In 1600, Giordano Bruno, an Italian theologian, philosopher, and cosmologist, was burnt at the stakes for insisting - among other "heresies" - that the universe contains an infinite number of stars, that they are all like the Sun, and that each one is circled by planets inhabited by intelligent beings like ourselves. At the end of the 19th century, the city of Rome erected a statue in his honour, and by the end of the 20th century the search for other worlds, for life and for intelligent beings had become a respectable, scientific venture.
For many centuries, the existence of other worlds and other beings was a controversial question, for which the various answers given by prominent thinkers mostly reflected their own philosophical and theological biases. There was indeed no firm scientific knowledge for thinkers to base their views upon; things have changed in the past half-century or so. It is interesting to review, even briefly, the history of views on the plurality of worlds. Perhaps the oldest reference we find on the question is the poetry of Orpheus, that legendary or mythical figure from around 1400BC who wrote that the Moon is a fertile land where cities and palaces have been built by aliens. Similar views on the Moon were expressed by the philosophers Xenophanes and Anaxagoras (circa 5th century BC). It is also said Plato believed that all planets are inhabited. Likewise, the Roman poet and philosopher Lucretius (first century BC) wrote that the heavens were full of worlds, with different peoples and animals of all kinds.
Greek philosophers with a scientific inclination based their views on more rigorous arguments; for example, Pythagoras (circa 5th century BC) insisted that the existence of life on other planets depended on environmental conditions, particularly the ambient temperature. Likewise, Plutarch (first century AD) argued that the Moon must be sterile and virgin, for it is too hot during the day and the air is very thin and dry.
During the Islamic era, it seems that few opinions were expressed on the question, other than attempts to interpret some Quranic verses that could be understood to imply the existence of other species in the cosmos, while other verses seemed to insist on the central importance of Man - hence the irrelevance of other intelligent and conscious species out there. During the European times of renaissance and scientific revolution, strong views on the question were expressed again, especially when telescopes made it clear that the heavens were full of stars, and new planets began to be discovered. William Herschel, the discoverer of Uranus, not knowing the true temperature of the Sun, thought it was populated by aliens. Christiaan Huygens, who was the first to resolve the nature of Saturn's rings and to discover the famous moon Titan, believed that aliens would resemble humans.
But it was in the second half of the 20th century that the search for extraterrestrial life and intelligence really took on the form of a scientific discipline. One important distinction that must always be made is between the search for "life" and the search for "intelligent species" (aliens). By "life", scientists usually mean any organism capable of metabolising and reproducing, no matter how small and simple it may be.
There are two ways to search for life: searching a place, as when we send spacecraft to Mars and dig the ground looking for any organisms or their signatures; or analysing the environmental conditions existing in some place from afar, as when we search for signatures of oxygen in the atmosphere of a planet or moon. One of the main issues that scientists find themselves confronting when attempting to define extra-terrestrial life is its characteristics. Many believe that "life" everywhere must have the same basis, most likely carbon; molecules gradually become more complex through chemical reactions in a favourable medium (most likely water); "biological" functions like metabolism and reproduction lead to the development of the species.
But why would life necessarily follow the exact same plan it followed on Earth? First, because carbon is the most common element forming complex molecules - it is a very versatile atom, which can bond easily with many other atoms, and it is one of the most abundant elements in the universe. Second, because water is not only common, it has the best properties for facilitating chemical reactions and biological processes. And that is why scientists are naturally drawn to searching for other planets like Earth and for water, in liquid form preferably.
Other scientists point out, however, that we have found on Earth places of extreme physical, chemical and environmental conditions where life has existed and prospered for millions of years. We have found microbes and other organisms near undersea volcanoes and hydrothermal vents, species thriving in extreme conditions that are called "extremophiles". And that is why astrobiologists have decided to take a closer look at places in the solar system that they previously had given up on, including Mars, Venus and moons like Europa and especially Titan. With evidence of water present in the past on Mars, the European-led ExoMars mission is being planned for 2013. The Titan Explorer is scheduled for launch in 2018, to arrive in 2024.
The search for intelligent aliens is a different story. It took a quantum leap in 1959 when Philip Morrison and Giuseppe Cocconi, two scientists at Cornell University, published a paper in the journal Nature titled "Searching for Interstellar Communications", in which they asked what would be the best method for aliens to communicate with us? They concluded that radio waves would be most effective because they can travel large distances without much loss, focusing on the frequency of 1428 MHz.
The following year, Frank Drake, then a graduate student in radioastronomy at Cornell, put together the first Seti (search for extra-terrestrial intelligence) project, named Ozma (after the Wizard of Oz). Mr Drake later wrote an equation meant to estimate the number of intelligent civilisations, but the inputs are so hard to quantify that the result is almost always something between zero and millions.
Seti projects became very popular during the 1970s, among scientists as well as the general public, with charismatic scientists like Carl Sagan taking leadership roles and writing books on the subject. Sagan's Contact was turned into a movie starring Jody Foster as a scientist who detects an alien intelligence. But by the late 1980s, no signal had been discovered, and the consensus was that there was very little chance of doing so because of the low-sensitivity equipment. In 1993, Nasa pulled the plug on Seti.
Soon afterward, private funds came to the rescue. The new projects used swarm computing, using personal home computers belonging to people who downloaded a software program to analyse the data collected from space. In its 10 years of operation, the project SETI@Home has enlisted almost 5.5 million participants. Among the enthusiasts is Paul Allen, the co-founder of Microsoft, who has donated US$13.5 million (Dh50 million) to build a bank of so-called radio telescopes, the first phase of which has been completed with 42 antennas; by the end of the multi-phase project, 350 antennas will be in operation. If we do not hear from extraterrestrials by then, we can probably conclude that nobody is out there.
Many scientists and thinkers already believe that we are alone, at least in the Milky Way. But our galaxy contains some 200 billion stars, many of which have planets. The argument is known as the Fermi Paradox, after Enrico Fermi, one of the greatest physicists of the 20th century: if intelligent life exists in a civilisation that predates our own, presumably it would have the capacity for interstellar travel or communication. Because we have not seen evidence so far, the assumption is that extraerrestrial life does not exist.
Counter arguments to this influential theory include: species may not be interested in us; they may be here but not be visible; or they may have self-destructed. The debate had lain dormant for centuries but exploded on the scene with the astronomical and technological developments of the past 50 years. There may never be a definitive answer, barring a clear signal like that received in Contact - a proposal that for now belongs only to the universe of science fiction.
Nidhal Guessoum is professor of physics and astronomy at the American University of Sharjah