With the emerging understanding that the stars are kin to the Sun came the possibility, even probability, that there were planetary systems other than our own. Surmise on this matter could run to the edge of the observable universe and beyond. This aspect of armchair cosmology reached almost fever pitch during the reign of Victoria, and divines of every sort pitched in with their sixpence worth. Indeed, since in the early part of the nineteenth century physical facts about the planets were scant, the discussion was more a matter of theology than physics.
The flowering of the Industrial Revolution and the growth of European empires brought about a surge of self-confidence that spilt over into complacency. The paramount position of Man in the divine creation was unquestionable. It was held that the function of physical and brute creation was the proliferation of Man, and that the propagation of that species was central to the Creator’s purpose. Thus, the service of fecund Man, however obscure the part played, was the function of all else, and so it followed that other planets must be habitable, if not inhabited. It was unthinkable that it might be otherwise, for deserts are there to be conquered, and other cultures to be converted—a position still held by some advocates of manned space flight even today. This position was pushed to the limit by one cleric who calculated the potential agrarian population of Saturn’s rings, utilising both sides, of course, and avoiding any discussion of artesian water rights.
The Deity’s purpose apart, we appear to have a horror of being alone in the depths of space, and a positive need to see the cosmos as populated, however sparsely. Perhaps the most extreme example of this tendency was our population of Mars with intelligent beings during the last 100 years.
In 1877 the Italian astronomer Giovanni Schiaparelli reported observing linear markings, like channels, on the surface of the planet. Two years later, at the next opposition, he elaborated this report by recording some of the extensive network as being composed of paired, parallel elements. His report of these canali or channels contained no implication that they were other than natural features. However the Italian for “channel” was mistranslated as “canals,” a word which bears the sense of an artificial construction, and this was further abetted by his report of pairing, a most unnatural feature.
These reports captured the imagination of Percival Lowell, who built and equipped the observatory at Flagstaff, Arizona, with its famous 60cm refractor for the further study of the red planet. Here he spent countless hours observing and recording the markings as they flickered in and out of visibility, and constructing globes showing the planet-wide network of canals which he surmised brought water down from the poles to the advancing deserts of the equator. Construction on such a scale argued not only advanced intelligence but also co-operation on a planet-wide scale: a world without war, a literally shining example to a planet on which the great powers were jostling one another towards the war to end all wars.
Although many astronomers doubted Lowell’s observations and rejected his conclusions, he was not alone in his conjecture. The Russian spectroscopist G. A. Tikhov established to his own satisfaction that the dark areas of Mars showed the same characteristics as that of terrestrial high latitude tundra. N. C. Flammarion, author of the very popular L’astronomie Populaire, which was also translated into English, wrote a book on Mars in 1892, reporting his observations supporting the existence of canals, and hence of an advanced technology.
The 1976 landings of the Viking probes failed to discover any signs of such macroscopic life—indeed, of any form of life at all. For the time being Martians live only in Wells’ War of the Worlds and Ray Bradbury’s elegiac Martian Chronicles. Nevertheless, there is still a slim chance that a future survey of Mars may detect some signs of life, even if long dead.
With Mercury too dry, Venus too hot, Mars too airless, Jupiter too insubstantial and Saturn and the outer planets too cold, the search has had to be extended beyond the solar system. There are two current approaches: the search for a suitable place, i.e. a planet orbiting another star, and the search for artificial electromagnetic emissions—the Search for Extraterrestrial Intelligence (SETI). The justification for these programmes is to be found in the Drake equation, a simple mathematical expression which enables us to calculate the number of advanced technical civilisations in our galaxy. The value one ascribes to the several variables in the equation produces results from one—lonely us—to about ten million.
When Frank Drake first wrote out this equation, N* x fp x ne x fl X fi; x fc x fL=N, it was a means of systematising one’s guesses at the resulting figure, and of showing how this figure would vary as one altered the value ascribed to the various parameters. Whatever the result, it is no more than a guess buttressed by the best information available.
In Drake’s equation N* = the number of stars in our, the Milky Way, galaxy, which is currently estimated at 4 x 1011—four hundred thousand million. fp, the fraction of the stellar population having planets, is probably quite large, a third or better. Taking the low estimate, then there are about 1.3 x 1011 such stars, and recent computer simulations suggest that rocky planets of the Venus, Earth, Mars type are a common feature of any planetary system. In other words, it is reasonable to assume that there are, on average, 2.5 planets in a planetary system upon which life of some sort could have developed—a total of 3 x 1011
The next steps are much more conjectural. f = the fraction of potentially life-supporting planets upon which life actually arises, and the answer has varied from 3 x 10-11, from the Earth-is-unique school, to Carl Sagan’s 0.3 or even higher. Settling for Sagan’s estimate, then the number of planets actually supporting life is about 1011, which is a very wide stage to strut upon. But of all life-bearing planets, what fraction has seen the development of intelligent life? This value, fi., is pure guess-work, as is that for fc , the fraction of such planets on which intelligent life develops into a communicative, technically sophisticated culture. If fi x fc. x = one in a thousand, then we have 108, one hundred million planets upon which a technical civilisation has developed.
The final factor, fL, the percentage of the lifetime of a planet when such a civilisation has existed, is again highly debatable, but if we take our own species with its potential for self-destruction as typical, we get a rather small fraction. The age of the earth is 5 x 109 years, and our technical civilisation, with its ability to make high-power radio transmissions, is but 70 years old, and it is far from certain that it will survive another 30 years. If we generalise and say that technically advanced civilisations may exist for only 5 x 10-7 of a planet’s life, then there may only be five advanced civilisations in the whole galaxy to talk to us, and the probability of any of them being near enough for us to pick up anything but deliberately directed signals would seem to be just about zero. However, take a more optimistic view and hundreds of thousands, or even tens of millions of such civilisations are possible.
All this has been hypothetical, for we have yet to establish the existence of even a single planet beyond our own system. Last year saw a flurry of excitement when Lynn and his co-workers at Jodrell Bank reported systematic perturbations of pulsar PSR 182910 which seemed to be explicable only by positing a planetary mass companion of about ten Earth masses. Shortly afterwards, Wolszczan and Frail announced that the pulsar PSR 1257+12, which they had discovered in 1990, showed small differences in the time of arrival of its radio pulses which could be explained if it had two planets orbiting it.
At a meeting of the American Astronomical Society at the beginning of this year Lynn had to report that there was a flaw in his data analysis, and he withdrew his claim: “Our embarrassment is unbounded, and we are very sorry,” he said. Wolszczan’s claim stands, but is yet to be verified. And so we still ride the seas of space without so much as another masthead in sight, as lonely as Magellan inching across the Pacific.
The nearest star to us is Proxima Centauri, a dwarf and faint third member of the Alpha Centauri system. It is 4.2 light years away, and is the commonest type of star within 12 light years of the Sun. Of the 25 stars less than this distance away, 14 are such dwarves, having brightnesses of 1/10,000th to 1/100,000th that of the Sun, and are flare stars. This type of star shows irregular bursts of activity which may be equivalent to double the star’s mean output of radiation, and are akin to flares on the Sun. However, the Sun being so much larger and larger and brighter, such flares are an insignificant proportion of its radiation and of no significance to us apart from radio interference. But for a planet orbiting a dwarf, these flares would be catastrophic; such great variations of the radiation flux from the primary would almost certainly be inimical to the development of complex life forms.
Of course, the volume of a 12 light years-radius sphere is a minute proportion of the two hundred billion cubic light years of the disc of our galaxy. Although failure to detect any signs of artificially produced electromagnetic radiation from neighbouring stars does nothing to dampen the enthusiasm of the hunters as they scan deeper into space, so does the significance of their work decline. Should we detect emissions from a star only 100 light years away, two-way communication between us is going to be a glacially slow business, for two hundred years must elapse between the sending of the question and receipt of its answer. Given the strides we have made towards planetary uninhabitability and the violent self-immolation of our species, the successful completion of even one question and answer cycle must be regarded as doubtful, let alone an extended conversation.
With the vast majority of stars lying thousands and tens of thousands of light years away, signals of such age are going to have little or no practical effect upon us, for we shall never exchange information, let alone ambassadors. At best, the conjecture that there are other intelligent beings in the galaxy will have been confirmed, but we will still be effectively alone—alone to solve the lethal problems we have created in the short span of our own technological civilisation.