The concept of extraterrestrial life, embodied in the discipline known as exobiology, astrobiology, or bioastronomy, is one of the oldest in the history of science. Although the search for life beyond Earth has always been intrinsically difficult, and intermingled with philosophy and theology, it has usually been a reflection of the science of its times.
The idea of an infinite number of worlds was part of the ancient Greek atomist system, but was opposed by the physical principles of the philosopher Aristotle. Beginning in the sixteenth century, Copernican theory made the Earth a planet, and the other planets potential Earths. In the seventeenth and eighteenth centuries, Cartesian cosmology extended the idea to other planetary systems, as did Newtonian cosmology, at first in conjunction with natural theology and later with the nebular hypothesis. Only in the twentieth century, and especially with the space age, has empiricism become a major component of the search for extraterrestrial life. It has done so under the banner of cosmic evolution, the idea that the universe has evolved from the Big Bang to planets, stars, galaxies, life, and intelligence. Although at the limits of science, the question of the abundance of life in the universe has been passionately pursued because it bears so strongly on humanity's place in the universe. As such, "the biological universe," as it has been called, has generated considerable and ever increasing theological and philosophical discussion.
Research on extraterrestrial life
Research on extraterrestrial life is pursued in at least four areas: planetary science, planetary systems science, origins of life studies, and the Search for Extraterrestrial Intelligence (SETI). Not surprisingly, the planet Mars was the focus for much of the twentieth century, beginning with the canals of Mars controversy and culminating with the Viking missions in 1976. The latter demonstrated to the satisfaction of most scientists that no organic molecules were present on the surface of Mars at the Viking lander sites, samples of which were analyzed down to parts per billion. Surprisingly, the problem of life on Mars was again highlighted in 1996 when scientists at NASA announced possible fossils in an ancient Mars rock found in the Antarctic. This conclusion caused an uproar and is still hotly debated. Meanwhile, the discovery of a possible liquid ocean under the ice of the Jovian moon Europa raised the unexpected possibility of life in the solar system beyond the ecosphere usually considered hospitable for life.
Although Earth-like conditions have not been found elsewhere in the solar system, extrasolar planets provide potential abodes of life. After a long and fruitless search during most of the twentieth century, in 1995 the first planet was found around the solar-type star 51 Pegasi. In the following six years, with increasingly refined technology, more than eighty planets were found around sunlike stars within a few hundred light years of Earth. More are being discovered monthly. The evidence is indirect, and the planets discovered through 2001 are gas giants like Jupiter, many in highly eccentric orbits or very close to their parent stars. The search continues for Earth-size planets, and the technology is advancing to the point where their detection may be possible within the next decade.
Because there is no guarantee that life will arise even on Earth-like planets, an understanding of the mechanisms of the origin of life is essential. Unfortunately, these mechanisms are not well understood, even on Earth. Nevertheless, the discovery of life in extreme environments—inside rocks, several kilometers below the surface of the Earth, and deep in the ocean—indicates that extraterrestrial life might develop under conditions considerably broader than thought possible. In particular, the exploration of deep-sea hydrothermal vents and their associated life, including tube worms several meters in length, has provided insights into the limits of life, and even the possible origin of life on Earth. Delivery of organic materials, and even life, from beyond the Earth remains an alternative possibility for the origin of life.
SETI programs seek artificial signals emanating from planets around sun-like stars, using a variety of assumptions about signal frequency and targets. For forty years, beginning with Frank Drake's Project Ozma in 1960, SETI relied mainly on radio telescope technology. During the 1990s, the search began expanding to include optical techniques, even as dedicated facilities were being built and increasingly complex software constructed for signal detection. While there have been some false alarms, no unambiguous signals from extraterrestrial intelligence have been detected. In fact, aside from the controversial Mars rock, no form of extraterrestrial life has yet been discovered.
Implications for religion and humanity
The human implications of the existence of extraterrestrial life have almost always been discussed in terms of extraterrestrial intelligence. For centuries religious implications have been a center of attention, especially in the Christian context, beginning with medieval commentaries on Aristotle and increasing significantly in the wake of heliocentrism. In the post-Newtonian era, scriptural objections were largely met by demonstrations of the benefits of extraterrestrials to natural theology, in which a universe full of life was seen as a demonstration of God's omnipotence and the magnificence of divine creation. During the nineteenth century, in the absence of techniques to resolve the scientific question of life on other worlds, the religious implications were explored in considerable detail. Three options were considered and adopted: religion and extraterrestrial life could be reconciled, certain religious doctrines could be rejected, or the idea of inhabited worlds could be rejected. No consensus has been reached on these options. Nor is there consensus on the effect of the discovery of extraterrestrials on Christian thought, particularly the doctrines of redemption, incarnation, and salvation. A few religions, including Mormonism, have incorporated the concept of life on other worlds into their religious doctrine. The implications for non-Western religions have only begun to be explored, but in general it seems that the effect on non-Adamist religions would be less than on those that teach salvation through a single God-head.
In a broader sense, the biological universe may be seen as a worldview analogous to the Copernican and Darwinian worldviews. It is possible that the implications for humanity will be similarly widespread, and may follow the general outlines of the reception of past world views, for which there is a rich literature in the history of science. Science fiction, both literature and film, has also addressed the human implications. In particular the work of Olaf Stapledon, Arthur C. Clarke, Stanislaw Lem, Carl Sagan, and Maria Dorrit Russell, have treated the theme of alien life in a thoughtful manner.
Finally, extraterrestrial intelligence may relegate all human knowledge to the status of a specific instance of a more general knowledge possessed, and perhaps shared, by civilizations scattered throughout the universe. On the other hand, in the absence of extraterrestrial intelligence, human destiny may be to spread throughout the galaxy on some variation of the model adopted in Isaac Asimov's Foundation series of novels, as opposed to the complex interactions with extraterrestrials found in Clarke's work. Either way, the implications are sobering.
See also Exobiology; Science Fiction
crowe, michael j. the extraterrestrial life debate, 1750–1900: the idea of a plurality of worlds from kant to lowell. cambridge, uk: cambridge university press, 1986.
dick, steven j. plurality of worlds: the origins of the extraterrestrial life debate from democritus to kant. cambridge, uk: cambridge university press, 1982.
dick, steven j. the biological universe: the twentieth century extraterrestrial life debate and the limits of science. cambridge, uk: cambridge university press, 1996.
dick, steven j. life on other worlds: the twentieth century extraterrestrial life debate. cambridge, uk: cambridge university press, 1998.
dick, steven j., ed. many worlds: the new universe, extraterrestrial life, and the theological implications. philadelphia: templeton foundation press, 2000.
goldsmith, donald, and owen, tobias. the search for life in the universe, 3rd edition. mill valley, calif.: university science books, 2001.
harrison, albert. after contact: the human response to extraterrestrial life. new york and london: plenum, 1997.
tough, allen, ed. when seti succeeds: the impact of high-information contact. bellevue, wash.: foundation for the future, 2000.
ward, peter, and brownlee, donald. rare earth: why complex life is uncommon in the universe. new york: springer-verlag, 2000.
steven j. dick
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