about his views on church ministers. However, one of these charges was explicitly for claiming the so-called “plurality of worlds,” the idea that there are other Earth-like planets providing homes for creatures in the Universe.
During the early Enlightenment in the seventeenth century, a major technological step forward was taken with the invention of the telescope. This allowed scientists to see new moons and planetary bodies in our Solar System and to develop a more accurate view of how the Solar System was structured. One might be convinced that this should have reduced speculation, since with much more data available about what was in the Solar System, thoughts about conditions on other worlds could be more constrained. However, the effect was the opposite. With solid evidence for the presence of other worlds in the Solar System that were planetary bodies like the Earth and Moon, but with little information about their surfaces and whether they were appropriate for life, speculation went wild.
Christiaan Huygens (1629–1695), who discovered Saturn's moon Titan, and who invented the pendulum clock, wrote extensively on extraterrestrial life and the habitability of other planets in his book Cosmotheoros, published posthumously in 1698. As well as speculating about astronomers on Venus, he also suggested that other intelligences would understand geometry. About music, he said: “This is a very bold assertion, but it may be true for aught we know, and the inhabitants of the planets may possibly have a greater insight into the theory of music than has yet been discovered among us.”
His book followed on the heels of Conversations on the Plurality of Worlds, published by Bernard le Bovier de Fontenelle (1657–1757) in 1686, an enormously popular book at the time about the inhabitants of the Moon and other planetary bodies. The book is set up as a conversation in a moonlit garden with an intrigued marquise keen to know about the Solar System and how the cosmos works. It is a timelessly delightful and short book. These popular works did much to ignite the public imagination.
William Herschel (1738–1822), discoverer of Uranus and infrared radiation, after observing the strangely circular craters of the Moon speculated about them, in an age when their impact origin was completely unknown: “By reflecting a little on this subject I am almost convinced that those numberless small Circuses we see on the moon are the works of the Lunarians and may be called their Towns.”
Further enthusiasm for the possibility of extraterrestrial life was advanced by Camille Flammarion (1842–1925) in a series of books including La Pluralité Des Mondes Habités (The Plurality of Habitable Worlds) in which he emphasized that life elsewhere should adapt to its environment and would be channeled by the environmental characteristics of different planets, although he stressed that we could probably not predict exactly how it might evolve.
As late as 1909, Percival Lowell (1855–1916), observer of the infamous Martian “canals” (Figure 1.11) said of Mars in his book Mars as the Abode of Life: “Every opposition has added to the assurance that the canals are artificial; both by disclosing their peculiarities better and better and by removing generic doubts as to the planet's habitability.”
Figure 1.11 The “canals” of Mars as depicted by astronomer Percival Lowell. He was convinced he could see artificial canals, built by Martians, on the surface of the planet.
Source: Reproduced with permission of Perelman, https://commons.wikimedia.org/wiki/File:Lowell_Mars_channels.jpg.
We could continue with many such quotes (and many other eminent scientists and philosophers were convinced of alien life), but these examples are adequate to make two points. First, we would have to wait for the space age and the direct and close-up observation of planetary bodies to truly force astrobiology into an empirical era. Second, these quotes are a warning from the past. The desire to believe in alien life should not trump empirical observation. Life should always be the last explanation after all non-biological explanations have been exhausted.
Herschel's observations on lunar craters being the fortresses of lunar towns are a particularly interesting lesson. At the time when he was writing, asteroid and comet impacts were not understood to be an important geological phenomenon. It would be much later that we would understand that these events do occur and that they make craters. Even more interesting is the knowledge that most impacts, unless they occur at a very oblique angle, tend to create circular craters so that a planetary surface, after being pummeled by impacts for billions of years, will record many of these beautifully round features (Figure 1.12).
Figure 1.12 Lunar craters. In the eighteenth century, the almost perfectly circular craters on the Moon, prior to any understanding of impact processes, looked suspiciously artificial. This view shows the Capuanus crater, which is 60 km in diameter, in the lower left.
Source: Reproduced with permission of NASA.
In hindsight, the perfectly circular structures Herschel observed on the Moon seemed unnatural, the products of an advanced civilization. However, assuming one's geological knowledge is incomplete is always a safer and more parsimonious way to interpret data than explaining observations using biology. This is an application of Ockham's razor, a philosophical principle that the explanation that requires the fewest assumptions or speculations is the better one. This principle was propounded by William of Ockham, a Franciscan friar who studied logic in the fourteenth century. It's a very useful principle when you are searching for evidence of life anywhere, in ancient Earth rocks or on other planets.
It was only at the beginning of the space age (Figure 1.13) that the photographic study of planetary surfaces yielded new and more empirically constrained views of the surfaces of other planets. In general, they showed other planets to be devoid of life, and this led to a strong retreat from previous optimism. Nevertheless, astrobiology entered into the realms of experimental testing with a range of pioneering experiments and discoveries that would take it from its previous philosophical underpinnings to its present-day status as a branch of science.
Figure 1.13 One of the first orbital photographs of Mars, taken by the Mariner 4 craft on July 15, 1965. This and other photographs suggested a dead, desiccated environment unfit for life. The area shown is 262 × 310 km and is a heavily cratered region south of Amazonis Planitia, Mars.
Source: Reproduced with permission of NASA.
Laboratory experiments from the 1950s onwards brought studies of the origin of life into mainstream scientific investigations. Scientists simulated conditions on early Earth and demonstrated the production of amino acids and other building blocks of life from simple gaseous precursors. The publication of evidence, in the 1980s and onwards, of fossil microbial life on Earth, preserved for more than three billion years, turned the search for ancient life on Earth and the timing of the emergence of life into a scientific quest.
The first experimental search for life on another world was undertaken by the robotic Viking biology experiments, which landed on Mars in 1976. The consensus is that their observations are explained by reactive chemical compounds in the Martian soil, not life. However, the landers demonstrated that we can go to other planets and implement the scientific method in the search for life.
Attempts were made in the 1970s to transmit radio messages to other civilizations with all of its social and ethical implications. Despite the lack of response, the efforts to search for, and communicate
