Our Galactic Significance

As noted in Keith Windschuttle’s Chronicle (May 2012), Bob Brown received almost universal media derision for his speculations concerning the absence of contact with possible intra-galactic technological life-forms, and his support for “one-world democratic government”, somehow immune from takeover by some form of global autocracy. While Senator Brown’s thoughts concerning a global government seem in the “far out crazy” category, his comments regarding the “Great Silence” from some fifty years of searching (by SETI—“search for extra-terrestrial intelligence”— radio-astronomers) for some indication of another technology-based civilisation “out there”, are worthy of more serious consideration.

As the American radio-astronomer Frank Drake has pointed out, we can multiply together a sequence of estimated probabilities, none of which can be zero, to calculate the number of technology-based civilisations—at least one—among the Milky Way galaxy’s billions of stars, many (perhaps most) of which are accompanied by planets or planetary systems. Astronomy texts frequently include Drake’s “equation” in their discussion of the question “Are we alone?” In Australia, SETI researchers have used the Parkes radio telescope to study nearby sun-like stars (within 1000 light-years) and their possible planetary systems: to date, its dish has detected reflections from overflying galahs, but nothing from extra-terrestrials. However, the Drake equation remains worthy of serious consideration.

To help us to organise and simplify our thinking—our attempt to make sense of the question, “Are we alone in the immensity of spacetime?”—we can use this equation to estimate the probability of any other Electromagnetically Telecommunicating Civilisations (ETC) co-existing with us, elsewhere in our galaxy. Drake’s equation, for the number of ETCs (at least one) in the galaxy:

N = S × P × T × B × I × C × L

Drake proposed seven sequential factors—three astronomical, two biological and two sociological (these last two cause most of the arguments). For each factor, we can estimate a non-zero probability (low to high), or a value (for S, T and L). The chance of finding another ETC in the Milky Way is then estimated by multiplying together the values assigned to each factor.

S is the rate of formation of stars, per year. The number of stars in the galaxy is between 200 billion and 400 billion, formed during some 12 billion years. Drake estimates that about twenty new single long-life (Sun-like) stars form every year within the galaxy’s potentially bio-friendly spiral arms, as detected by infra-red and optical observations of star formation within its “giant molecular clouds” (dark nebulae).

P is the fraction of these new stars with planetary systems. Angular momentum conservation during planetary accretion (from cosmic dust to “sticky fluff” to “putty balls” to planetesimals to planets); observations of proto-planetary gas/dust discs around young stars; “wobble-watching” and gravitational micro-lensing studies of nearby stars: these indicate a high probability for P: 0.9 (90 per cent) or greater.

T is the average number of terrestrial-type planets per planetary system, able to support life. Our own Solar System, and detection of more than 500 extrasolar planets, imply the existence of bio-friendly terrestrial-type planets. “Inner solar systems” seem necessary, protected from infalling comets and asteroids by outer Jupiter-type “punching-bag” gas giants, with at least one rocky silicate/carbon inner water world orbiting within the “ecosphere” of a long-life parent star: a “Goldilocks” world, enough gravity to retain an atmosphere, warm enough for liquid water. However: excess carbon over oxygen in planet-forming nebulae would result in bio-unfriendly worlds—graphite crust, diamond interior, tar oceans. T could be 0.1 (that is, one terrestrial planet for every ten systems).

B is the fraction of these “bio-friendly water-worlds” on which a self-sustainable living biosphere originates and develops. Based on the ubiquitous presence of small organic molecules (H₂O, CO, HCOH, HCN, alcohols, amino-acids and many others) in dark nebulae, chondrite meteorites and cosmic dust grains, and the apparent ease of synthesis of “protobiont” organic molecules in experiments with primitive reducing atmospheres and oceans, it seems that B could be high: 0.5 or greater. However, it seems likely that any such life on Mars was short-lived. Are other “exobiologies” possible, not based on water and carbon? We know only one life-bearing planet: Earth (the “n=1 problem”).

I is the fraction of these primitive (procaryote, bacterial?) biospheres which evolve to include complex conscious self-aware “intelligent” life-forms. Some 9 billion years for supernovae to generate enough “metals” (elements heavier than hydrogen and helium), plus 4 billion years of biological evolution on Earth, have been needed. This number could be very small. Are we among the first to attain intelligence? Hence, SETI’s “great silence”?

C is the fraction of intelligent life-forms which discover the necessary communications technology, and are willing and able to send or search for radio and other electromagnetic signals from other ETCs. On Earth, among many cultures living in settled communities, those using Western science-based technology have entered this stage of development. C, on Earth, has depended on the evolution of non-nomadic societies (agriculture-based city-states with stable institutions—universities or similar), able to undertake evidence-based research into nature’s workings, unobstructed by totalitarian or other orthodoxies. Can we assume that, given time (some 100,000 years, if Earth is any guide), C is greater than 0.5? Again, the “n=1 problem”: might any other ETCs conduct their affairs more rationally and responsibly than we do, avoiding millennial delays caused by misuse of resources, prevention of research and development, and destructive warfare?

L is the estimated life-span of an ETC, from its emergence to its extinction. The “n=1 problem” again looms large. Our civilisation has existed for some seventy years since radio-communication came into wide use and the first radio-astronomers detected extra-terrestrial signals (Jansky in the 1930s found radio noise originating from Jupiter’s turbulent atmosphere and the Sun’s corona). Provided that our technological civilisation is able to sustain itself, avoiding collapse due to planetary climatic impacts, over-use of limited resources, or catastrophic conflict, it could conceivably last for another billion years or so before Earth becomes uninhabitable due to gradual solar warming (about 10 per cent per billion years). Or it could collapse to a primitive state, with loss of radio-astronomy and other advanced technologies, within (say) 100 years. Would a life-span somewhere within this huge range be typical of other ETCs? Would they be interested in searching for signals from other ETCs?

Multiplying all the estimated values, we arrive at an estimate that ETCs are probably exceedingly rare, at best none within thousands of light-years of Earth, far beyond the nearest stars. Indeed, we could be the only one among the Milky Way’s 200-plus billion stars. Other galaxies are too remote for radio contact to be possible.

In which case, the means by which we manage our small world and conduct our affairs could be of galactic, even cosmic significance. Conscious mind, capable of “bringing self-awareness to the Universe”, could be an exceedingly rare phenomenon. We may usefully recall Monty Python’s pithy injunction: “So pray that there’s intelligent life somewhere out in space, because there’s b— all down here on Earth.” 

Another thought, from the Harvard cosmologist Abraham Loeb (Scientific American, November 2006): 

When I look up into the night sky, I often wonder whether we humans are too preoccupied with ourselves … the universe puts things in perspective … my own life, for example … it gives me a sense of longevity … because of the big picture. Perhaps the greatest triumph of the past century has been a model of the universe that is supported by a large body of data. The value of such a model to society is sometimes underappreciated. When I open the daily newspaper … I often see lengthy descriptions of conflicts between people about borders, possessions or liberties. Today’s news is often forgotten a few days later. But when one opens ancient texts that have appealed to a broader audience over a longer time, such as the Bible, what does one find in the opening chapter? A discussion of how the constituents of the universe—light, stars, life—were created. Although we humans are often caught up with mundane problems, we are curious … as citizens of the universe, we cannot help but wonder how the first sources of light formed, how life came into existence and whether we are alone as intelligent beings in this vast space. Astronomers in the 21st century are uniquely positioned to answer these big questions.