Extraterrestrial life

Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the universe known to support life. Its existence is currently hypothetical; there is yet no evidence of extraterrestrial life that has been widely accepted by the scientific community.

Speculative forms of extraterrestrial life range from humanoid and monstrous beings seen in works of science fiction to life at the much smaller scale of bacteria. Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens or ETs. The putative study and theorisation of ET life is known as astrobiology or xenobiology.

Historical Belief in Extraterrestrial Life

Astronomy played a significant role in ancient societies, the arrangment of heavenly bodies inspiring cultural beliefs and especially mythology. Beliefs in creatures and lifeforms that populated the sky world became almost common place in the ancient world. The atomists of Greece took up the idea of an infinite universe, with an infinity of populated worlds. The Jewish Talmud states that there are at least 18,000 other worlds. The Babylonians, Egyptians, Sumerians, Chinese and Aztecs all held somewhat similar ideas of multiple worlds that existed amongst the stars. However, these beliefs were often spiritually focused, as the sky and stars was often the domain of dieties and supernatural powers. The notion of alien biological lifeforms as they are thought of today, did take shape for centuries.

Giordano Bruno, De l'Infinito, Univirso e Mondi, 1584

This situation changed, however, with the 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 Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own solar system; he was eventually burned at the stake for heretical ideas. Dominican monk Tommaso Campanella wrote about a Solarian alien race in his Civitas Solis. 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 our 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 hosting aliens.

Extraterrestrials and the Modern era

This enthusiasm towards the possibility of alien life continued well into the 20th century. As long as human kind was unable to travel into space, extraterrestrial life seemed as probable to scientists and the general public alike. However, a split in the belief in E.T. life forms occured in the beginning half of the 20th century. With advancement in the scientific areas of physics, astronomy and evolutionary biology, scientists started to understand how complicated and fragile it was for life to develop on Earth, let alone elsewhere in a largely dangerous and inhospitable universe. Furthermore, the early explorations of space by probes showed no signs of life on the barren planet of Mars and exposed Venus to be a planet of such dramatic climate it is not condusive to support any known forms of life. The major scientific arguement against extraterrestrial life was formed; Earth was unique in its evolution, randomly composed of the exact conditions to support the fragile, and somewhat random itself, process that led to life. Most scientists believed, and still do that for such conditions to happen twice, even in a universe as big as ours, is unlikely (See Scientific Debate below)

Yet, as science was dismissing the notion of extraterrestrial life, it was becoming more popular in the public's imagination. The connection between extraterrestrials, the sudden frequent reports of UFOs, and the anthropromorphizing of extraterrestrals into the common identity of aliens, as they are known today, happened in a time period between 1920 and 1950. The possibility of intelligent life forms became an almost alluring mystery to the public. Early science fiction of the period used aliens frequently in its stories. Even those who claimed not to believe in aliens, shared a widespread fear of malevolent creatures from space, possibly best demonstrated by Orson Welles' infamous radio broadcast of The World of the Wars in which hundreds of people in the New York and New Jersey area were led to believe an invasion by aliens was taking place.

Artistic depiction of a Grey Alien

Belief in aliens has become one of the most interesting and complicated aspects of pop culture, the paranormal and the new age movement in the later half of the 20th century. Not only is commonly believed that UFOs are in fact alien space craft, but reports of contact between humans and aliens have been prevalent since the 1950s. One popular phenomeon is the abduction of humans by aliens, during which humans are unwittingly brought aboard space craft and subjected to medical examinations. Conspiracy theorists have claimed that world governments have clandestine relationships with aliens or are otherwise covering up alien existence. Some new age socities believe aliens hold a special place in the spiritual realm. These beliefs are fueled by overwhelming reports, observations, and the media, in which aliens are commonplace elements of movies and television shows, as well as books and documentaries. Commonly, these aliens are seen as small, with greyish skin, large head and black, lidless eyes. They are sometimes refered to as Greys. Other depictions percieve aliens as repitilian, insect-like, or even resembling people of Nordic descent.

File:Grey alien close1.jpg

A grey alien close-up

Aliens are depicted in numerous different ways; some portray and believe aliens exist to help mankind, while other times aliens are seen as hostile and using mankind for their own advancement. Either way, the notion that there are other life forms that possess similar intelligence and cognitant abilities as humans seems to strike a cord in humanity. Perhaps it is an overactive imagination projecting onto the infinity of the cosmos, of which we still know little. Or perhaps the fear of the ultimate unknown, that which is alien to mankind that helps fuel the belief.

Scientific Approach

Since there is no solid proof for the existence of extraterrestrial life, most of the scientific community since the mid 20th century have not given the idea much thought, especially with the stigma of the more outlandish pop culture perspectives on the matter. However, there have been, and continue to be, scientists who engage the idea, on many different levels.

Basis of Life

In order for something to be considered a living organism, there are five biological criteria that must be met: ability to reproduce and grow, it consume nutrients in order to survive, have mobility and respond to external stimuli Further, all living organisms on earth are based on the element carbon, depend on water as a meduim for crucial chemical reactions, and are made up of long strands of amino acids, known as DNA — Deoxyribonucleic acid. The difficulty with trying to understand possible other life forms, is that science currently is unaware of any other composition of life (there has been some speculation that instead of carbon, the element silicon may consistitue the basis of extraterrestrial life). Therefore, either life in the universe can only exist on planets with similar water and carbon levels and tempeture of Earth, or life exists in some form that we currently do not know about, and raises the issue of how to look for something that does not correspond to current understandings.

Speculation

Without concrete facts to analyze or direct phenomena to observe, most scientists studying extraterrestrial life must do so by speculation (Most speculation is, however, based on what is accepted knowledge of astronomy and biology, as well as careful scientific postulating). Even with so many unknown variables, such postulating requires that specific guidelines be established in order to further pursue the notion. For example, scientists must first try to understand if the evolution of extraterrestrial life would have occurred independently in different places in the universe, or if, as a minority of scientists contend, life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet, known as panspermia.

In considering the subject more seriously, a useful division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not 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 have evolved several times here on Earth with differing materialization. There are a huge variety of eyes, for example, many of which have radically different working schematics as well as different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, by contrast, are essentially arbitrary evolutionary forms which often serve little utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Parochials include the five digits of mammals, the genitalia and sexual mechanics of animals, as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals.

A consideration of which features are ultimately parochial challenges many taken for granted notions about morphological necessity. Skeletons, in some form, are likely to be replicated elsewhere, yet the vertebrate spine—while a profound development on Earth—is just as likely to be unique. Similarly, it is reasonable to expect some type of egg laying amongst off-Earth creatures but the mammary glands which set apart mammals may 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 Earth life foregrounds even greater variety in space, others point out that convergent evolution dictates substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism," respectively.

Currently, Astrobiology is a field of study that has gained legitimacy and acceptance in the scientific community. Its main objectives include the search for extraterrestrial life, but in doing so seek to develop a better understanding of life on Earth

The Arecibo message is a digital message sent to globular star cluster M13, and is a well-known symbol of human attempts to contact extraterrestrials.

The Search For Extraterrestrials

Since the last quarter of the 20th century, scientists have been actively searching the universe for other lifeforms with all the different types of technology availble. Yet, even if all of NASA and the The European Space Agency's budgets were directed towards this one goal, the universe is still too large and our technological means still not adequate enough to exhaustively search those areas in space that we know are possibily harboring life. Still, there are undaunted scientists who contuine to search for possible life, whether it be bacteria or advanced civilizations. There are two different methods scientists use in this search: the direct and indirect method.

The Direct Method

Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors that have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a 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. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial. Independently in 1996 structures resembling bacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial and scientific debate continues. (See Viking biological experiments.)

In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as 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. However, only a few days after Stoker and Lemke made their claims, scientists from the European Space Agency reported that their own measurements of methane on Mars suggested an organic origin.

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 in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there.

Terrestrial Planet Finder - Infrared interferometer concept of finding Earth-like extrasolar planets

Astronomers also search for extrasolar planets that would be conducive to life, especially those like 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 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). Darwin is an ESA mission designed to find Earth-like planets, and analyse their atmosphere. It has been argued that one of the best candidates for the discovery of life-supporting planets may be Alpha Centauri, the closest star system to Earth, given that two of the three stars in the system are broadly sun-like.

Indirect search

It is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radio activity that 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.

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma," after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. Nothing of great interest was found.

SETI (pronounced ['sɛti]) is the acronym for Search for Extra-Terrestrial Intelligence; organized efforts by humans to detect intelligent aliens. A number of efforts with "SETI" in the project name have been organized, including projects funded by the United States Government. The generic approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet - an approach widely endorsed by the scientific community as hard science.

There are great challenges in searching across the sky to detect a first transmission that can be characterised as intelligent, since its direction, spectrum and method of communication are all unknown beforehand. SETI projects necessarily make assumptions to narrow the search, and thus no exhaustive search has so far been conducted. Visiting another civilization on a distant world is presently beyond human capabilities (see Project Orion and Project Daedalus for some hypothetical explorations of the concept). However, it is currently technologically feasible to develop a communications system which uses a powerful transmitter and a sensitive receiver to search the sky for extraterrestrial worlds whose citizens have a similar inclination as terrestrials.

SETI is not generally viewed by scientists as a trivial task. Our galaxy, the Milky Way, is 100,000 light years across and contains approximately a hundred billion stars. Searching the entire sky for some far-away and faint signal is an exhausting exercise. A number of assumptions are needed for SETI to be feasible.

A basic assumption of SETI is that of "Mediocrity": the idea that humanity is not exotic in the cosmos but in a sense "typical" or "medium" when compared with other intelligent species.^{[citation needed]} This would mean that humanity has sufficient similarities with other intelligent beings that communications would be mutually desirable and understandable. If this basic assumption of Mediocrity is correct, and other intelligent species are present in any number in the galaxy at our technological level or above, then communications between the two worlds should be inevitable.

Another assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that intelligent life would likely not have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally locked to the star, with one side of the planet perpetually baked and the other perpetually frozen. (However, some speculate that a thick cloud cover may mitigate these differences. [1])

About 10% of the stars in the Milky Way galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of the Sun. These stars would be useful primary targets for interstellar listening.

However, we know of only one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct, and so as a second priority the entire sky must be searched.

Searching the Electromagnetic Spectrum

In order to find an electromagnetic transmission from an alien civilization we also have to search through most of the useful radio spectrum, as there is no way to know what frequencies aliens might be using. Trying to transmit a powerful signal over a wide range of wavelengths is impractical, and so it is likely that such a signal would be transmitted on a relatively narrow band. This means that a wide range of frequencies must be searched at every spatial coordinate of the sky.

There is also the problem of knowing what to listen for, as we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it might be encoded. Narrow-bandwidth signals that are stronger than background noise and constant in intensity are obviously interesting, and if they have a regular and complex pulse pattern are likely to be artificial.

However, while studies have been performed on how to send a signal that could be easily deciphered, there is no way to know if the assumptions of those studies are valid, and deciphering the information from an alien signal could be very difficult.

Arecibo message

In 1974, a largely symbolic attempt was made to send a message to other worlds. To celebrate a substantial upgrading of the 305 meter Arecibo Radio Telescope in Puerto Rico, a coded message of 1,679 bits was transmitted towards the Globular Cluster M13, about 25,100 light years away.

The pattern of 0s and 1s contained in the message defines a 23 × 73 two dimensional grid which when plotted reveals some data about our location in the Solar System, a stylized figure of a human being, chemical formulae and an outline of the radio telescope itself. The 23 by 73 grid was chosen because both 23 and 73 are prime numbers, which makes it easier to decode the message.

Given the limitations of the speed of light for message transmission, no reply would be possible before the year 52,174 (approximately) and hence has been dismissed by some as a publicity stunt. A controversy arose because the transmission raised the serious question of whether a small group should be allowed to speak for Earth.

Criticism of SETI

SETI has occasionally been the target of criticism by those who suggest that it is a form of pseudoscience. In particular, critics allege that no observed phenomena suggest the existence of extraterrestrial intelligence, and furthermore that the assertion of the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability [2]. Science fiction writer Michael Crichton, in a 2003 lecture at Caltech, stated that "The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion." [3].

In response, SETI advocates note, among other things, that the existence of intelligent life on Earth is a plausible reason to expect it elsewhere, and that individual SETI projects have clearly defined "stop" conditions. Concerning the latter argument, the justification for SETI projects doesn't necessarily require an acceptance of the Drake equation. The search for extra-terrestrial intelligence is not an assertion that extra-terrestrial intelligence exists, and conflating the two can be seen as a straw man argument. There is an effort to distinguish the SETI projects from UFOlogy, the study of UFOs considered to be pseudoscience by many.

In 1983 Stanislaw Lem, disappointed with the lack of results of SETI in spite of the huge effort, has noted that he considers the Universe silent, coining the moniker silencium universi.

Further reading

• Exers, Ronald, D. Cullers, J. Billingham, L. Scheffer (editors) (2003). SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence. SETI Press. ISBN 0-9666335-3-9. 
• McConnell, Brian and Chuck Toporek (2001). Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations. O'Reilly. ISBN 0-596-00037-5. 
• Roth, Christopher F., "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In E.T. Culture: Anthropology in Outerspaces, ed. by Debbora Battaglia. Durham, N.C.: Duke University Press, 2005.
• Sagan, Carl. 1996. The Demon-Haunted World: Science as a Candle in the Dark: chapter 4: "Aliens"
• David Grinspoon (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 0-06-018540-6. 
• Jack Cohen and Ian Stewart (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2. 
• John C. Baird (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 0-87451-406-1. 
• Donald Goldsmith (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 0-525-94336-6. 
• Michael T. Lemnick (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book. 
• Cliff Pickover (2003). The Science of Aliens. New York: Basic Books. ISBN 0-465-07315-8. 
• Roth, Christopher F. (2005) "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In E.T. Culture: Anthropology in Outerspaces, ed. by Debbora Battaglia. Durham, N.C.: Duke University Press.
• Crowe, Michael J.. The Extraterrestrial Life Debate, 1750—1900. Dover Publications. 
• Sagan, Carl and I.S. Shklovskii (1966). Intelligent Life in the Universe. Random House. 
• Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. 

External links

• NASA: Astrobiology
• PBS: Life Beyond Earth a film by Timothy Ferris
• PBS: Exploring Space - The Quest for Life by Scott Pearson
• Xenopsychology by Robert A. Freitas Jr.
• "What Aliens Might Look Like" from National Geographic
• Top stars picked in alien search
• SETI Institute
• The Planetary Society
• The SETI League
• International Academy of Astronautics
• SETI Principles as defined from the Department of Space Studies of the Southwest Research Institute (PDF File)
• SETI: Searching for Life – Article series in Sky & Telescope magazine
• Cirkovic, Milan M., and Bradbury, R. J., 200n, "Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI."
• Searching for Good Science: The Cancellation of NASA's SETI Program (PDF) – By Stephen J. Garber, NASA History Office. Journal of the British Interplanetary Society, Vol. 52, pp. 3-12, 1999. Provides more details on the elimination of SETI funding by the US Congress in 1993.