Were earthlings to embark on such a scientific (or expansionist) adventure with little more than present-day technology, and assuming that each newly settled colony constructed its own craft and founded its own colonies, it would take less than 50 million years to put a New Zealand flag on every half-decent piece of real estate in the Milky Way. That seems like a long time but is nothing in a 10,000 m-year-old galaxy, and by Fermi’s reckoning such a feat should have been accomplished a long time ago.
Our Solar System is only half the age of the Milky Way. That gives all of the elderly races closer to the galactic core a few billion years head start on us, and yet, as Fermi noted, we have uncovered no unequivocal evidence of extraterrestrial (ET) civilisations.
Astronomer and ET hunter Frank Drake produced an equation to estimate a likelihood and distribution of intelligent ETs. He and fellow astronomer Carl Sagan calculated that there should be around a million civilisations in our Galaxy. If that is so, not one of them appears to have any urge to explore, needs more space for an expanding population, or has the feral aspirations to establish hegemony over a swathe of the universe. There is much that can be read into the silence of the stars.
It seems reasonable to ask: why should there be any aliens anyway? Life here on Earth seems like nothing short of a miracle. No self-evident rationale explains why, when you take a sample of raw elements from the periodic table and apply heat, add a pinch of sulphur and a splash of metals and stir for a bit, you can produce molecular architecture that will replicate slightly inaccurately over vast time spans until it eventually resembles a spider, a peacock or Yehudi Menuhin.
Carbon to Chopin we are enormously complex configurations of basic material. Four of the six most common elements in space—hydrogen, carbon, oxygen and nitrogen (the other two, helium and neon, are inert)—are also the top four ingredients for life on Earth. If, instead of this formula, terrestrial life had coalesced from higher elements—say silicon, gadolinium, krypton and palladium—it would be tempting to assume that our planet and its cargo were unique. Their being made of common stuff, while not ruling out the possibility of life forming out of less common elements, greatly enhances the likelihood of extraterrestrial life in the cosmos. We know that life, as an expression, is not inhibited by a need for extreme circumstances or an unlikely congruence of rare elements. Simply throw together the basics—water, carbon, energy—and leave to simmer.
Earth formed from an accretion of space debris 5–4.5 billion years ago. Water, in liquid form, made its appearance as the planet cooled more than 4 b years ago (possibly as early as 4.4 b). Chemical traces from the oldest sedimentary rocks found—3.85 b years old, in Greenland—show indications of carbon fixation and perhaps photosynthesis too, suggesting an almost immediate onset of biological activity.
The earliest microfossils are of simple cyanobacteria, and they first register in the geological record 300 m years later. Forming microbial mats, these prokaryotes produced stromatolitic mounds in shallow seas and along intertidal zones. In the Pilbara region of Western Australia, 3.45 b-year-old fossil stromatolites are the earliest direct evidence of life on Earth—and stromatolites still inhabit the waters of nearby Shark Bay. Cyanobacteria manufacture useful molecules from atmospheric carbon dioxide and nitrogen, pumping oxygen into the atmosphere as a waste product.
The early oceans contained a lot of iron that acted as an oxygen sink, soaking up O2 and forming bands of rust. As the large quantities of iron in the ocean were depleted, the atmosphere became infused with free oxygen, beginning 2 b years ago and reaching high saturation levels 1.5 b years ago.
Excessive O2 led to the mass extinction of organisms for which the gas was toxic. To avoid drowning in their own waste, prokaryotes either had to hide from O2 or adapt to aerobic conditions. Indeed, 1.5 b years ago—about the mid-point of the history of life on Earth—eukaryotic organisms appeared. Within these cells, genetic material was now contained in a nucleus, bounded by a membrane, and aerobic energy production was carried out in mitochondria—organelles that arose from the incorporation of aerobic bacteria into an anaerobic bacterium. Although some prokaryotes had become aerobic, all the eukaryotes were dependent on oxygen, generating energy more efficiently than anaerobic metabolism.
The next crucial step was taken with the appearance of multicellular organisms about 700 m years ago. Whereas single-celled organisms can be found grouped in colonies—stromatolites are a good example—the step up to a multicellular configuration involved something new and innovative: an ability to work in concert for group survival, thereby augmenting individual survival prospects en masse. In this new interdependency, tasks could be allocated to specific cell groups, which meant organisms developed asymmetries —organs, limbs and other features that could be tailored to exploit their surroundings.
Some 530 m years ago a frenzy of diversification occurred, an event known as the Cambrian Explosion. Almost all living animal phyla appeared in a mere 10 m years. A boom in phytoplankton led to further increases in oxygen levels, strengthening the ozone layer. Shielded now from damaging solar UV radiation, life was able to spread out of the oceans onto land.
The Palaeozoic Era, which began with the Cambrian period, ended in a massive extinction event 245 m years ago, following which giant reptiles assumed dominance. The Mesozoic Era is popularly referred to as the Age of the Dinosaurs, and 66.4 m years ago it too ended with a mass extinction of species, delineated at the K/T Boundary (the time zone comprising the end of the Cretaceous and the beginning of the Tertiary periods).
Since then, the modern era—the Cenozoic —has been notable for a radiation of mammals, which came to dominate the megafauna as the reptiles had earlier. A mere 100,000 years ago, certain hominids—having speciated from apes 3–5 m years earlier—began their climb to supreme predator status. In the last 12,000 years Homo sapiens has mastered horticulture and 6000 years ago began writing in cuneiform script before advancing to the myriad written languages in use today. Less than a hundred years ago, we transmitted our first radio message.
This extraordinary progression occurred against daunting orders of probability. Earth’s unstable environment—punctuated by meteor impacts, geothermal and tectonic activity, cosmic irradiations, atmospheric changes induced by the metabolisms of its inhabitants and the steady pulse of climatic variation—has doubtless helped drive evolutionary change.
Accepting the sheer improbability of emerging from bacterial slime to stare through telescopes, many astrobiologists believe that whereas life could be a universal phenomenon, intelligence will prove to be far less common. We are the only example out of the 10 b or so species in Earth’s history to have evolved to our level of technological mastery. Intelligence as an evolutionary strategy has not enjoyed a high take-up rate.
On the other hand, perhaps natural disasters ensure that intelligence will always arise, for when it does it can transcend habitat dependency. One can argue that the true hit rate—one intelligent species per suitable planet—is clearly present in the data. Nevertheless, extrapolating from a sample size of one is, at best, a speculative exercise.
Given such a chaotic progress, it is unlikely that life elsewhere in the cosmos will have followed the same developmental path as it has on Earth. It may not necessarily be carbon-based (organic), and it is possible that we might not even readily recognise it as life.
Looking skyward there are an estimated 70 sextillion (70 ,000,000,000,000,000,000,000) stars in the known Universe and around 300 billion in the Milky Way Galaxy (estimates range from 100 b to 500 b). An unknown percentage of these will have systems with one or more orbiting planets in a life-sustaining envelope.
Even if just one star in a million has Earth-like planets, there should be a lot of life out there. Non–Earth-like planets that allow biological evolution are also possible. A quick scan of planets and moons in our backyard hints at the possibilities.
Last year, one of the Mars rovers, Opportunity, established the presence of Martian rocks formed out of sulphate salts. The only way to make rocks with this mineral configuration is through the evaporation of salt-laden seas, and such seas would have been present for quite some time. It seems likely, then, that Mars was once a benign planet with large oceans—perhaps even as Earth’s surface steamed and bubbled with magma.
In February this year a massive frozen sea of what appears to be water ice, strikingly similar in appearance to broken pack ice, was discovered beneath the surface near the equator of Mars. This frozen sea measures 800 x 900 km up to 45 m deep.
At the Martian equator, temperatures often rise above freezing, making such an ocean a plausible habitat for microbial life. More fuel has been added to the debate by revisiting the equivocal Viking results of 1976, which reveal carbon14-containing methane in soil samples, and the identification in recent months by a Fourier spectrometer on board the Mars Express spacecraft of significant levels of atmospheric methane in the thin Martian atmosphere. Because UV radiation breaks down atmospheric methane in a few hundred years, this discovery implies that methane is still being produced on Mars. Whether the source is microbial or volcanic is an open question.
NASA scientists working with Opportunity’s data conclude so much water was involved in the ancient geologic processes on Mars that life could easily have existed there. They wonder if life might have arisen on Mars and ejecta raining down on our primordial seas subsequently seeded Earth. It is estimated that around 10 per cent of the meteors ejected from Mars have collided with Earth.
Ice deposits on Mars might contain fossilised or even dormant viable life forms. Martian life would serve as the equivalent of an astrobiologist’s Rosetta Stone, helping to determine the nature, prevalence and potential diversity of life in the cosmos.
For all its apparent diversity, life on Earth is really something of a monoculture—many different expressions of the same thing. Out of all the amino acids available, the same short list of 20, and the same DNA and RNA form the basis of all terrestrial life. When we consider the possibility of independently evolved life, the odds on it using sets of molecules identical, or even very similar, to those involved in terrestrial biology are small.
If we do find fossils on Mars, therefore, and they match the molecular profile of terrestrial life, the clear implication will be that men really are from Mars.
A question mark also hangs over Europa, the Galilean moon with an entire surface reminiscent of polar pack ice. There is mounting evidence that a large body of salty water, many kilometres deep, sits beneath Europa’s icy crust. Europa is heated by tidal flexing. Strong gravitational forces caused by the motions of Jupiter and its larger moons, particularly Io and Ganymede, pull Europa in different directions. The extreme rigidity of Europa’s surface means that ice in the subsurface is continuously compressed and extended as strong tides rise and fall. This continuous flexing—a potential mechanism for liquefying the ice—is primarily responsible for the many deep cracks on the moon’s surface.
A combination of interior heat, water and infall of possible hydrocarbons from comets and meteorites provides Europa with all the key ingredients for life.
Earth’s polar marine ecosystems are some of the planet’s richest, and the discovery of deep-ocean life around hydrothermal vents supplies us with a model that might also apply to Europa.
Hold the line caller if there are intelligent aliens in the Galaxy, and if they share our desire to communicate across interstellar space, then radio waves could be the communication band of choice. Radio waves traverse the galaxy unimpeded by interstellar gas and dust clouds.
In 1960 Project Ozma, which evolved into Project SETI—the Search for Extraterrestrial Intelligence—was set up by Frank Drake and began scanning deep space for any messages that ETs might have broadcast in our direction, either intentionally or by accident. Given that humanity, in the normal course of television and radio broadcasting, pumps a thick band of electromagnetic spectrum data at the stars, it was considered likely that similar or more advanced civilisations in our galactic neighbourhood could be detectable in this spectrum.
In 1999, the SETI@home project was launched. This encourages PC users to download a SETI screen-saver from the internet. The screen saver is actually a program that assesses radio-survey data from recordings made at the Aricibo Radio Telescope, in Puerto Rico, and then reports the results back over the internet. It cleverly utilises the unharnessed computing power of PCs when they are in screen-saver mode.
Over five million computer users in hundreds of countries have collectively contributed more than 14 billion hours of computer processing time to this cause, greatly increasing the scope of the search for extraterrestrials.
The most famous radio signal in SETI history was detected on the night of August 15, 1977, at the Ohio State University Big Ear Observatory. Big Ear had completed a major wideband mapping of the night sky, and in 1973 had been converted to make a similar map of narrowband sources. Readings, recorded as strings of letters and numbers, cascaded down printout sheets (as in The Matrix). Big Ear volunteer Jerry Ehman, a professor at Franklin University in Columbus, regularly checked these.
Reviewing the August 15 data a few days later, Ehman was astonished to find a series of characters that formed 6EQUJ5. He circled the code and added “Wow!” in the margin, after which it became known as the “Wow!” signal.
The numbers and letters were not a message as such but indicated the radio signal’s strength. The radio frequency was set at 1420 MHz, the frequency of hydrogen. Since hydrogen is the most abundant element in the universe, it was thought that aliens wanting to attract attention might use this frequency. It is also a frequency which is out of bounds to broadcasters, so would be clear of local noise.
The scanning beam registered a reading every six seconds. A number from 1 to 9 denoted each step-up in intensity above the background noise, and the range was extended to 35 by inserting letters of the alphabet.
The sequence 6EQUJ5, therefore, denoted a signal that grew in strength to level U and then gradually subsided. This meant that the signal increased from zero to very intense and then decreased again to zero over 37 seconds.
Thirty-seven seconds was precisely the time it took the Big Ear scanning beam to cross a point in deep space, so the “Wow!” signal exactly matched the predicted profile for a signal of deep-space origin.
The signal was also intermittent. Big Ear had two separate beams that scanned the same area of sky in succession several minutes apart, but the signal appeared on one of the beams and not the other. A strong, focused, intermittent signal coming from the direction of the constellation Sagittarius (and the galactic centre): could it be that Big Ear had detected an alien?
For a month following the discovery the Big Ear crew tried to relocate the signal without success. Other SETI searches have also focused on the “Wow!” coordinates, but no more broadcasts from this location have ever been detected.
To this day we do not know the source of the strongest and clearest signal ever to come from a SETI search. Since 6EQUJ5 was undoubtedly artificial, and almost certainly of celestial origin, there remains a possibility that it was a true signal from an alien civilisation. But unless the signal is detected again, we will never know for sure.
Radio signals are not the only possible indicators of life. Chemical signatures picked up in the spectrographic analysis of distant bodies contain information riches and an abundance of free oxygen in a planet’s atmosphere, should we detect it, would be a strong indication that something really interesting was going on. Since oxygen is highly reactive, atmospheric oxygen is taken out of circulation by iron and other surface chemicals. For it to be detectable on a mineral-rich rocky planet, there must be some regenerating mechanism to counter this tendency. Water and carbon dioxide can be broken down to yield oxygen, but the reactions involved usually require enzymes that are only known to occur in living organisms.
Fermi’s paradox in 1950 Enrico Fermi, lunching with some friends, chewed over the idea that the Galaxy should be heavily populated. The general consensus was yes but Fermi was not so sure. “Where is everybody?” he asked.
On the surface this looks like a simple-minded objection, but Fermi was being perceptive his paradox is a very strong argument. Even if creatures are reluctant to travel, he reasoned, what about artificial intelligences (effectively immortal) or simplistic remote devices (von Neumann probes)? If there are any other advanced civilisations, then signs of ETIs (extraterrestrial intelligences) should be everywhere. Their chatter should fill the galactic plane.
Solutions to Fermi’s Paradox fall into two main sets: those that state ETIs don’t exist, and those offering explanations as to why we have failed to detect them. A third set, which is really a subset of the second, could be termed the X-file group and includes scenarios like the Zoo Hypothesis (ETs have cordoned us off and set up some mirrors).
A major difficulty with the first set is that all its candidate explanations require Earth to be not just special but unique—alone in the immensity of the cosmos. The difficulty with the second (and third) set is that nearly all solutions require an unwarranted uniformity of motive for all ET civilisations over very long time spans, or an assumption they are operating under a Star Trek styled “Prime Directive,” which is, by its very nature, unprovable.
The leading candidate among the first set is the Rare Earth Hypothesis, championed by astronomer Donald Brownlee and palaeontologist Peter Ward. It argues that intelligence has developed on Earth through an impossible-toreplicate serendipity, and that Earth-type planets must be exceedingly rare anyway.
It reasons thus: Earth-like planets can only exist having formed around a metal-rich star, which means being not too close and not too far from the galactic centre, not too old and not too young. The star must be in the correct range, types F1 to K7, which comprises just half a per cent of all stars, and it must maintain a circular galactic orbit to avoid sterilising radiation from the galactic centre, where a super-massive black hole is believed to reside.
During early formation, a Mars-sized body must collide with it at just the right angle. This is believed to have happened to the young Earth, resulting in the development of plate tectonics (needed for an active geology) and the positioning of our large moon, which gravitationally stabilises Earth’s orbit. Such a collision might also produce an oversized planetary core, responsible for generating a magnetic field that deflects dangerous radiation. A large moon is also a primary shield from asteroids.
Jupiter’s gravity cleared away the debris from Solar System formation, so a Jupiter-sized body in a Jupiter-like orbit would be a big advantage too.
Then there were a series of well-timed changes to the ecology of our planet during its history that kept evolution on track. These included the critical mass-extinction events that allowed evolution to change tack whenever it risked stagnating.
While much of the Rare Earth Hypothesis involves a compelling study of the emergence of life and then intelligence here on Earth, it is essentially anthropic, as it does not concede the possibility of benign habitats arising in other ways, of animals evolving within more rapidly changing habitats, or intelligence emerging from differently configured biologies. Critics point out that “Rare Earth” contains a circular argument, exhibits carbon chauvinism and has been unduly influenced by the creationist values of some of its architects.
In the second set of solutions, many reasonable scenarios explain why one civilisation, or even a percentage of civilisations, might be invisible to us, but these rationales generally fail on the basis that not all ETIs would develop according to the same cultural template.
We leak less radio-frequency energy into space today than we did a few decades ago because we have developed more efficient technologies, such as cable and satellite. As communication technology improves further these systems are likely to be superseded by point-to-point laser and microwave communication. The Earth, to an outside observer, will then have fallen silent. Perhaps a new technological culture is only visible in the radio spectrum for a hundred years? If so, the lack of accidental broadcast noise around the galactic plane would indicate there are no such hundred‑year cultures within range of our telescopes. Suppose that more advanced races choose to inhabit Dyson Spheres—artificial habitats that enclose their star, maximising its energy while producing a nearly invisible electromagnetic radiation signature: we wouldn’t see them either.
Some argue that intelligent life cannot avoid self-destruction almost as soon as it has the know-how—a legacy of an evolutionary psychology that made it the dominant predator in the first place. We narrowly missed this fate in the Cold War and are, in our new age of nuclear posturing, still a long way from being out of the woods.
An interesting variation on this idea is the Technological Singularity scenario, which supposes that the inevitable construction of artificial intelligence (AI) is followed by an abrupt end to biological evolution. Vastly superior AIs wouldn’t bother with radio communication and might not choose to interact with biological primitives either.
There is also a subset of theories that propose intermittent galaxy-scale sterilising events, so that the evolutionary clock is regularly restarted on life-bearing planets. Among these ideas are berserker pogroms, aliens with attitude who stamp out rival intelligence wherever it is detected, and cosmic phenomena that effectively do the same job.
Gamma-ray bursters are one disturbing cosmic phenomenon that might saturate a largish portion of our Galaxy with sterilising radiation at regular intervals. Bursters, believed to be caused by the creation of black holes out of hypernovae or through the collision of neutron star binaries, expel doses of gamma radiation so intense that they are easily detected from distant galaxies.
Periodic Milky Way gamma bursts would be a serious setback for any evolving life within range of the event (a good portion of the Galaxy). Intelligence, if it does emerge, may not be able to establish a galactic foothold due to the frequency of these events. In this scenario humanity could be near the end of a lucky roll, emerging into a sterile sector of the Galaxy while teetering on the brink of the next gamma-burst episode.
Finally, there are solutions to Fermi’s Paradox built out of mathematical models. One such solution proposed by NASA researcher Geoffrey Landis uses Percolation Theory to demonstrate that the colonisation of the Galaxy will follow a fractal distribution pattern where colonising and non-colonising civilisations co-exist.
Add gamma-bursts to the equation and empires could fragment, producing large voids (uncolonised zones) in the Galaxy. Landis suggests that the Solar System could be located in one of these uncolonised zones, hence the impression that not much is happening out there.
Back to the future although the search has yielded no definitive answers to date, an ongoing mission to find ETI is considered by many to be vital to humanity. Whether we discover that we are part of a galactic community with whom we can share science, art and philosophy, or learn that we are profoundly alone after all, the value of this knowledge is incalculable.
We cannot foresee how, if we do contact another intelligent species, it will alter our self-perception, or what cultural changes we will have to accept. We don’t know whether contact will mark the end of a golden age or signify the beginning of a renaissance.
Humans are intellectually curious. Our ancestors came out of Africa in search of other worlds, their passage our unwitting fledgling steps toward the stars. Embracing the unknown is our legacy. It has made us who we are.