No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are contingent, in the sense that they are the result of evolutionary accidents such that elsewhere a different sequence of events might have led to different properties of life. In this respect the discovery of even a single example of extraterrestrial life, no matter how elementary in form or substance, would represent a fundamental revolution in science. Do a vast array of biological themes and counterpoints exist in the universe, or are there places with living fugues, compared with which Earth's one tune is a bit thin and reedy? Or is Earth's the only tune around?

Life on Earth, structurally based on carbon, hydrogen, nitrogen, and other elements, uses water as its interaction medium. phosphorus, as phosphate bound to an organic residue, is required for energy storage and transport; sulfur is involved in the three-dimensional configuration of protein molecules; and other elements are present in smaller concentrations. Must these particular atoms (atom) be the atoms of life everywhere, or might there be a wide range of atomic possibilities in extraterrestrial organisms? What are the general physical constraints on extraterrestrial life?

In approaching these questions, several criteria can be used. The major atoms should tend to have a high cosmic abundance. Structural molecules (molecule) of organisms at the temperature of the planet in question should not be so extremely stable that chemical reactions (chemical reaction) are impossible, but neither should they be extremely unstable, or else the organism would fall to pieces. A medium for molecular interaction must be present. Solids are inappropriate because of their inertness. The medium, most likely a liquid but possibly a very dense gas, must be stable in a number of respects. It should have a large temperature range (for a liquid, the temperature difference between freezing point and boiling point should be large). The liquid should be difficult to vaporize and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent solvent. A fluid phase must be present on the planet in question, for material must cycle to the organism as food and away from the organism as waste.

The planet should therefore have an atmosphere and some liquid near the surface, although not necessarily a water ocean. If the intensity of ultraviolet light (ultraviolet radiation) or charged particles from its sun is intense at the planetary surface, then some area, perhaps below the surface, should be shielded from this radiation (although some forms or intensity of radiation might permit useful chemical reactions to occur). Finally, it is imperative that conditions allow the existence of autotrophy (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary compounds.

One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV. Covalent bonds, where electrons (electron) are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than van der Waals forces, which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average kinetic energy corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously.

Specific atoms have circumscribed functions in modern biology, but, aside from structure and the need for the liquid interaction medium, they may not be fundamental. The energy-rich phosphate bonds in adenosine triphosphate (ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular degradation or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond.

In The Fitness of the Environment (1913), American biochemist Lawrence Joseph Henderson (Henderson, Lawrence Joseph) first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances.

There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils (fossil). Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, physiology, and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance.

The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be illuminating. Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases ( atmosphere and hydrosphere) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments.

Chemical, mechanical, or spectral disequilibria may also be sought. Earth's atmosphere contains large amounts of molecular oxygen and about 1.7–2 parts per million (10⁶) of methane, but in thermodynamic equilibrium the abundance of methane should be less than one part in 10³⁵. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic hydrocarbons (hydrocarbon) could have been produced nonbiologically early in Earth's history, and slow degradation may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to bacteria. Some methanogenic bacteria live in wetlands (wetland) (hence the term marsh gas for methane), and others live in the intestinal tracts of cows (cow) and other ruminants (ruminant). Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (terpene) (hydrocarbons given off by plants and found over forests (forest)) could be used as a test for life.

In situ studies by vehicles that enter Earth's atmosphere and land on the surface could detect life at many places on Earth. However, there are many other places where large organisms are infrequent such that life-detection attempts based solely on television searches for large animals would be inconclusive. Of course, if such an experiment were successful—for example, if the camera recorded a cavorting dolphin, a strolling camel, or a flying peacock—it would provide quite convincing evidence of life.

Although the open ocean, the Gobi Desert (Gobi), and Antarctica are relatively devoid of large life-forms, they are—like other, less-barren ecosystems (ecosystem)—replete with microorganisms. A television camera coupled to an optical or electron microscope might be an optimal life detector. The 17th-century Dutch microscopist Antonie van Leeuwenhoek (Leeuwenhoek, Antonie van) had no difficulty in identifying as alive the little “animalcules” he found in a drop of water, even though nothing similar had been seen before in human history.

Metabolic and chemical criteria might be used for detecting life with in situ studies. The fixation of gas (such as carbon dioxide) when illuminated might be due to photosynthesis or chemosynthesis. Direct tests of soil or water for optical activity might be made. Organic molecules could certainly be sought with gas chromatography, mass spectrometry, or remote analytic chemistry. The detection of organic matter would then lead to experiments that would determine if it was biological in origin.

In general, many tests for life are intrinsically ambiguous. There remains the omnipresent problem of contamination. Any spacecraft might carry living organisms from the home planet and report them as detected on the target planet. Great care must be taken to ensure that the spacecraft is rigorously sterilized and travels without life from home.

No single unambiguous “life detector” exists. Instruments of great generality that make few ambiguous assumptions about the nature of extraterrestrial life require luck (e.g., an animal or protist must walk or swim by during the operating lifetime of the camera) or the solution of difficult instrumental problems (e.g., the acquisition and preparation of samples for remote electron microscopic examination). Highly sensitive instruments, such as metabolism detectors, are directed at organisms presumably vastly more abundant than animals. These instruments critically depend on assumptions that are basically informed guesses (e.g., that extraterrestrial organisms eat sugars (sugar)). Therefore, an array of both very general and very specific instruments is recommended to establish, or preclude, the existence of extraterrestrial life in the solar system.

The seasonal changes on Mars have been reliably observed, not only visually but also photometrically. There is a conspicuous springtime increase in the contrast between the bright and dark areas of Mars. Colour changes with season have also been reported. Space probes have found no vegetation on Mars, but seasonally variable dust storms provide a convincing explanation of the colour changes.

Tens of thousands of comets (comet), as well as some thousands of asteroids and asteroidal fragments revolving about the Sun between the orbits of Mars and Jupiter, contain organic molecules. The asteroids are the presumed sources of the carbonaceous chondrites' organic matter. Pluto has a predominantly nitrogen atmosphere covering a surface of frozen nitrogen, carbon dioxide, and methane. The intense cold and paucity of solar radiation on Pluto and the lack of atmosphere and liquid waters on the asteroids argue against the likelihood of finding life on these bodies.

Since the Renaissance, fashionable belief has fluctuated. Practically all informed opinion in the late 18th century held that each planet was populated by intelligent beings. However, except for those who followed Percival Lowell (Lowell, Percival), the prevailing informed opinion in the early 20th century held that chances for extraterrestrial intelligent life were insignificant. The subject of extraterrestrial intelligent life is for many people a touchstone of their beliefs and desires. Some urgently desire evidence for extraterrestrial intelligence, and others equally fervently deny the possibility of its existence. The subject should be approached in as unbiased a frame of mind as possible. The probability of advanced technical civilizations in the Milky Way Galaxy depends on many controversial issues.

Owing to limitations of current detection methods, most of the planets discovered so far have masses at least as great as the solar system's largest planets, Jupiter and Saturn. Most of these planets are also very close to their stars, much closer than Earth is to the Sun, so life systems similar to Earth's could not exist on them. This difference has led scientists to consider new models for planetary formation. For example, Gliese 876, a red dwarf star one-third the mass of the Sun and 15 light-years away in Aquarius, has three planets: a gas giant half the mass of Jupiter that orbits Gliese 876 every 30 Earth days, another twice the mass of Jupiter that orbits exactly once for every two orbits of its neighbour, and a third six times the mass of Earth that orbits every 2 Earth days. HD 168443, lying 123 light-years away in Serpens, has one planet 8 times Jupiter's mass and another 18 times the mass of Jupiter, which is beyond the scale that had been considered possible for a planet; this giant may be a brown dwarf. Several planets even smaller than Saturn have been found. Gliese 581, at 20 light-years away, has three planets, one of which is only five times the mass of Earth. NASA's Kepler mission, slated for launch in 2009, will use space-based telescopes to observe Sun-like stars that may host Earth-sized planets.

Because the wobble method can detect only a planet that has been observed for a significant portion of its orbit, finding a planet like Jupiter with an orbital period of 12 Earth years requires several years of observations. Nevertheless, some planetary systems similar to the solar system have been found. HD 190360A, at 52 light-years away, is very similar to the Sun and has a detectable planet similar in size and orbital distance to Jupiter. Systems like HD 190360A could also have smaller planets arranged like those in the solar system. The existence of large planets around so many nearby stars demonstrates that a significant fraction of stars do indeed have planets in orbit around them.

Another indication that planetary formation is a general process throughout the universe is the satellite systems of the major planets of the solar system. Jupiter with 63 satellites, Saturn with 60, and Uranus with 27 resemble miniature solar systems. Considering the wide range of temperatures that seem to be compatible with life, it can be tentatively concluded that f_pn_e is about 1. However, since liquid water is considered to be crucial to life's origin and evolution, f_pn_e probably has a significantly smaller value.

Similar arguments are made for f_c, the fraction of technical civilizations. Intelligence and technical civilization are clearly not equivalent. For example, dolphins (dolphin) appear to be intelligent, but their lack of manipulative organs limits their technology. Both intelligence and technical civilization evolved about halfway through the lifetime of Earth and the Sun. Some, but by no means all, evolutionary biologists would conclude that 1/100 is a conservative estimate for the product f_if_c.

Still more uncertain is the value of the final parameter, L, the lifetime of a technical civilization. A technical civilization here is defined as one capable of interstellar radio communication. Thus, human technical civilization is only a few decades old. Technical civilizations may tend, through the use of weapons of mass destruction, to destroy themselves shortly after they come into being. If L is then taken to be 10 years, multiplication of all the factors assumed above leads to the conclusion that only one technical civilization exists in the Milky Way Galaxy—our own. But if technical civilizations do not produce massively destructive weapons or use them to annihilate themselves, then the lifetimes of technical civilizations may be very long. In that case, the number of technical civilizations in the Milky Way Galaxy may be immense. If even 1 percent of developing civilizations make peace with themselves, then about 1,000,000 technical civilizations may be extant in the Milky Way Galaxy. If such civilizations were randomly distributed in space, the nearest would be several hundred light-years (light-year) from Earth. These conclusions are very uncertain.

Problems would definitely surface in the establishment of such radio communication. The frequency, target star, longevity, and character of the message would all have to be selected by the transmitting planet so that the receiving planet would be able to deduce them without too much effort. None of these problems seems insuperable. One choice might be to listen to stars of approximately solar spectral type. Certain natural radio frequencies, such as the 1,420-megahertz (21-cm (21-centimetre radiation)) line of neutral hydrogen, might also be used. In the absence of any symbols or language in common, messages that use the neutral hydrogen line might be the most appropriate for discerning intelligent origin and intellectual content from life-forms that do not share human evolutionary history. Very few anthropocentric assumptions would be needed.

Because Earth's technologies are relatively new, it makes little sense to transmit messages to hypothetical planets of other stars. But it does make sense to listen for radio transmissions from planets of other stars. Other communication techniques include laser transmission and interstellar spaceflight, but these may not be feasible. American engineers Christopher Rose and Gregory Wright have argued that sending a physical artifact is a preferable communication technique because radio waves tend to disperse, whereas physical artifacts retain their information in compact form and are more likely to be readable when they arrive at their destination. However, such “messages in a bottle” would travel 1,000 times slower than light. If the measure of effectiveness is the amount of information communicated across a broad area per unit cost, then radio transmission is the method of choice.

A scientific search for intelligent extraterrestrial life that could communicate beyond its own celestial home was first called for in 1959 by Italian physicist Giuseppe Cocconi and American physicist Philip Morrison. Using the radio telescope at Green Bank, W.Va., in 1960, Drake mounted the first (very brief) search, Project Ozma (Ozma, Project), which was oriented to two nearby stars, Epsilon Eridani and Tau Ceti. On the basis of the Drake equation, it would be very unlikely that success would greet an effort aimed at two stars only 12 light-years away. Not surprisingly, Project Ozma was unsuccessful. Related programs organized on a larger scale were mounted with great enthusiasm in the 1960s in the U.S.S.R.

After Project Ozma ended, various government and private projects continued the search for extraterrestrial intelligence (SETI). The Planetary Society, founded in 1980 by American astronomer Carl Sagan (Sagan, Carl), planetary scientist Bruce Murray, and aerospace engineer Louis Friedman, has as one of its aims the bringing together of professionals and amateurs in support of SETI. Funding by American movie director Steven Spielberg (Spielberg, Steven) permitted the society to start the first privately funded SETI project, the Megachannel Extraterrestrial Assay, in 1982.

SETI is an extraordinary pursuit, in part because of the potential significance of success. SETI brings unity to a wide range of scientific disciplines as well. astrobiology, which includes SETI, as the study of the origin and evolution of stars, planets, and life and of the evolution of intelligence and of technical civilizations, is arguably the most important science for understanding the human condition. Astrobiology includes the political problem of recognizing ourselves less as members of tribes and more as citizens of the universe. To pursue these studies, a number of modern methods—molecular evolution via computational proteomics and genomics, geochronological analyses, chemical element detections coupled with scanning electron microscopy (microscope), immunocytochemistry for study of protein dynamics, to name only a few—promise to refine definitions of life as well as detect life under extreme conditions on Earth and beyond.

Although there is only an infinitesimal possibility that humanlike beings will be discovered in outer space (to serve as a cosmic example of convergent evolution (evolution)), the discovery of any other living matter anywhere else in the cosmos would be of the utmost scientific significance. Moreover, if no evidence at all for life beyond Earth is found after a significant search, this too would be of great scientific moment. The absence of the evolving matter-energy flow systems that are life would reinforce the awesome responsibility of protecting its diversity in this biosphere, which includes that precious, cosmically fragile, and recent growth form, human civilization.