Extraterrestrial life
is hypothetical life which may occur outside of Earth and which did not
originate on Earth. Such life might range from simple prokaryotes (or
comparable life forms) to beings with civilizations far more advanced than humanity.
The Drake
equation speculates about the existence of intelligent life elsewhere in
the universe. The science of extraterrestrial life in all its forms is known as
astrobiology.
Since the mid-20th century, active ongoing research has
taken place to look for signs of extraterrestrial life. This encompasses a
search 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, chiefly in
works of science fiction. Over the years, science fiction has introduced a
number of theoretical ideas, each having a wide range of possibilities. Many
have piqued public interest in the possibilities of extraterrestrial life. One
particular concern is the wisdom of attempting communication with
extraterrestrial intelligence. Some encourage aggressive methods to make
contact with intelligent extraterrestrial life. Others argue to do so may give
away the location of Earth, making an invasion possible in the future.
General
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, as well as notable
personalities such as Winston Churchill,
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.
Since the 1950s, astronomers have proposed that "habitable zones" around stars
are the most likely places for life to exist. Numerous discoveries of such
zones since 2007 have generated numerical estimates of many billions of planets
with Earth-like compositions. As of 2013, only a few planets had been
discovered in these zones. 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
nearest such planet may be 12 light-years away, according to the scientists. Astrobiologists have also considered a "follow the energy" view of
potential habitats.
Evolution
A study published in 2017 suggests that due to how complexity
evolved in species on Earth, the level of predictability for alien evolution
elsewhere would make them look similar to life on our planet. One of the study
authors, Sam Levin, notes "Like humans, we predict that they are
made-up of a hierarchy of entities, which all cooperate to produce an alien. At
each level of the organism there will be mechanisms in place to eliminate
conflict, maintain cooperation, and keep the organism functioning. We can even
offer some examples of what these mechanisms will be." There is also research in assessing the
capacity of life for developing intelligence. It has been suggested that this
capacity arises with the number of potential niches a planet contains, and that
the complexity of life itself is reflected in the information density of
planetary environments, which in turn can be computed from its niches.
Biochemical basis
Life on Earth requires water as a solvent in which
biochemical reactions take place. Sufficient quantities of carbon and other
elements, along with water, might enable the formation of living organisms on
terrestrial planets with a chemical make-up and temperature range similar to
that of Earth. Life based on ammonia
(rather than water) has been suggested as an alternative, though this solvent
appears less suitable than water. It is also conceivable that there are forms
of life whose solvent is a liquid hydrocarbon, such as methane, ethane or
propane.
About 29 chemical elements play active roles in living
organisms on Earth. About 95% of living matter is built upon only six elements:
carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements
form the basic building blocks of virtually all life on Earth, whereas most of
the remaining elements are found only in trace amounts. The unique
characteristics of carbon make it unlikely that it could be replaced, even on
another planet, to generate the biochemistry necessary for life. The carbon
atom has the unique ability to make four strong chemical bonds with other
atoms, including other carbon atoms. These covalent bonds have a direction in
space, so that carbon atoms can form the skeletons of complex 3-dimensional structures
with definite architectures such as nucleic acids and proteins. Carbon forms
more compounds than all other elements combined. The great versatility of the
carbon atom, and its abundance in the visible universe, makes it the element
most likely to provide the bases—even exotic ones—for the chemical composition
of life on other planets.
Planetary
habitability in the Solar System
Some bodies in the Solar
System have the potential for an environment in which extraterrestrial life
can exist, particularly those with possible subsurface oceans. Should life be discovered elsewhere in the Solar System, astrobiologists suggest
that it will more likely be in the form of extremophile microorganisms.
According to NASA's 2015 Astrobiology
Strategy, "Life on other worlds
is most likely to include microbes, and any complex living system elsewhere is
likely to have arisen from and be founded upon microbial life. Important insights
on the limits of microbial life can be gleaned from studies of microbes on
modern Earth, as well as their ubiquity and ancestral characteristics."
Researchers found a stunning array of
subterranean organisms, mostly microbial, deep underground and estimate that
approximately 70 percent of the total number of Earth's bacteria and archaea
organisms live within the Earth's crust. Rick
Colwell, a member of the Deep Carbon
Observatory team from Oregon State
University, told the BBC: "I
think it’s probably reasonable to assume that the subsurface of other planets
and their moons are habitable, especially since we’ve seen here on Earth that
organisms can function far away from sunlight using the energy provided
directly from the rocks deep underground".
Mars may have niche subsurface environments where microbial
life might exist. A subsurface marine
environment on Jupiter's moon Europa
might be the most likely habitat in the Solar
System, outside Earth, for extremophile microorganisms.
The panspermia hypothesis proposes that life elsewhere in
the Solar System may have a common
origin. If extraterrestrial life was found on another body in the Solar System, it could have originated
from Earth just as life on Earth could have been seeded from elsewhere
(exogenesis). The first known mention of
the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras. In the 19th
century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), Kelvin (1871), Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903). Sir
Fred Hoyle (1915–2001) and Chandra
Wickramasinghe (born 1939) are important proponents of the hypothesis who
further contended that life forms continue to enter Earth's atmosphere, and may
be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary
for macroevolution.
Directed panspermia concerns the deliberate transport of
microorganisms in space, sent to Earth to start life here, or sent from Earth
to seed new stellar systems with life. 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.
Venus
In the early 20th century, Venus was often thought to be similar to Earth in terms of
habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to
Earth-like life. However, between the altitudes of 50 and 65 kilometers, the
pressure and temperature are Earth-like, and it has been speculated that
thermoacidophilic extremophile microorganisms might exist in the acidic upper
layers of the Venusian atmosphere. Furthermore, Venus likely had liquid water
on its surface for at least a few million years after its formation.
Mars
Life on Mars has
been long speculated. Liquid water is widely thought to have existed on Mars in
the past, and now can occasionally be found as low-volume liquid brines in
shallow Martian soil. The origin of the
potential biosignature of methane observed in Mars' atmosphere is unexplained,
although hypotheses not involving life have also been proposed.
There is evidence that Mars had a warmer and wetter past:
dried-up river beds, polar ice caps, volcanoes, and minerals that form in the
presence of water have all been found. Nevertheless, present conditions on
Mars' subsurface may support life. Evidence obtained by the Curiosity rover studying Aeolis
Palus, Gale Crater in 2013
strongly suggests an ancient freshwater lake that could have been a hospitable
environment for microbial life.
Current studies on Mars by the Curiosity and Opportunity rovers
are searching for evidence of ancient life, including a biosphere based on
autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well
as ancient water, including fluvio-lacustrine environments (plains related to
ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy
(related to fossils), and organic carbon on Mars is now a primary NASA
objective.
Ceres
Ceres, the only
dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. Frost on the surface may also have been
detected in the form of bright spots. The presence of water on Ceres had led to
speculation that life may be possible there.
Jupiter system
Jupiter
Carl Sagan and
others in the 1960s and 1970s computed conditions for hypothetical
microorganisms living in the atmosphere of Jupiter. The intense radiation and other conditions,
however, do not appear to permit encapsulation and molecular biochemistry, so
life there is thought unlikely. In
contrast, some of Jupiter's moons may have habitats capable of sustaining life.
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 EJSM/Laplace mission is planned to
determine the habitability of these environments.
Europa
Internal structure of Europa. The blue is a
subsurface ocean. Such subsurface oceans could possibly harbor life.
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. Hydrothermal vents on
the bottom of the ocean, if they exist, may warm the water and could be capable
of supporting nutrients and energy to microorganisms. It is also possible that
Europa could support aerobic macrofauna using oxygen created by cosmic rays
impacting its surface ice.
The case for life on Europa was greatly enhanced in 2011
when it was discovered that vast lakes exist within Europa's thick, icy shell.
Scientists found that ice shelves surrounding the lakes appear to be collapsing
into them, thereby providing a mechanism through which life-forming chemicals
created in sunlit areas on Europa's surface could be transferred to its interior.
On 11 December 2013, NASA
reported the detection of "clay-like
minerals" (specifically, phyllosilicates), often associated with
organic materials, on the icy crust of Europa. The presence of the minerals may have been the
result of a collision with an asteroid or comet according to the
scientists. The Europa Clipper, which
would assess the habitability of Europa, is planned for launch in 2025. Europa's subsurface ocean is considered the
best target for the discovery of life.
Saturn system
Like Jupiter, Saturn is
not likely to host life. However, Titan
and Enceladus have been speculated to have possible habitats supportive of
life.
Enceladus
Enceladus, a moon
of Saturn, has some of the conditions for life, including geothermal activity
and water vapor, as well as possible under-ice oceans heated by tidal effects. The
Cassini–Huygens probe detected
carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during
its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The
temperature and density of the plumes indicate a warmer, watery source beneath
the surface.
Titan
Titan, the
largest moon of Saturn, is the only known moon in the Solar System 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 stable bodies of
surface liquid discovered outside Earth.
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.
Small Solar System
bodies
Small Solar System
bodies have also been speculated to host habitats for extremophiles. Fred Hoyle and Chandra Wickramasinghe have proposed that microbial life might
exist on comets and asteroids.
Other bodies
Models of heat retention and heating via radioactive decay
in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may
have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases is the
fact that the models indicate that the liquid layers are in direct contact with
the rocky core, which allows efficient mixing of minerals and salts into the
water. This is in contrast with the oceans that may be inside larger icy
satellites like Ganymede, Callisto, or Titan, where layers of high-pressure
phases of ice are thought to underlie the liquid water layer.
Hydrogen sulfide has been proposed as a hypothetical solvent
for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface.
Scientific search
The scientific search for extraterrestrial life is being
carried out both directly and indirectly. As of September 2017, 3,667
exoplanets in 2,747 systems have been identified, and other planets and moons
in our own solar system hold the potential for hosting primitive life such as
microorganisms.
Direct search
Lifeforms produce a variety of biosignatures that may be detectable
by telescopes.
Scientists search for biosignatures within the Solar System
by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial
life has existed on Mars. An experiment
on the two Viking Mars landers
reported gas emissions from heated Martian soil samples that some scientists
argue are consistent with the presence of living microorganisms. Lack of corroborating
evidence from other experiments on the same samples suggests that a
non-biological reaction is a more likely hypothesis. In 1996, a controversial report stated that
structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from
Mars.
In February 2005 NASA scientists reported they may have
found some evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke
of NASA's Ames Research Center, based
their claim on methane signatures found in Mars's 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 distanced NASA from the scientists' claims, and Stoker herself
backed off from her initial assertions. Though such methane findings are still
debated, support among some scientists for the existence of life on Mars
exists.
In November 2011 NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and
present habitability on Mars using a variety of scientific instruments. The
rover landed on Mars at Gale Crater in
August 2012.
The Gaia hypothesis stipulates that any planet with a robust
population of life will have an atmosphere in chemical disequilibrium, 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 their star are necessary before such spectroscopic methods
can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014
and is dedicated to the atmospheric characterization of exoplanets in circumstellar
habitable zones. Planetary spectroscopic data will be obtained from telescopes
like WFIRST and ELT.
In August 2011, findings by NASA, based on studies of
meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and
related organic molecules), building blocks for life as we know it, may be
formed extraterrestrially in outer space.
In October 2011, scientists reported that cosmic dust contains complex
organic matter ("amorphous organic
solids with a mixed aromatic-aliphatic structure") that could be
created naturally, and rapidly, by stars. One of the scientists suggested that these
compounds may have been related to the development of life on Earth and said
that, "If this is the case, life on
Earth may have had an easier time getting started as these organics can serve
as basic ingredients for life."
In August 2012 and in a world first, astronomers at Copenhagen University reported the
detection of a specific sugar molecule, glycolaldehyde, in a distant star
system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light
years from Earth. Glycolaldehyde is
needed to form ribonucleic acid, or RNA, which is similar in function to DNA.
This finding suggests that complex organic molecules may form in stellar
systems prior to the formation of planets, eventually arriving on young planets
early in their formation.
Indirect search
Projects such as SETI are monitoring the galaxy for
electromagnetic interstellar communications from civilizations on other worlds.
If there is an advanced extraterrestrial
civilization, there is no guarantee that it is transmitting radio
communications in the direction of Earth or that this information could be
interpreted as such by humans. The length of time required for a signal to
travel across the vastness of space means that any signal detected would come
from the distant past.
The presence of heavy elements in a star's light-spectrum is
another potential biosignature; such elements would (in theory) be found if the
star was being used as an incinerator/repository for nuclear waste products.
Extrasolar planets
Some astronomers search for extrasolar planets that may be
conducive to life, narrowing the search to terrestrial planets within the
habitable zone of their star. Since 1992
over two thousand exoplanets have been discovered (4,126 planets in 3,067
planetary systems including 671 multiple planetary systems as of 1 November
2019). The extrasolar planets so far
discovered range in size from that of terrestrial planets similar to Earth's
size to that of gas giants larger than Jupiter. The number of observed exoplanets is expected
to increase greatly in the coming years.
The Kepler space telescope has also detected a few thousand
candidate planets, of which about 11% may be false positives.
There is at least one planet on average per star. About 1 in 5 Sun-like stars
[a] have an "Earth-sized"
[b] Planet in the habitable zone,
[c] With the nearest expected to be within 12 light-years
distance from Earth. Assuming 200
billion stars in the Milky Way,
[d] That would be 11 billion potentially habitable
Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are
included.
The rogue planets in the Milky Way possibly number in the
trillions.
The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in
the southern constellation of Centaurus.
As of March 2014, the least massive planet known is PSR B1257+12 A, which is about twice the
mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b, about 29
times the mass of Jupiter, although according to most definitions of a planet,
it is too massive to be a planet and may be a brown dwarf instead. Almost all
of the planets detected so far are within the Milky Way, but there have also
been a few possible detections of extragalactic planets. The study of planetary
habitability also considers a wide range of other factors in determining the
suitability of a planet for hosting life.
One sign that a planet probably already contains life is the
presence of an atmosphere with significant amounts of oxygen, since that gas is
highly reactive and generally would not last long without constant
replenishment. This replenishment occurs on Earth through photosynthetic
organisms. One way to analyze the atmosphere of an exoplanet is through
spectrography when it transits its star, though this might only be feasible
with dim stars like white dwarfs.
Terrestrial analysis
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."
The Drake equation
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. The
equation is best understood not as an equation in the strictly mathematical
sense, but to summarize all the various concepts which scientists must contemplate
when considering the question of life elsewhere. The Drake
equation is:
N = R ∗ ⋅ f p ⋅
n e ⋅ f ℓ ⋅
f i ⋅ f c ⋅ L {\displaystyle N=R_{\ast }\cdot f_{p}\cdot
n_{e}\cdot f_{\ell }\cdot f_{i}\cdot f_{c}\cdot L} N=R_{\ast }\cdot f_{p}\cdot
n_{e}\cdot f_{\ell }\cdot f_{i}\cdot f_{c}\cdot L
where:
N = the number of
Milky Way galaxy civilizations already capable of communicating across
interplanetary space
And
·
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
Drake's proposed estimates are as follows, but numbers on
the right side of the equation are agreed as speculative and open to
substitution:
10,000 = 5 ⋅ 0.5 ⋅ 2 ⋅
1 ⋅ 0.2 ⋅ 1 ⋅
10,000 {\displaystyle 10{,}000=5\cdot 0.5\cdot 2\cdot 1\cdot 0.2\cdot 1\cdot
10{,}000} {\displaystyle 10{,}000=5\cdot 0.5\cdot 2\cdot 1\cdot 0.2\cdot 1\cdot
10{,}000}
The Drake equation has proved controversial since several of
its factors are uncertain and based on conjecture, not allowing conclusions to
be made. This has led critics to label
the equation a guesstimate, or even meaningless.
Based on observations from the Hubble Space Telescope, there are between 125 and 250 billion
galaxies in the observable 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
observable universe. Even if it is assumed that only one out of a billion of
these stars has planets supporting life, there would be some 6.25 billion
life-supporting planetary systems in the observable universe.
A 2013 study based on results from the Kepler spacecraft
estimated that the Milky Way contains at least as many planets as it does
stars, resulting in 100–400 billion exoplanets. Also based on Kepler data, scientists estimate
that at least one in six stars has an Earth-sized planet.
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.
Cultural impact
Cosmic pluralism
Cosmic pluralism, the plurality of worlds, or simply
pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor
extraterrestrial life. Before the development of the heliocentric theory and a
recognition that the Sun is just one of many stars, the notion of pluralism was
largely mythological and philosophical. The earliest recorded assertion of
extraterrestrial human life is found in ancient scriptures of Jainism. There
are multiple "worlds"
mentioned in Jain scriptures that support human life. These include Bharat Kshetra, Mahavideh Kshetra, Airavat
Kshetra, Hari kshetra,etc. Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic
pluralism on the basis of the Qur'an.
With the scientific and Copernican
revolutions, and later, during the Enlightenment,
cosmic pluralism became a mainstream notion, supported by the likes of Bernard le Bovier de Fontenelle in his
1686 work Entretiens sur la pluralité des
mondes. Pluralism was also
championed by philosophers such as John
Locke, Giordano Bruno and astronomers such as William Herschel. The astronomer Camille Flammarion promoted the notion of cosmic pluralism in his
1862 book La pluralité des mondes
habités. None of these notions of
pluralism were based on any specific observation or scientific information.
Early modern period
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 Earth
was merely one planet amongst countless bodies in the universe, the theory of
extraterrestrial life started to become a topic in the scientific community.
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 wrote that
other worlds "have no less virtue or a nature different to that of our
earth" and, like Earth, "contain
animals and inhabitants".
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 Earth, in proportion to the [characteristics] of the two spheres".
In Baroque literature such as The Other World: The Societies and Governments of the Moon by Cyrano de Bergerac, extraterrestrial
societies are presented as humoristic or ironic parodies of earthly society.
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). 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". 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 who believed that the Solar System is 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.
19th century
Speculation about life on Mars increased in the late 19th
century, following telescopic observation of apparent Martian canals—which
soon, however, turned out to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the
work of a long-gone civilization. The
idea of life on Mars led British writer H.
G. Wells to write the novel The War
of the Worlds in 1897, telling of an invasion by aliens from Mars who were
fleeing the planet's desiccation.
Spectroscopic analysis of Mars's atmosphere began in earnest
in 1894, when U.S. astronomer William
Wallace Campbell showed that neither water nor oxygen was present in the
Martian atmosphere. By 1909 better telescopes and the best perihelic opposition
of Mars since 1877 conclusively put an end to the canal hypothesis.
The science fiction genre, although not so named during the
time, developed during the late 19th century. Jules Verne's Around the Moon (1870) features a
discussion of the possibility of life on the Moon, but with the conclusion that
it is barren. Stories involving extraterrestrials are found in e.g. Garrett
P. Serviss's Edison's Conquest of
Mars (1898), an unauthorized sequel to The
War of the Worlds by H. G. Wells
was published in 1897 which stands at the beginning of the popular idea of the "Martian invasion" of Earth
prominent in 20th-century pop culture.
20th century
Most unidentified flying objects or UFO sightings can be
readily explained as sightings of Earth-based aircraft, known astronomical
objects, or as hoaxes. Nonetheless, a
certain fraction of the public believe that UFOs might actually be of
extraterrestrial origin, and, indeed, the notion has had influence on popular
culture.
The possibility of extraterrestrial life on the Moon was
ruled out in the 1960s, and during the 1970s it became clear that most of the
other bodies of the Solar System do not harbor highly developed life, although
the question of primitive life on bodies in the Solar System remains open.
Recent history
The failure so far of the SETI program to detect an
intelligent radio signal after decades of effort has at least partially dimmed
the prevailing optimism of the beginning of the space age. Notwithstanding,
belief in extraterrestrial beings continues to be voiced in pseudoscience,
conspiracy theories, and in popular folklore, notably "Area 51" and legends. It has become a pop culture trope
given less-than-serious treatment in popular entertainment.
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 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, detected in 1977 by a
SETI project, remains a subject of speculative debate.
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, whereas microbial life is common. Ward and Brownlee are
open to the idea of evolution on other planets that is not based on essential
Earth-like characteristics (such as DNA and carbon).
Theoretical physicist Stephen
Hawking in 2010 warned that humans should not try to contact alien life
forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would
be much as when Columbus landed in America, which didn't turn out well for the
Native Americans", he said. Jared Diamond had earlier expressed similar
concerns.
In 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 may be 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."
In 2013, the exoplanet Kepler-62f was discovered, along with
Kepler-62e and Kepler-62c. A related special issue of the journal Science,
published earlier, described the discovery of the exoplanets.
On 17 April 2014, the discovery of the Earth-size exoplanet
Kepler-186f, 500 light-years from Earth, was publicly announced; it is the first Earth-size planet to be
discovered in the habitable zone and it has been hypothesized that there may be
liquid water on its surface.
On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake
and David Brin) at a convention of the American
Association for the Advancement of Science, discussed Active SETI and
whether transmitting a message to possible intelligent extraterrestrials in the
Cosmos was a good idea; one result was a
statement, signed by many, that a "worldwide
scientific, political and humanitarian discussion must occur before any message
is sent".
On 20 July 2015, British physicist Stephen Hawking and Russian billionaire Yuri Milner, along with the SETI Institute, announced a well-funded
effort, called the Breakthrough Initiatives, to expand efforts to search for
extraterrestrial life. The group contracted the services of the 100-meter Robert C. Byrd Green Bank Telescope in West Virginia in the United States and the 64-meter Parkes Telescope in New South Wales, Australia.
