Astrobiology

Astrobiology

Discussion
Ask a question about 'Astrobiology'
Start a new discussion about 'Astrobiology'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia

Astrobiology is the study of the origin, evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

, distribution, and future of life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

. This interdisciplinary field encompasses the search for habitable environments in our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 and habitable planets
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 outside our Solar System, the search for evidence of prebiotic chemistry
Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

, laboratory and field research into the origins and early evolution of life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

, and studies of the potential for life to adapt to challenges on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and in outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

. Astrobiology addresses the question of whether life exists beyond Earth, and how humans can detect it if it does. (The term exobiology is similar but more specific — it covers the search for life beyond Earth, and the effects of extraterrestrial environments on living things)

Astrobiology makes use of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

, ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

, planetary science
Planetary science
Planetary science is the scientific study of planets , moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation,...

, geography
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

, and geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 to investigate the possibility of life on other worlds and help recognize biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

s that might be different from the biosphere on Earth. Astrobiology concerns itself with interpretation of existing scientific data
Scientific method
Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...

; given more detailed and reliable data from other parts of the universe, the roots of astrobiology itself—physics, chemistry and biology—may have their theoretical bases challenged. Although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

Earth is the only place in the universe known to harbor life. However, recent advances in planetary science have changed fundamental assumptions about the possibility of life in the universe, raising the estimates of habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

s around other stars and the search for extraterrestrial microbial life. The possibility of life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...

, either currently or in the past, is an active area of research.

Overview


Astrobiology is etymologically derived from the Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 , astron, "constellation, star"; , bios, "life"; and , -logia, study. The synonyms of Astrobiology are diverse; however, the synonyms were structured in relation to the most important sciences implied in its development: astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

. A synonym is exobiology from the Greek , "external"; Βίος, bios, "life"; and λογία, -logia, study. Another term used in the past is xenobiology, ("biology of the foreigners") a word coined in 1954 by science fiction writer Robert Heinlein in his work The Star Beast
The Star Beast
The Star Beast is a 1954 science fiction novel by Robert A. Heinlein about a high school senior who discovers that his late father's extraterrestrial pet is more than it appears to be...

.

While it is an emerging and developing field, the question of whether life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 exists elsewhere in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 is a verifiable hypothesis and thus a valid line of scientific
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 inquiry. Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. Planetary scientist David Grinspoon
David Grinspoon
David H. Grinspoon is an American astrobiologist. He is the current curator of Astrobiology at the Denver Museum of Nature and Science. He has published numerous works, such as Lonely Planets: The Natural Philosophy of Alien Life, which won the 2004 PEN literary award for nonfiction.Currently, he...

 calls astrobiology a field of natural philosophy, grounding speculation on the unknown, in known scientific theory. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's interest in exobiology first began with the development of the U.S. Space Program. In 1959, NASA funded its first exobiology project, and in 1960, NASA founded an Exobiology Program; Exobiology research is now one of four elements of NASA's current Astrobiology Program. In 1971, NASA funded the Search for Extra-Terrestrial Intelligence (SETI) to search radio frequencies of the electromagnetic spectrum for signals
Interstellar communication
Interstellar communication is the transmission of signals between planetary systems. Sending Interstellar messages is potentially much easier than interstellar travel, being possible with technologies and equipment which are currently available...

 being transmitted by extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

 outside the Solar System. NASA's Viking missions
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...

 to Mars, launched in 1976, included three biology experiments
Viking biological experiments
The two Viking spacecraft each carried four types of biological experiments to the surface of Mars in the late 1970s. These were the first Mars landers to carry out experiments to look for biosignatures of life on Mars. The landers used a robotic arm to put soil samples into sealed test containers...

 designed to look for possible signs
Biosignature
A biosignature is any substance -such as an element, isotope, or molecule - or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of...

 of present life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...

. The Mars Pathfinder
Mars Pathfinder
Mars Pathfinder was an American spacecraft that landed a base station with roving probe on Mars in 1997. It consisted of a lander, renamed the Carl Sagan Memorial Station, and a lightweight wheeled robotic rover named Sojourner.Launched on December 4, 1996 by NASA aboard a Delta II booster a...

 lander in 1997 carried a scientific payload intended for exopaleontology in the hopes of finding microbial fossils entombed in the rocks.

In the 21st century, astrobiology is a focus of a growing number of NASA and European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 Solar System exploration missions. The first European workshop on astrobiology took place in May 2001 in Italy, and the outcome was the Aurora programme
Aurora Programme
The Aurora programme is a human spaceflight programme of the European Space Agency established in 2001 with the primary objectives of creating, and then implementing, a European long-term plan for exploration of the Solar System using robotic spacecraft and human spaceflight...

. Currently, NASA hosts the NASA Astrobiology Institute
NASA Astrobiology Institute
The NASA Astrobiology Institute was established in 1998 by the National Aeronautics and Space Administration "to develop the field of astrobiology and provide a scientific framework for flight missions". The NAI is a virtual, distributed organization that integrates astrobiology research and...

 and a growing number of universities in the United States (e.g., University of Arizona
University of Arizona
The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...

, Penn State University, Montana State University, University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

, and Arizona State University
Arizona State University
Arizona State University is a public research university located in the Phoenix Metropolitan Area of the State of Arizona...

), Britain (e.g., The University of Glamorgan
University of Glamorgan
The University of Glamorgan is a university based in Pontypridd, Rhondda Cynon Taf, Wales with campuses in Treforest, Glyntaff, Merthyr Tydfil, Tyn y Wern and Cardiff...

), Canada, Ireland, and Australia (e.g., The University of New South Wales
University of New South Wales
The University of New South Wales , is a research-focused university based in Kensington, a suburb in Sydney, New South Wales, Australia...

) now offer graduate degree programs in astrobiology. The International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 regularly organizes international conferences through its Bioastronomy Commission.

Advancements in the fields of astrobiology, observational astronomy and discovery of large varieties of extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

s with extraordinary capability to thrive in the harshest environments on Earth, have led to speculation that life may possibly be thriving on many of the extraterrestrial bodies in the universe. A particular focus of current astrobiology research is the search for life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...

 due to its proximity to Earth and geological history. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 on its surface, water being considered an essential precursor to the development of carbon-based life.

Missions specifically designed to search for life include the Viking program
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...

 and Beagle 2
Beagle 2
Beagle 2 was an unsuccessful British landing spacecraft that formed part of the European Space Agency's 2003 Mars Express mission. All contact with it was lost upon its separation from the Mars Express six days before its scheduled entry into the atmosphere...

 probes, both directed to Mars. The Viking results were inconclusive, and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter
Jupiter Icy Moons Orbiter
The Jupiter Icy Moons Orbiter was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system...

, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. Recently, the Phoenix lander
Phoenix (spacecraft)
Phoenix was a robotic spacecraft on a space exploration mission on Mars under the Mars Scout Program. The Phoenix lander descended on Mars on May 25, 2008...

 probed the environment for past and present planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 of microbial life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...

, and to research the history of water there.

In November 2011, NASA plans to launch the Mars Science Laboratory
Mars Science Laboratory
The Mars Science Laboratory is a National Aeronautics and Space Administration mission with the aim to land and operate a rover named Curiosity on the surface of Mars. The MSL was launched November 26, 2011, at 10:02 EST and is scheduled to land on Mars at Gale Crater between August 6 and 20, 2012...

 (MSL) rover which will continue the search for past or present life on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 using a variety of scientific instruments. The MSL is scheduled to land on Mars at Gale Crater
Gale (crater)
Gale is a crater on Mars, near the border of the lowlands of Elysium Planitia at . It is 154 km in diameter and believed to be about 3.5 to 3.8 billion years old...

 in August 2012. The European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 has been developing the ExoMars
ExoMars
ExoMars is a European-led robotic mission to Mars currently under development by the European Space Agency with collaboration by NASA...

 astrobiology rover, which is to be launched in 2018.

Planetary habitability


When looking for life on other planets like the earth, some simplifying assumptions are useful to reduce the size of the task of the astrobiologist. One is to assume that the vast majority of life forms in our galaxy are based on carbon chemistries
Carbon-based life
Carbon forms the backbone of biology for all of life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen, and carbon is able to bond with all of these because of its four valence electrons. Carbon is abundant on earth...

, as are all life forms on Earth. While it is possible that non-carbon-based life forms
Alternative biochemistry
Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define...

 exist, carbon is well known for the unusually wide variety of molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s that can be formed around it. Carbon is the fourth most abundant element
Abundance of the chemical elements
The abundance of a chemical element measures how relatively common the element is, or how much of the element is present in a given environment by comparison to all other elements...

 in the universe and the energy required to make or break a bond is just at an appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of arbitrarily long and complex molecules
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

.

The presence of liquid water is a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life. Some researchers posit environments of ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, or more likely, water-ammonia mixtures.

A third assumption is to focus on sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

-like star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s. This comes from the idea of planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

. Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s 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. Without a thick atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

, one side of the planet would be perpetually baked and the other perpetually frozen. In 2005, the question was brought back to the attention of the scientific community
Scientific community
The scientific community consists of the total body of scientists, its relationships and interactions. It is normally divided into "sub-communities" each working on a particular field within science. Objectivity is expected to be achieved by the scientific method...

, as the long lifetimes of red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

s could allow some biology on planets with thick atmospheres. This is significant, as red dwarfs are extremely common. (See Habitability of red dwarf systems
Habitability of red dwarf systems
Determining the habitability of red dwarf systems could help reveal the likelihood of extraterrestrial life, as red dwarfs make up most stars in the Milky Way Galaxy...

).

It is estimated that 10% of the stars in our galaxy are sun-like; there are about a thousand such stars within 100 light-years of our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. These stars would be useful primary targets
Project Phoenix (SETI)
Project Phoenix is a SETI project: a search for extraterrestrial intelligence by analyzing patterns in radio signals. It is run by the independently funded SETI Institute of Mountain View, California, USA....

 for interstellar listening. Since Earth is the only planet known to harbor life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, there is no evident way to know if any of the simplifying assumptions are correct.

Communication attempts


Research on communication with extraterrestrial intelligence (CETI) focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. Communication attempts by humans have included broadcasting mathematical languages, pictorial systems such as the Arecibo message
Arecibo message
The Arecibo message was broadcast into space a single time via frequency modulated radio waves at a ceremony to mark the remodeling of the Arecibo radio telescope on 16 November 1974. It was aimed at the globular star cluster M13 some 25,000 light years away because M13 was a large and close...

 and computational approaches to detecting and deciphering 'natural' language communication. The SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...

 program, for example, uses both radio telescope
Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...

s and optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

s to search for deliberate signals from extraterrestrial intelligence.

While some high-profile scientists, such as Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

, have advocated the transmission of messages, scientist Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 has warned against it, suggesting that aliens might simply raid Earth for its resources and then move on.

Astronomy


Most astronomy-related astrobiological research falls into the category of extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-like exoplanets are under development, most notably NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Terrestrial Planet Finder
Terrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...

 (TPF) and ESA's
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 Darwin
Darwin (ESA)
Darwin was a suggested ESA Cornerstone mission which would have involved a constellation of four to nine spacecraft designed to directly detect Earth-like planets orbiting nearby stars and search for evidence of life on these planets...

 programs. Additionally, NASA has launched the Kepler mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 in March 2009, and the French Space Agency has launched the COROT
Corot
Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...

 space mission in 2006. There are also several less ambitious ground-based efforts underway. (See exoplanet).

The goal of these missions is not only to detect Earth-sized planets, but also to directly detect light from the planet so that it may be studied spectroscopically
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface; given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory, is using computer modeling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life. The photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 temporal variability of extrasolar planets may also provide clues to their surface and atmospheric properties.

An estimate for the number of planets with intelligent extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

 can be gleaned from the Drake equation
Drake equation
The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...

, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:


where, N = The number of communicative civilizations, R* = The rate of formation of suitable stars (stars such as our Sun), fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun), ne = The number of Earth-like worlds per planetary system, fl = The fraction of those Earth-like planets where life actually develops, fi = The fraction of life sites where intelligence develops, fc = The fraction of communicative planets (those on which electromagnetic communications technology develops), L = The "lifetime" of communicating civilizations.

However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable error limits any time soon. The first term, Number of Stars, is generally constrained within a few orders of magnitude. The second and third terms, Stars with Planets and Planets with Habitable Conditions, are being evaluated for the sun's neighborhood. The problem of the formula is that it is not usable to emit hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 because it contains units that can never be verified. It has permitted that the applications of the formula had been related to simplistic or pseudoscientific arguments. Another associated topic is the Fermi paradox
Fermi paradox
The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations....

, which suggests that if intelligent life is common in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

, then there should be obvious signs of it. This is the purpose of projects like SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...

, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations.

Another active research area in astrobiology is Planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

 formation. It has been suggested that the peculiarities of our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 (for example, the presence of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 as a protective shield) may have greatly increased the probability of intelligent life arising on our planet. No firm conclusions have been reached so far.

Biology


Biology and chemistry, as opposed to physics, do not admit ideological contexts: either the biological phenomena are real, or they are abstract. Biologists cannot say that a process or phenomenon, by being mathematically possible, have to exist forcibly in the real nature. For biologists, the ground of speculations is well noticeable, and biologists specify what is speculative and what is not.

Until the 1970s, life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 was believed to be entirely dependent on energy from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. Plants on Earth's surface capture energy from sunlight to photosynthesize
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 sugars from carbon dioxide and water, releasing oxygen in the process, and are then eaten by oxygen-respiring animals, passing their energy up the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. A world's ability to support life was thought to depend on its access to sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin
DSV Alvin
Alvin is a manned deep-ocean research submersible owned by the United States Navy and operated by the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. The vehicle was built by General Mills' Electronics Group in the same factory used to manufacture breakfast cereal-producing...

, scientists discovered colonies of giant tube worm
Giant tube worm
Giant tube worms, Riftia pachyptila, are marine invertebrates in the phylum Annelida related to tube worms commonly found in the intertidal and pelagic zones...

s, clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

s, and other assorted creatures clustered around undersea volcanic features known as black smokers. These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain is a form of bacterium that derives its energy from oxidization of reactive chemicals, such as hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 or hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, that bubble up from the Earth's interior. This chemosynthesis
Chemosynthesis
In biochemistry, chemosynthesis is the biological conversion of one or more carbon molecules and nutrients into organic matter using the oxidation of inorganic molecules or methane as a source of energy, rather than sunlight, as in photosynthesis...

 revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist.

Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

 able to survive kilometers below the ocean's surface near hydrothermal vent
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...

s and microbes that thrive in highly acidic environments. It is now known that extremophiles thrive in ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life. It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the universe. According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth - and we know they're very hardened to radiation." Some organisms able to withstand exposure to the vacuum and radiation of space include the lichen fungi Rhizocarpon geographicum and Xanthoria elegans
Xanthoria elegans
Xanthoria elegans, commonly known as the elegant sunburst lichen, is a lichenized species of fungus in the genus Xanthoria, family Teloschistaceae. Recognized by its bright orange or red pigmentation, this species grows on rocks, often near bird or rodent perches. It has a circumpolar and alpine...

, the bacterium Bacillus safensis
Bacillus safensis
Bacillus safensis is a bacterium, highly resistant to gamma and UV radiation, that has raised some concerns over a possibility of having been brought to the planet Mars with the two space probes Spirit and Opportunity in 2004....

, Deinococcus radiodurans
Deinococcus radiodurans
Deinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records.-Name...

, Bacillus subtilis
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...

, yeast Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

, seeds from Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

('mouse-ear cress'), as well as the invertebrate animal Tardigrade
Tardigrade
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...

.

On 2 December 2010, it was announced that an extremophile bacterium (GFAJ-1
GFAJ-1
GFAJ-1 is a strain of rod-shaped bacterium in the family Halomonadaceae. The extremophile was isolated from the hypersaline and alkaline Mono Lake in eastern California by a research team led by NASA astrobiologist Felisa Wolfe-Simon...

) can be coaxed into partially substituting arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 for phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 in some of its basic chemistry under laboratory conditions. Also related, on 27 June 2011, it was reported that a new E. coli bacterium was produced from an engineered DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 in which approximately 90% of its thymine
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...

 was replaced with the synthetic building block 5-chlorouracil, a substance "toxic to other organisms". Some scientists believe that these discoveries might give weight to the long-standing idea that life on other planets may have a radically different chemical makeup and may help in the search for alien life. Jupiter's moon, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, and Saturn's moon, Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

, are now considered the most likely locations for extant extraterrestrial life in the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

.

The origin of life
Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

, distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane
J. B. S. Haldane
John Burdon Sanderson Haldane FRS , known as Jack , was a British-born geneticist and evolutionary biologist. A staunch Marxist, he was critical of Britain's role in the Suez Crisis, and chose to leave Oxford and moved to India and became an Indian citizen...

 postulated that the conditions on the early Earth were conducive to the formation of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s from inorganic elements
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...

 and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry
Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

, has made some progress, but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative theory of panspermia is that the first elements of life may have formed on another planet with even more favorable conditions (or even in interstellar space, asteroids, etc.) and then have been carried over to Earth by a variety of means.
In October 2011, scientists found that the cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 permeating the universe contains complex organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. As one of the scientists noted, "Coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 and kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

 are products of life and it took a long time for them to form ... How do stars make such complicated organics under seemingly unfavorable conditions and [do] it so rapidly?" Further, the scientist 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."

Astrogeology


Astrogeology
Geology of solar terrestrial planets
The geology of solar terrestrial planets mainly deals with the geological aspects of four planets of the Solar system namely, Mercury, Venus, Earth, and Mars and one terrestrial dwarf planet, Ceres...

 is a planetary science
Planetary science
Planetary science is the scientific study of planets , moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation,...

 discipline concerned with the geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 of the celestial bodies such as the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s and their moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

s, asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, and meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s. The information gathered by this discipline allows the measure of a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

's or a natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

's potential to develop and sustain life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, or planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

.

An additional discipline of astrogeology is geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

, which involves study of the chemical
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 composition of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and other planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, chemical processes and reactions that govern the composition of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s and soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

s, the cycles of matter and energy and their interaction with the hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

 and the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 of the planet. Specializations include cosmochemistry
Cosmochemistry
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples...

, biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

 and organic geochemistry
Organic geochemistry
Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. The study of organic geochemistry is usually traced to the work of Alfred E. Treibs, "the father of organic geochemistry." Treibs first isolated metalloporphyrins from petroleum. This discovery...

.

The fossil record provides the oldest known evidence for life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

. By examining this evidence, paleontologists are able to understand better the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

 and the McMurdo Dry Valleys
McMurdo Dry Valleys
The McMurdo Dry Valleys are a row of snow-free valleys in Antarctica located within Victoria Land west of McMurdo Sound. The region is one of the world's most extreme deserts, and includes many interesting features including Lake Vida and the Onyx River, Antarctica's longest river.-Climate:The Dry...

 of Antarctica, are also considered to be geological analogs to regions of Mars, and as such, might be able to provide clues on how to search for past life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...

.

Life in the Solar System




People have long speculated about the possibility of life in settings other than Earth, however, speculation on the nature of life elsewhere often has paid little heed to constraints imposed by the nature of biochemistry. The likelihood that life throughout the universe is probably carbon-based is encouraged by the fact that carbon is one of the most abundant of the higher elements. Only two of the natural atoms, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

, are known to serve as the backbones of molecules sufficiently large to carry biological information. As the structural basis for life, one of carbon's important features is that unlike silicon it can readily engage in the formation of chemical bonds with many other atoms, thereby allowing for the chemical versatility required to conduct the reactions of biological metabolism and propagation. The various organic functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus, sulfur, and a host of metals, such as iron, magnesium, and zinc, provide the enormous diversity of chemical reactions necessarily catalyzed by a living organism. Silicon, in contrast, interacts with only a few other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe of organic macromolecules. Indeed, it seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail. Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique. If life has had a comparable impact elsewhere in the solar system, the relative abundances of chemicals key for its survival - whatever they may be - could betray its presence. Whatever extraterrestrial life may be, its tendency to chemically alter its environment might just give it away.
Thought on where in the Solar System life might occur was limited historically by the belief that life relies ultimately on light and warmth from the Sun and, therefore, is restricted to the surfaces of planets. The three most likely candidates for life in the Solar System are the planet Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, the Jovian moon Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, and Saturn's moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

. More recently, Saturn's moon Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 may be considered a likely candidate as well. This speculation of likely candidates of life is primarily based on the fact that (in the cases of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it, for its use as a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 in cells. Water on Mars is found in its polar ice caps, and newly carved gullies recently observed on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 suggest that liquid water may exist, at least transiently, on the planet's surface, and possibly in subsurface environments such as hydrothermal springs
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

 as well. At the Martian low temperatures and low pressure, liquid water is likely to be highly saline. As for Europa, liquid water likely exists beneath the moon's icy outer crust. This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably tidal heating
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

.

Another planetary body that could potentially sustain extraterrestrial life is Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's largest moon, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

. Titan has been described as having conditions similar to those of early Earth. On its surface, scientists have discovered the first liquid lakes outside of Earth, but they seem to be composed of ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

 and/or methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, not water. After Cassini data was studied, it was reported on March 2008 that Titan may also have an underground ocean composed of liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

. Additionally, Saturn's moon Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 may have an ocean below its icy surface and, according to NASA scientists in May 2011, "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Rare Earth hypothesis


This hypothesis states that based on astrobiological findings, multi-cellular life forms found on Earth may actually be more of a rarity than scientists initially assumed. It provides a possible answer to the Fermi paradox which suggests, "If extraterrestrial aliens are common, why aren't they obvious?" It is apparently in opposition to the principle of mediocrity, assumed by famed astronomers Frank Drake
Frank Drake
Frank Donald Drake PhD is an American astronomer and astrophysicist. He is most notable as one of the pioneers in the search for extraterrestrial intelligence, including the founding of SETI, mounting the first observational attempts at detecting extraterrestrial communications in 1961 in Project...

, Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

, and others. The Principle of Mediocrity suggests that life on Earth is not exceptional, but rather that life is more than likely to be found on innumerable other worlds.

The anthropic principle
Anthropic principle
In astrophysics and cosmology, the anthropic principle is the philosophical argument that observations of the physical Universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the Universe has the age and the fundamental...

 states that fundamental laws of the universe work specifically in a way that life would be possible. The anthropic principle supports the Rare Earth Hypothesis by arguing the overall elements that are needed to support life on Earth are so fine-tuned that it is nearly impossible for another just like it to exist by random chance (note that these terms are used by scientists in a different way from the vernacular conception of them). However, Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

 compared the claim that the universe is fine-tuned for the benefit of our kind of life to saying that sausages were made long and narrow so that they could fit into modern hot dog bun
Hot dog bun
A hot dog bun is a type of soft bun shaped specifically to contain a hot dog. The original purpose of this bun was to make it possible to eat hot dogs without burning ones hands....

s, or saying that ships had been invented to house barnacle
Barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s.

Research


The systematic search for possible life outside of Earth is a valid multidisciplinary scientific endeavor. The University of Glamorgan
University of Glamorgan
The University of Glamorgan is a university based in Pontypridd, Rhondda Cynon Taf, Wales with campuses in Treforest, Glyntaff, Merthyr Tydfil, Tyn y Wern and Cardiff...

, UK, started just such a degree in 2006, and the American government funds the NASA Astrobiology Institute
NASA Astrobiology Institute
The NASA Astrobiology Institute was established in 1998 by the National Aeronautics and Space Administration "to develop the field of astrobiology and provide a scientific framework for flight missions". The NAI is a virtual, distributed organization that integrates astrobiology research and...

. However, characterization of non-Earth life is unsettled; hypotheses and predictions as to its existence and origin vary widely, but at the present, the development of theories to inform and support the exploratory search for life may be considered astrobiology's most concrete practical application.

Biologist
Biologist
A biologist is a scientist devoted to and producing results in biology through the study of life. Typically biologists study organisms and their relationship to their environment. Biologists involved in basic research attempt to discover underlying mechanisms that govern how organisms work...

 Jack Cohen
Jack Cohen (scientist)
Jack Cohen, FIBiol is a British reproductive biologist also known for his popular science books and involvement with science fiction.-Life:...

 and mathematician
Mathematician
A mathematician is a person whose primary area of study is the field of mathematics. Mathematicians are concerned with quantity, structure, space, and change....

 Ian Stewart
Ian Stewart (mathematician)
Ian Nicholas Stewart FRS is a professor of mathematics at the University of Warwick, England, and a widely known popular-science and science-fiction writer. He is the first recipient of the , awarded jointly by the LMS and the IMA for his work on promoting mathematics.-Biography:Stewart was born...

, amongst others, consider xenobiology separate from astrobiology. Cohen and Stewart stipulate that astrobiology is the search for Earth-like life outside of our solar system and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining characteristics of life. (See carbon chauvinism
Carbon chauvinism
Carbon chauvinism is a neologism meant to disparage the assumption that the chemical processes of hypothetical extraterrestrial life must be constructed primarily from carbon , as carbon's chemical and thermodynamic properties render it far superior to all other elements.-Concept:The term was used...

).

Research outcomes



, no proof of extraterrestrial life has been identified. Examination of the ALH 84001 meteorite, which was recovered in Antarctica in 1984 and believed to have originated from Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, is thought by David McKay
David S. McKay
David S. McKay is Chief Scientist for Astrobiology at the Johnson Space Center. McKay was the first author of a scientific paper postulating past life on Mars on the basis of evidence in Martian meteorite ALH84001 found in Antarctica. This paper has become one of the most heavily cited papers in...

, Chief Scientist for Astrobiology at NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Johnson Space Center, as well as other scientists, to contain microfossils of extraterrestrial origin; this interpretation is controversial. On 5 March 2011, Richard B. Hoover
Richard B. Hoover
Richard B. Hoover is a NASA scientist who has authored 33 Volumes and 250 papers on astrobiology, extremophiles, diatoms, solar physics, X-ray/EUV optics and meteorites...

, a scientist with the Marshall Space Flight Center
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1
CI1 fossils
CI1 fossils refer to alleged morphological evidence of microfossils found in five CI1 carbonaceous chondrite meteorite fall: Alais, Orgueil, Ivuna, Tonk and Revelstoke. The research was published in March 2011 in the Journal of Cosmology by Richard B. Hoover, an engineer...

 carbonaceous meteorites. However, NASA formally distanced itself from Hoover's claim. According to American astrophysicist Neil deGrasse Tyson
Neil deGrasse Tyson
Neil deGrasse Tyson is an American astrophysicist, a science communicator, the Frederick P. Rose Director of the Hayden Planetarium at the Rose Center for Earth and Space, and a Research Associate in the Department of Astrophysics at the American Museum of Natural History...

: "At the moment, life on Earth is the only known life in the Universe, but there are compelling arguments to suggest we are not alone."

Methane
In 2004, the spectral signature of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...

 probe. Because of solar radiation and cosmic radiation, methane is predicted to disappear from the Martian atmosphere within several years, so the gas must be actively replenished in order to maintain the present concentration. The Mars Science Laboratory
Mars Science Laboratory
The Mars Science Laboratory is a National Aeronautics and Space Administration mission with the aim to land and operate a rover named Curiosity on the surface of Mars. The MSL was launched November 26, 2011, at 10:02 EST and is scheduled to land on Mars at Gale Crater between August 6 and 20, 2012...

 rover will perform precision measurements of oxygen and carbon isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 ratios in carbon dioxide (CO2) and methane (CH4) in the atmosphere of Mars in order to distinguish between a geochemical
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

 and a biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 origin.

Planetary systems
It is possible that some planets, like the gas giant Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 in our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, may have moons with solid surfaces or liquid oceans that are more hospitable. Most of the planets so far discovered outside our solar system are hot gas giants thought to be inhospitable to life, so it is not yet known whether our solar system, with a warm, rocky, metal-rich inner planet such as Earth, is of an aberrant composition. Improved detection methods and increased observing time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Kepler Mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 seeks to discover Earth-sized planets around other stars by measuring minute changes in the star's light curve
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...

 as the planet passes between the star and the spacecraft. Progress in infrared astronomy
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...

 and submillimeter astronomy has revealed the constituents of other star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...

s. Infrared searches have detected belts of dust and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s around distant stars, underpinning the formation of planets.

Planetary habitability

Efforts to answer questions such as the abundance of potentially habitable planets in habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

s and chemical precursors have had much success. Numerous extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s have been detected using the wobble method
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....

 and transit method, showing that planets around other star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s are more numerous than previously postulated. The first Earth-like extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

, which was found using radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

.

Missions


Research into the environmental limits of life and the workings of extreme ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s is also ongoing, enabling researchers to predict what planetary environments might be most likely to harbor life. Missions such as the Phoenix lander
Phoenix (spacecraft)
Phoenix was a robotic spacecraft on a space exploration mission on Mars under the Mars Scout Program. The Phoenix lander descended on Mars on May 25, 2008...

, Mars Science Laboratory
Mars Science Laboratory
The Mars Science Laboratory is a National Aeronautics and Space Administration mission with the aim to land and operate a rover named Curiosity on the surface of Mars. The MSL was launched November 26, 2011, at 10:02 EST and is scheduled to land on Mars at Gale Crater between August 6 and 20, 2012...

, ExoMars
ExoMars
ExoMars is a European-led robotic mission to Mars currently under development by the European Space Agency with collaboration by NASA...

 to Mars, the Cassini probe to Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, and the "Ice Clipper" mission to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's moon Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

 hope to further explore the possibilities of life on other planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s in our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

.

An upcoming mission, the Chemistry and Mineralogy instrument, or CheMin for short, will identify and measure the abundances of various minerals on Mars. Examples of minerals found on Mars so far are olivine, pyroxenes, hematite, goethite, and magnetite. Minerals are indicative of environmental conditions that existed when they formed. For example, olivine and pyroxene, two primary minerals in basalt, form when lava solidifies. Jarosite, found in sedimentary rocks by NASA's Opportunity rover on Mars, precipitates out of water. Using CheMin, scientists will be able to study further the role that water, an essential ingredient for life as we know it, played in forming minerals on Mars. For example, gypsum is a mineral that contains calcium, sulfur, and water. Anhydrite is a calcium and sulfur mineral with no water in its crystal structure. CheMin will be able to distinguish the two. Different minerals are linked to certain kinds of environments. Scientists will use CheMin to search for mineral clues indicative of a past Martian environment that might have supported life.

See also


  • Abiogenesis
    Abiogenesis
    Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

  • Active SETI
    Active SETI
    Active SETI is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message...

  • Alien language
    Alien language
    Alien language is a generic term used to describe a possible language originating from a hypothetical alien species. The study of such a hypothetical language has been termed xenolinguistics, although alternative terminology such as exolinguistics has found its way into use through the medium of...

  • Astrosciences
  • Aurelia and Blue Moon
    Aurelia and Blue Moon
    Aurelia and Blue Moon are hypothetical examples of a planet and a moon on which extraterrestrial life could evolve. They are the outcome of a collaboration between television company Blue Wave Productions Ltd. and a group of American and British scientists who were collectively commissioned by...

  • Back-contamination
    Back-contamination
    Back-contamination is the informal but widely employed name for the hypothetical introduction of microbial extraterrestrial organisms into Earth's biosphere. It is assumed that any such contact will be disruptive or at least have consequences over which human beings will have little control...

  • Cosmic dust
    Cosmic dust
    Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

  • Extraterrestrial life
    Extraterrestrial life
    Extraterrestrial life is defined as life that does not originate from Earth...

  • Forward-contamination
    Forward-contamination
    Forward-contamination is the contamination of other worlds with Terrestrial microbes. The risk of forward-contamination is twofold: that human beings may accidentally seed a previously sterile world, thus creating "extraterrestrials" that are really of terrestrial origin ; or that an...

  • Gravitational biology
    Gravitational biology
    Gravitational biology is the study of the effects gravity has on living organisms. Throughout the history of the Earth life has evolved to survive changing conditions, such as changes in the climate and habitat. The only constant factor in evolution since life first began on Earth is the force of...

  • Hypothetical types of biochemistry
  • Living Interplanetary Flight Experiment
    Living Interplanetary Flight Experiment
    The Living Interplanetary Flight Experiment is an interplanetary mission developed by the Planetary Society. It consists of sending selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft in 2011, which is a sample-return...

  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • Purple Earth Hypothesis
    Purple Earth Hypothesis
    Purple Earth hypothesis is an astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based thus making Earth appear purple rather than green...

  • Rare Earth hypothesis
    Rare Earth hypothesis
    In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

  • Shadow biosphere
    Shadow biosphere
    The term "shadow biosphere" was coined by Carol Cleland and Shelley Copley. A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life...


External links