presented its findings, 75 percent of the scientists in attendance
were reported to believe that life once existed on Mars, and 25 percent reported
a belief that life currently exists there.
hypothesis stipulates that any planet with a robust population of life will
have an atmosphere not in chemical equilibrium, which is relatively easy to
determine from a distance by spectroscopy. However, significant advances in the
ability to find and resolve light from smaller rocky worlds near to their star
are necessary before this can be used to analyze extrasolar planets.
It is theorized that any technological society in space will be transmitting
information, although this is arguable, as there are generally no human systems
intentionally, randomly, transmitting information into deep space, so there is
no guarantee that any other species would do so, either. Also, the length of
time required for a signal to travel across the vastness of space means that any
signal detected or not detected would come from the distant
Nevertheless, projects such as SETI
are conducting an astronomical search for radio activity which would confirm the
presence of intelligent life. A related suggestion is that aliens might
broadcast pulsed and continuous laser
signals in the optical, as well as infrared, spectrum;
laser signals have the advantage of not "smearing" in the interstellar medium,
and may prove more conducive to communication between the stars. While other
communication techniques, including laser transmission and interstellar
spaceflight, have been discussed seriously and may well be feasible, the measure
of effectiveness is the amount of information communicated per unit cost,
resulting with radio as the method of choice.
Astronomers also search for extrasolar planets that they believe would be
conducive to life, such as Gliese 581 c, Gliese 581 d and OGLE-2005-BLG-390Lb, which have been found
to have Earth-like qualities. Current radiodetection methods have been inadequate for such a search, as the
resolution afforded by recent technology is inadequate for a detailed study of
extrasolar planetary objects. Future telescopes should be able to image planets
around nearby stars, which may reveal the presence of life — either directly or
through spectrography which would reveal
key information, such as the presence of free oxygen in a planet's atmosphere:
It has been argued that Alpha Centauri, the closest star system to Earth, may
contain planets which could be capable of sustaining life.
On April 24, 2007, scientists at the European Southern Observatory in La Silla,
Chile said they had found the first Earth-like planet. The planet, known as
Gliese 581 c, orbits
within the habitable
zone of its star Gliese
581, a red dwarf star which
is 20.5 light
years (194 trillion km) from the Earth. It was initially thought that this
planet could contain liquid water, but recent computer simulations of the
climate on Gliese 581c by Werner von Bloh and his team at Germany's Institute
for Climate Impact Research suggest that carbon dioxide and methane in the
atmosphere would create a runaway greenhouse effect. This would warm the planet
well above the boiling point of water (100 degrees Celsius/212 degrees
Fahrenheit), thus dimming the hopes of finding life. As a result of greenhouse
models, scientists are now turning their attention to Gliese 581 d, which lies just outside of the
star's traditional habitable zone.
On May 29, 2007, the Associated Press released a report stating
that scientists identified twenty-eight new extra-solar planetary bodies. One of
these newly-discovered planets is said to have many similarities to Neptune.
As of March 4, 2010, 429 extrasolar planets have been discovered (with 48
multi-planet systems), and new discoveries occur monthly.
In 1961, University of California,
Santa Cruz astronomer and astrophysicist Dr. Frank Drake devised the Drake equation. This controversial equation
multiplied estimates of the following terms together:
Drake used the equation to estimate that there are approximately 10,000
planets in the Milky Way galaxy
containing intelligent life with the possible capability of communicating with
Based on observations from the Hubble Space Telescope, there are at least 125
billion galaxies in the universe. It is estimated that at least ten percent of
all sun-like stars have a system of planets,
i.e. there are 6.25×1018 stars with planets orbiting them in the
universe. Even if we assume that only one out of a billion of these stars have
planets supporting life, there would be some 6.25×109 (billion)
life-supporting planetary systems in the universe.
Many bodies in the Solar System have been suggested as being capable of
containing conventional organic life. The most commonly suggested ones are
listed below; of these, five of the ten are moons, and are thought to have large
bodies of underground liquid (streams), where life may have evolved in a similar
fashion to deep sea vents.
- Mars — Life on Mars has been long speculated. Liquid
water is widely thought to have existed on Mars in the past, and there may still
be liquid water beneath the surface. Methane was found in the atmosphere of Mars. By July
2008, laboratory tests aboard NASA's Phoenix Mars Lander had identified water in
a soil sample. The lander's robotic arm delivered the sample to an instrument
which identifies vapors produced by the heating of samples. Recent photographs
from the Mars
Global Surveyor show evidence of recent (i.e. within 10 years) flows of a
liquid on the Red Planet's frigid surface.
- Mercury — The
MESSENGER expedition to Mercury
has discovered that a large amount of water exists in its exosphere.
- Europa — Europa may
contain liquid water beneath its thick ice layer. It is possible that vents on
the bottom of the ocean warm the ice, so liquid could exist beneath the ice
layer, perhaps capable of supporting microbes and simple plants, just like in Earth's hydrothermal
- Jupiter — Carl Sagan in the 1960s and
70s computed conditions for hypothetical amino acid-based macroscopic life in the atmosphere of
Jupiter, based on observed conditions of this atmosphere. These
investigations inspired some science fiction stories.
- Ganymede —
Possible underground ocean (see Europa).
- Callisto —
Possible underground ocean (see Europa).
- Enceladus —
Geothermal activity, water vapor. Possible under-ice oceans heated by tidal
- Titan (Saturn's largest moon) — The only known moon with a
significant atmosphere. Data from the Cassini-Huygens mission refuted the
hypothesis of a global hydrocarbon ocean, but later demonstrated the
existence of liquid
hydrocarbon lakes in the polar regions - the first liquid lakes discovered
outside of Earth.
Analysis of data from the mission has uncovered aspects of atmospheric chemistry
near the surface which are consistent with – but do not prove – the hypothesis
that organisms there are consuming hydrogen, acetylene and ethane, and producing
- Venus — Recently, scientists have
speculated on the existence of microbes in the stable cloud layers 50 km above
the surface, evidenced by hospitable climates and chemical disequilibrium.
Numerous other bodies have been suggested as potential hosts for microbial life. Fred Hoyle has proposed that life might exist on
comets, as some Earth microbes managed to survive on a lunar probe for many
years. However, it is considered highly unlikely that complex multicellular
organisms of the conventional chemistry of terrestrial life (i.e. animals and
plants) could exist under these living conditions.
Even if microbial
extraterrestrial life were found on another body in the Solar System, it would still need to be proven
that such life did not originate from Earth in the recent or distant past. For example, an
alternate explanation for the hypothetical existence of microbial life on Titan has already been
formally proposed — theorizing that
microorganisms could have left Earth when it suffered a massive asteroid or comet impact (such as the impact that created Chicxulub crater only
65 mya), and
survived a journey through space to land
on Titan 1 million years later. The Living Interplanetary
Flight Experiment, developed by the Planetary Society
and due to be launched in 2011, has been designed to test similar theories.