Alien life forms are generally regarded as the stuff of science fiction and fantasy. So much so, that serious scientists interested in the possibility that intelligent extraterrestrial life exists now eschew the term ‘alien’. Unfortunately for them, their preferred alternative has itself been hijacked by Steven Spielberg; people who know about aliens only from the movies may be surprised to learn that SETI, the search for extraterrestrial intelligence, was a serious scientific proposal long before the initials ET were attached to that cute fictional alien.
Now, as part of NASA’s Exobiology Program, which seeks to understand the origin, evolution and distribution of life in the Universe, researchers are about to begin the SETI Microwave Observing Project. Starting next year, radio telescopes around the world will search for signals produced by other intelligences. The project is led by NASA’s Ames Research Center in California, and involves scientists there and at the Jet Propulsion Laboratory in Pasadena, California .
In one respect, the task the NASA researchers have set themselves is simple. While philosophers and biologists may debate the meaning of the term ‘intelligence’, for the purposes of the SETI project intelligence simply means the ability to build large radio telescopes and transmitters. There may be all kinds of intelligent life out there in the Galaxy, but only those civilisations that emit radio signals have any chance of detection in this search.
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There are two schools of thought concerning the prospects of success. On the one hand, it is argued that life on Earth, and especially intelligent life, is the result of an incredibly unlikely set of circumstances; there is no intelligent life anywhere else in our Milky Way Galaxy, perhaps none in the entire Universe. But according to the opposing argument, there are so many stars and planets in the Galaxy that, provided there is even a small chance of intelligence developing on any one planet it must have happened many times on many different planets. Nobody seems to take the middle view, that life is restricted to just a few planets in our Galaxy; either it exists solely on Earth, or there are many inhabited planets. If the SETI project detects just one signal, the implication will be that we are not alone, and that evolutionary biology is an inherent characteristic of certain locations in the Universe – planets like Earth.
Why restrict the discussion to ‘other Earths’? This is simply because the researchers are trying to err on the side of caution. We know that intelligence can evolve on a planet like the Earth, because we know that it has happened on Earth itself. So the calculations start out from an estimate of the number of planets like Earth that may be found in our Galaxy.
Observations show that about 10 per cent of all bright stars are roughly similar to the Sun. In our own Galaxy alone, that means 40 billion stars of the right type. Moreover, theorists believe that, in general, planets are a natural by-product of the formation of a star like the Sun from a collapsing cloud of gas and dust; observations made with the Infrared Astronomy Satellite have revealed discs of material around the stars Beta Pictoris and Vega. These dusty discs are thought to be typical of the regions in which theorists say planets form.
So, according to current thinking, there may be tens of billions of stars like our Sun in our Galaxy, most or all of them with retinues of planets. What are the chances that any of those planets will be enough like Earth to harbour life as we know it? The evidence from our own Solar System suggests that the odds are quite high.
We know that there is one planet in the Solar System capable of supporting life – Earth. Studies of the two nearest planets, Venus and Mars, show that we orbit the Sun roughly in the middle of a band within which conditions range from being too hot for life (Venus) to being too cold (Mars). Although this habitable range does correspond to the distance from the Sun, this may not be the only important variable. Ames researchers James Kasting, Owen Toon and James Pollack have recently made a convincing case that another key factor has been the different strengths of the greenhouse effect on each of these planets.
On Earth, temperatures were low enough from the outset for water vapour emitted in large quantities by early volcanoes to condense and form oceans of water. These oceans dissolved much of the carbon dioxide also produced by volcanic activity, ultimately laying it down as carbonate rocks through growth and death of plankton. This thinned the atmosphere, keeping the temperature in the range suitable for liquid water and life to exist. Venus is almost the same size as the Earth and its atmosphere may have gained no more primordial gases from volcanic eruptions, but because Venus is closer to the Sun, it was never cool enough for liquid water to exist. Both water vapour and carbon dioxide stayed in the atmosphere, producing the strong greenhouse effect that makes Venus a hot desert.
Mars is much smaller than Earth, and its gravitational pull has been too weak to hold on to much atmosphere. The planet is cold and lifeless. If Earth were in the orbit of Mars, however, its additional atmosphere would provide a strong enough greenhouse effect to compensate significantly for the extra distance from the Sun, keeping the planet warm enough for life to flourish. Indeed, although the Viking spacecraft that landed on Mars failed to find any evidence of life there today, features of the Martian geography that may have been carved by running water suggest that the planet could have been warmer long ago – perhaps because of an enhanced greenhouse effect from a thicker early atmosphere.
The range of possible orbits for life-bearing planets in our Solar System certainly encompasses both Earth and Mars; optimists argue that it very nearly embraces Venus as well, for Mars might very well be habitable if it were as close to the Sun as Venus. It seems reasonable to suggest that other solar systems also have one or two planets suitable for our kind of life. If only 10 per cent of all solar systems like ours actually have one planet like Earth each, that still means there are billions of such comfortable homes for life in our Galaxy.
Life on Earth is based on the chemistry of carbon, the element that forms the greatest number of complex molecular compounds, many of which, such as DNA, incorporate long chains of carbon molecules. Some scientists, not to mention science fiction writers, have speculated on the possibility of life not as we know it: completely different forms of life chemistry, perhaps based on silicon rather than carbon, perhaps using ammonia in place of our water. Others have pointed out that the ultimate definition of intelligence is the ability to store and manipulate information, and on that basis intelligence might, in principle, be found anywhere from a tenuous interstellar cloud to the high gravity conditions that exist on the surface of a neutron star.
The Ames team, headed by John Billingham, prefers to be more cautious. Echoing the conservatism that restricts estimates of the number of possible homes for life to the number of planets like Earth, Billingham prefers, along with most of his colleagues, to restrict speculation about other forms of life to those based, like ourselves, on the chemistry of carbon. Billingham stresses that this is not to say that other chemistries for life do not exist, but that we simply do not know of any workable alternative to carbon. If more exotic forms do exist, all well and good – that will simply increase the number of likely planets in the Universe where intelligence might evolve.
As well as carbon, life as we know it requires many other heavy elements, from oxygen to iron. These elements are manufactured inside stars, and hurled out into space when stars explode (described in ‘The birth of the elements’, ¿ìè¶ÌÊÓƵ, 16 December 1989). There, they may form some of the ingredients that go into later generations of stars. The first stars that formed after the big bang in which the Universe was born contained only hydrogen and helium; they could not have had any planets. Later generations of stars, with retinues of planets built from the essential heavy elements, began to appear about 10 billion years ago. On the conservative assumptions made by Billingham and his colleagues, that is the earliest date for the possible appearance of life in the Universe. Because planets have been forming ever since, there may also be planets on which life has just begun.
The SETI project is based on the assumption that among the planets that formed steadily over a span of some 10 billion years, there is an immense number suitable for life. The nature of the search, involving radio telescopes, means that we do not need to know what kind of life forms built or operate the transmitters. But Billingham and his colleagues are willing to establish some ground rules for the kind of intelligences they hope to contact, setting out the basis of their belief that intelligent life is widespread by looking at the sequence of evolution inferred from studies of our own planet and its inhabitants.
The first assumption is that life is likely to originate on planets like the Earth, orbiting stars like the Sun. Life began on our planet between 3500 and 4000 million years ago, although the planet itself only formed about 4500 million years ago. This suggests that life is, in a sense, waiting in the wings to take over a planet where the conditions suit. You can imagine going back in time and rerunning the ‘experiment’, watching the Earth form again from a ring of dusty material around the young Sun. It seems highly probable that life would begin again, although the fine details would surely be different. If there are billions of planets like Earth orbiting billions of stars like the Sun, the Galaxy has, in effect, repeated the terrestrial experiment time and time again. Where similar planetary and stellar conditions exist, the occurrence of life is as probable as it was on the early Earth. And even on planets where conditions are not quite right, there are still billions of years in which life might yet gain a grip.
But then there is the problem of survival. As soon as life begins, extinction threatens. If the planetary environment changes, life forms that have established themselves under one set of conditions may be unable to cope with the new order. This may have happened on Mars. If conditions there were indeed more like Earth billions of years ago, before most of its atmosphere leaked away, it is quite possible that the waters on the young Mars contained the same sort of single-celled life as the seas of the young Earth. Earth’s rocks preserve traces of this early life as fossils as much as 3500 million years old. If and when human explorers reach Mars, or when more sophisticated probes than the Viking landers arrive there, the search for such microfossils will be an important part of the mission.
Although the details of the chemistry, form and function of alien species will be different from those of Earth species, it is difficult to imagine any way in which they could be immune from the general laws of evolutionary biology as we know them for Earth. Any flourishing and diverse biota will surely undergo evolutionary processes based on mutation, speciation and natural selection, just as Darwin described. But these processes need not take place at the same rate that we see on Earth. There may be planets where evolution from simple organisms to complexity and intelligence takes place more rapidly than it has here; equally, we can imagine planets where conditions are far from ideal, and primitive life forms gain only a toehold, diversifying very little in billions of years. We have no idea where terrestrial life fits on this scale of mutability. But we do have some idea of the directions in which evolution is likely to lead.
Extraterrestrial species will always be different from ours. But they may have general characteristics in common with each other and with Earth life. At the molecular level, the need to store complex genetic information strongly suggests a need for long-chain molecules like DNA, even if not DNA itself. At a gross level, animals need to take in information about their surroundings, to find food and avoid predators, suggesting the evolution of organs that can make use of the available light. Life on Earth is rich in examples of convergent evolution, in which organisms from different genetic stock have developed similar form and function in response to similar evolutionary pressures. Eyes, for example, have evolved independently in at least 40 different groups of animals. Similarly, the placental mammals of South America and the marsupial mammals of Australia have both evolved species that fit the niches of moles, mice, flying squirrels and anteaters elsewhere.
According to Billingham, the idea that convergent evolution might also happen between the faunas of different planets should not be dismissed lightly. Remember that we are, after all, restricting the discussion to Earth-like planets, orbiting Sun-like stars, so any organs of vision, for example, will use much the same wavelengths of light as we receive from our Sun. Perhaps we should expect general characteristics such as bilateral symmetry, photosynthesis, sexual reproduction, brains and intelligence to be commonplace on such planets.
But the SETI scientists are not suggesting that extraterrestrial biology will be identical to that on Earth. Even with convergent evolution, and even if life elsewhere happened to use DNA itself as the genetic molecule, no other living organism will have your three billion base pairs along the genetic molecule arranged in an identical sequence on identical chromosomes. Humans will not be able to interbreed with aliens, much as this will disrupt the plots of the wilder fantasy novels. Indeed, any species on Earth is sure to be unique in the Universe. There is only one Drosophila melanogaster and there was only one Tyrannosaurus rex. Extraterrestrial species of comparable complexity are equally likely to be unique.
This is an important point. The proposition that humans are unique, as a species, has been put forward by critics of SETI as an argument for not wasting time and money searching for something that is not there – other humans. But the argument is flawed; no one expects to find other humans. The SETI hypothesis is that the Universe holds many examples of intelligence of the human type – creatures with the power of abstract thought, beings who can construct at least partly successful models of the external world and who can use their skills to build things and (to some limited extent) predict the future. They may look something like us, a little like us, or nothing like us; they will never be the same as us.
And no one claims that convergent evolution is always the case. There could well be biologies so different from ours that we cannot imagine them. But if convergence is present in some cases, that is sufficient to encourage the search. Intelligence is likely to be one of the general characteristics in which a significant degree of convergence is likely; the ability to process information efficiently, understand the environment and manipulate tools must surely be an evolutionary advantage. Whether this would lead to a technological civilisation interested in radio astronomy is open to debate, but the SETI researchers point out that such civilisations only have to arise occasionally to be widespread in the Galaxy.
Even if our kind of intelligence is associated with just one star in every 100,000 of the same type as our Sun, then there are 400,000 technological civilisations in our Galaxy alone. Life can be a rare happening in terms of the percentage of stars involved, but still be abundant in a galaxy. SETI does not look specifically for humanoids. Had we evolved in a different way, we might nevertheless be searching for other forms of life. What SETI hopes to find traces of is the cognitive type of intelligence that humans have, which Billingham and his colleagues assume is occasionally shared by extra-terrestrial beings having a variety of different body plans, functions, genetic make-up and social organisation.
The way they have chosen to conduct the search is by using radio telescopes. Other civilisations may be transmitting microwave signals, perhaps in the same way that we communicate through radio, television and so on, or maybe in a deliberate attempt at interstellar communication. It is technically feasible, and relatively cheap, to search for these signals using existing radio telescopes. The SETI project will systematically search the sky for signals in the range 1 to 10 gigahertz, where emissions from natural and background sources are least obtrusive. There are two aspects to the programme. The Targeted Search will examine at high sensitivity about 800 stars like the Sun which lie relatively close to us, while the Sky Survey will slowly scan the entire sky, but with rather less sensitivity.
The search will be largely automated, and will use existing radio telescopes such as the 305-metre dish at Arecibo in Puerto Rico. The telescopes will be fitted with special signal processors that search for patterns of microwave noise that are not (as far as anybody knows) produced by natural astrophysical processes such as the regular bursts of radiation from pulsars. The high speed digital processors can search across a wide range of wavelengths and pick out subtle variations: one system can resolve frequencies separated by about 1 Hz from input that ranges across 10 MHz. The systems also discriminate automatically against radio frequency interference.
The project will start listening out for ET in 1992, and will continue for about 10 years. It will not detect any beings who have not developed our type of intelligence, because they will not be transmitting. But according to the arguments outlined here, there may be other species in our Galaxy anything up to a billion years older than us. There is little anyone can say about their biology and civilisation, apart from knowing that if they have survived for long they will have passed through our own stage of civilisation – in many cases, long ago – and achieved some sort of stability and longevity. In some cases, they may have advanced far beyond our level of cognitive intelligence; in other cases, not. But we should be aware that any species we detect may be far in advance of us in their evolutionary development.
Another possibility, in line with the definition of intelligence used here, is that we might make contact with machines. Some researchers believe that machines might be built which have all the characteristics of human intelligence – and more – and which might well be motivated to build radio telescopes.
If the project finds one source of extraterrestrial intelligence, there will surely be many. It is difficult to imagine circumstances which could lead to the existence of only two civilisations in the Universe, and it is likely that we could learn about others from the first one we contact. We could, perhaps, tune in to many other civilisations, once our presence is known, through a network of communicating intelligences. One of the most fascinating things that such a cosmic network will communicate is information about the beings that operate it. For the Earth-bound biologist, the prospect of understanding a single case of extraterrestrial life is exciting enough; having many to study is a compelling prospect. The extraterrestrials themselves may be equally interested in learning about our own biology.
But there is a limit to our speculation, for above all, SETI is an exploration. ‘A frequent consequence of exploration,’ says Billingham, ‘is the discovery of things that were not predicted. We should expect the unexpected.’
Background information on the SETI Microwave Observing Project supplied by John Billingham, of NASA. John Gribbin is coauthor of Cosmic Coincidences (Black Swan, 1991) which looks at the place of humanking in the Universe.