Extraterrestrial life, life that may exist or may have existed in the universe outside of Earth. The search for extraterrestrial life encompasses many fundamental scientific questions. What are the basic requirements for life? Could life have arisen elsewhere in the solar system? Are there other planets like Earth? How likely is the evolution of intelligent life?

Universal Criteria

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 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 of organisms at the temperature of the planet in question should not be so extremely stable that chemical reactions 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 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.

The Drake equation and extrasolar life

American astrophysicist Frank D. Drake devised a simple approach that illuminates the uncertainties involved in determining whether extraterrestrial intelligence is possible. The number of extant technical civilizations in the Milky Way Galaxy is estimated by the following equation (the so-called Drake equation, or Green Bank formula):
N = R*fpneflfifcL
where R* is the average rate of star formation over the lifetime of the Milky Way Galaxy, fp is the fraction of stars with planetary systems, ne is the mean number of planets per star that are ecologically suitable for the origin and evolution of life, fl is the fraction of such planets on which life arises, fi is the fraction of such planets on which intelligent life evolves, fc is the fraction of such planets on which a technical civilization develops, and L is the mean lifetime of a technical civilization. A consideration of the factors involved in the choice of numerical values for each parameter follows. These estimates are little better than informed guesses; no great reliability should be pretended for them.

There are about 200 billion stars in the Milky Way Galaxy. The age of the Milky Way Galaxy is about 10 billion years. A value of R* = 10 stars per year is probably fairly reliable. While most contemporary theories of star formation imply that the origin of planets accompanies the origin of stars, such theories are not developed well enough to merit much confidence. More than 250 extrasolar planets have been confirmed. They have been observed via several different means: by “wobble,” which detects the changing wavelength of a star’s light as the star gets closer and then farther away from Earth as a massive planet tugs it from the centre of the system; by transit, which detects the dimming of a star as a planet crosses between it and Earth, much like a solar eclipse; and by infrared observation, which observes a planet directly.

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 fpne is about 1. However, since liquid water is considered to be crucial to life’s origin and evolution, fpne probably has a significantly smaller value.

Because of the short time it took for life to arise on Earth, as implied by the fossil record, and because of the ease with which relevant organic molecules are produced in experiments that simulate the early Earth, the likelihood of life’s arising during a period of billions of years may be high. Some scientists believe that the appropriate value of fl, the fraction of planets with life, is about 1. For the quantities of fi, the fraction of planets with intelligent life, the parameters are even more uncertain. The evolutionary path leading to mammals depends on a great many specific circumstances and historical accidents; it is therefore highly unlikely that such a path will ever repeat. However, intelligence clearly has a great selective advantage and is not necessarily restricted to the single evolutionary path that occurred on Earth.

Similar arguments are made for fc, the fraction of technical civilizations. Intelligence and technical civilization are clearly not equivalent. For example, dolphins 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 fifc.

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 from Earth. These conclusions are very uncertain.

Searching for technical civilizations

How would technical civilizations enter into communication with one another? Independent of the value of L, the Drake formula cited above implies that about one technical civilization arises every 10,000 years in the Milky Way Galaxy. Accordingly, it would be extraordinarily unlikely for humans to find a technical civilization as backward as Earth’s. The rate of technical advance on Earth in the past few hundred years makes it clear that no serious and reliable projection of future scientific and technical advances can be made. Advanced civilizations are expected to have techniques and sciences unknown to 21st-century people. Nevertheless, humanity is already capable of communication by radio over interstellar distances. If Earth’s largest radio telescope, the 305-metre- (1,000-foot-) diameter dish at the Arecibo Observatory in Puerto Rico and its receivers, is employed and if identical equipment is employed on some transmitting planet, how far apart could the transmitting and receiving planets be for intelligible signals to be passed? The rather astonishing answer is 1,000 light-years. Within a volume centred on Earth with a radius of 1,000 light-years, there are more than 10,000,000 stars.

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) 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, West Virginia, in 1960, Drake mounted the first (very brief) search, Project Ozma, 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, 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 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, 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.

Science fiction routinely depicts extraterrestrial beings as thinly disguised men and women. The unique circuitous one-way path of evolution on Earth makes it extremely unlikely that any mammal or flowering plant, to say nothing of a child, would have evolved on a moon of Jupiter or an extrasolar planet. In the words of Loren Eiseley (from The Immense Journey [1957]), 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), 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.