It seems with each passing year, the Drake Equation is actualizing itself, right before our eyes, towards 1.00
Though only about dozen potentially habitable exoplanets have been detected so far, scientists say the universe should be teeming with alien worlds that could support life. The Milky Way alone may host 60 billion such planets around faint red dwarf stars, a new estimate suggests.
Based on data from NASA’s planet-hunting Kepler spacecraft, scientists have predicted that there should be one Earth-size planet in the habitable zone of each red dwarf, the most common type of star. But a group of researchers has now doubled that estimate after considering how cloud cover might help an alien planet support life.
“Clouds cause warming, and they cause cooling on Earth,” study researcher Dorian Abbot, an assistant professor in geophysical sciences at the University of Chicago, said in a statement. “They reflect sunlight to cool things off, and they absorb infrared radiation from the surface to make a greenhouse effect. That’s part of what keeps the planet warm enough to sustain life.
The notion of extraterrestrial intelligent life presupposes the existence of extraterrestrial life itself, and this brings up the problem of our definition of ‘life’, not to mention our definitions of ‘intelligence’ within the aforementioned ‘life’. In Extraterrestrials: Science and Alien Intelligence, philosopher Edward Regis writes:
There is no generally accepted definition of life, nor is there agreement regarding on what a correct definition ought to be based — whether, for example, upon physiology, metabolism, biochemistry, genetics, thermodynamics or indeed something else. Neither is there agreement on whether non-carbon-based life is possible… Nevertheless, many scientists contend that because of the abundance of carbon in the universe, and its ease of bonding with other elements thereby forming a variety of stable compounds, carbon is the element of choice for the origin of life. Another problem with life based on alternative biochemistries is that we might not be able to recognize, much less interact with, such life even should it exist…(Regis, Extraterrestrials: Science and Alien Intelligence, 1985, p. 19.)
To what extent our notions of reality, as perceived through our five senses along with concomitant metaphysical necessities (e.g., experiences of spatiality, temporality and causality), should be shared across the universe is generally an area of philosophical inquiry, and as such introduces questions of anthropomorphism. That tacit anthropomorphism within the SETI community may or may not be a problem — it all depends upon your epistemology, i.e., whether you believe the human mind is ultimately an accurate “mirror of nature” or rather that the mind is itself (with all its rational categories of space, time, form, causality, etc.) the product of Darwinian evolution. From even an”evolutionary epistemology” position, which at first sight may appear irremediably pessimistic about the very concept of extraterrestrial intelligence (ETI), may still maintain (in a Peircean tradition) the notion that Ultimate Reality (insofar as this is a logically coherent term) acts as somewhat of a guide, regulating our knowledge in a determinate sense.
Needless to say, most scientists involved with SETI simply talk about “life as we know it”, framing the probability of similar such life arising on other worlds in terms of an equation, one that incorporates the probabilities of many complex, relevant variables (from physics, astrophysics, biochemistry, evolutionary biology, and even psychology.)
Such is the lens called the Drake Equation. Named after the pioneering figure in the history of SETI, the Drake Equation is a mathematical string of multiplicative factors of the form:
N = R a b c d e L
The definitions of the factors are:
N: the number of currently extant hi-tech galactic civilizations;
R: the rate of galactic star formation;
a: the fraction of stars which have planets;
b: the number of earthlike planets per system;
c: the fraction of earths which will form life;
d: the fraction of ecologies which will evolve intelligences;
e: the fraction of ETI which will develop civilizations;
L: the mean lifetime of an advanced civilization.
Were we to concern ourselves with not just the chance of life elsewhere but intelligent life elsewhere, a more speculative Drake Equation would accommodate additional factors such as: thresholds of rationality that would motivate such intelligent life to send out SETI-like messages across the cosmos, the vastness of interstellar distances, whether gravity and space-time could conceivably be manipulated in non-relativistic terms, motivations for a comprehensive interstellar space exploration, etc.
As it stands, variables “R” and “a” appear to be among the least contentious of the above Drake Equation factors. That is, there seems to be much more agreement among the respective physical scientists as to the probabilities of these factors and, as news stories like today’s attest to, these factors are increasingly in probability all the time.
Variable ‘R’: The rate of star formation in just our galaxy alone is very much agreed upon, as it is arrived at in a rather straightforward manner. With a reasonable understanding of the process of starbirth, or by calculating the approximate history/time scale of the galaxy (along with an accurate star count), scientists estimate that our galaxy has averaged about 25 star births per year, and has perhaps slowed down to between one and ten star births per year in its current stage of development. Now, this variable “R” itself breaks down (as does all the other Drake Equation variables in an ideally-limited theoretic sense) into other factors: How many stars are suitable for life-formation? How many become unsuitable as their life histories progress? After eliminating those stars not conducive to planetary ecologies, we are left with six to fifteen billion sun-like stars in a galaxy of 250 billion stars (although estimates are increasing every year, as new information comes in.)
Variable “a”: The fraction of stars which have planets — is a factor whose speculation stems, in part, from solid empirical evidence right here in our own solar system. Not only do we have one “planetary system” of nine bodies revolving around the Sun, but we also have several mini-systems of moons revolving around Earth, Jupiter, etc., leading many to believe that large rotating centers-of-mass naturally acquire secondary bodies revolving about them. This view is further supported by empirical measurements of various stars in our galaxy (gravitational wobbles caused by large unseeable objects on the stars; the widespread galactic phenomenon of “double stars”, a variant of a planetary system, etc.) As more stories like today’s come out, current planetary theories suggest that planets surrounding a star should be the rule rather than the exception.
Variable “b”: The number of earthlike planets per system — is much more contentious. Here, we are defining earths as rocky, terrestrial planets which stably orbit their suns for long periods of time at a distance which allows a proper temperature/radiation input so as to keep the solvent-of-life, water, in its liquid state. The frequency of such earths occurring during the formation of a planetary system is still widely debated, with the pessimistic side being greatly influenced by the models of Michael Hart.
As a sun-like star condenses by gravity out of a heavy molecular cloud, it flattens and takes on its disc-like form. Lumps that aggregate on the star during this process break away from the star and eventually revolve around it in a flattened plane. The critical phase is when the planets cool: to be earthlike, a planet must lie in a certain Continuously Habitable Zone (CHZ) whereupon water remains to allow the processes leading to life to begin. If it’s too close to the sun-like star, it becomes a Venus; and if it’s too far away, it becomes a Mars. The questions is: just how wide is this CHZ?
Pessimists believe that the strip is so narrow that it is almost certain life as we know it is a fluke — that we are, in fact, alone in the universe, although this is a shrinking minority position.
What this means takes on greater significance when the spacing of planets in our own solar system is looked at. A mathematical formulation called the Bode-Titius equation attributes the gradually widening gaps between the planets as we go further away from the Sun to some underlying forces of gravity. If we then assume our planetary system to not be a deviant from the norm, we are then easily able to see how one could lay down the aforementioned CHZ grid over any initial arrangement of ordered planetary distances from a sun-like star. Using the optimistic end of CHZ speculation, it turns out that for our system, a planet falls in the life zone over 90% of the time. Hence, if our solar system is in the mean, it follows that the vast majority of other systems would have a planet in the CHZ.
It seems to me, however, that the real difficulty here is how the hosts of other sub-factors (such as the actual masses of both the sun-like star and, more importantly, the planets found in the CHZ; the periods of rotation of the CHZ planets, etc.) endlessly complicate speculations.
Variable “c”: The fraction of earths which will form life has an increasingly cut and dry consensus within the scientific community. The so-called combinatorial problem has become less of a problem ever since chemists began simulating the Earth’s primordial atmosphere, discovering that these original circumstances began to spontaneously create the chemicals of life.
The primitive conditions not only produce the right biochemicals but they seem to do so in a non-random way. Chemistry’s products are determined, and not just anything is possible. Certain atomic arrangements (for example, just certain amino acids or nucleic acid bases) are strongly favored over other arrangements in the same biochemical classes of compounds. There seems to be a limited set of biochemical units out of which earthlike life, and presumably all galactic life, can be constructed.
That such chemicals combined in a non-random way of course contradicts the long held idea that spontaneous biochemical life is the result of pure chance. We are given hints that some sort of principles of physics, or evolutionary necessities of some kind, are at work. Needless to say, the vast majority of scientists expect that an earthlike planet in the CHZ will develop simple life forms.
Variables “d” and “e”: The fraction of ecologies which will evolve intelligences, and the fraction of ETI which will develop civilizations, are hotly debated topics, for it is here that the concept of evolutionary probabilities is addressed, and with it, all the various debates about what should and shouldn’t necessarily entail from the different stages of a particular ecosystem.
The line of thought of the pessimists, to which many evolutionary biologists subscribe, is as follows: though the spontaneous origin of life may occur many times on many earths, it is highly unlikely (and probably close to nil) that a recognizable ‘intelligence’ will be found anywhere else. The classic exposition of this viewpoint was presented in George Gaylord Simpson’s 1964 paper, “The Nonprevalence of Humanoids” (Science, Vol. 143, No. 3608 (Feb. 21, 1964)), in which Simpson claimed that the random and highly idiosyncratic course of evolution on earth made the odds next to zero that intelligent life could ever be repeated even here. But the evolutionary biologists who have followed Simpson furnished their pessimism with more than just inculcated scientific conservatism — they are able to infer quite a bit from the evolutionary histories of different species right here on earth. And not all of them are entirely pessimistic. (A sociological survey of evolutionary biologists’ opinions, or a comprehensive review of the literature, would be necessary to say what the consensus among evolutionary biologists really is.)
Another philosophical problem is whether a rather liberal definition of intelligence (e.g., the ability to make use of previous experience in subsequent actions) problematic from the start. Ernst Mayr, who was Emeritus Professor of Zoology at Harvard, and one of the most eminent scientists in the field, makes the commonly heard observation that rudimentary forms of intelligence are widely distributed in our animal kingdom, but points out the incredible improbability of genuine intelligence emerging on another planet:
There were probably more than a billion species of animals on earth, belonging to many millions of separate phyletic lines, all living on this planet earth which is hospitable to intelligence, and yet only a single one of them succeeded in producing intelligence. (Mayr, in Regis, p. 28.)
Elsewhere, David Raup deems the complexity and diversity of our earth’s evolutionary record as showing that “there was anything but a neat and simple progression from single to complex or from unsophisticated to sophisticated.” (Raup, in Regis, p. 34)
Both Mayr and Raup note the apparent phenomenon of evolutionary convergence here on earth, most noticeably the examples of the sabertooth tiger and the widely referred to fact that many species have independently developed eyes. Fossils of the long-extinct sabertooth tiger, in the La Brea tar pits of Los Angeles, apparently reveal substantial information about its anatomy. In South America at about the same time geologically, there was a marsupial version of the sabertooth tiger. Surprisingly similar anatomy evolved independently in the two mammalian groups, and although placental and marsupial mammals have a common ancester in the Mesozoic era, they had been genetically separate for tens of millions of years before the sabertooth form appeared. Raup views such evolutionary convergence as responses to similar environmental pressures and/or opportunities, and argues:
It is presumed that convergence is most common in situations where there are only a few ways of solving a particular problem, thus increasing the probability that independent lineages will adopt the same solution. (Raup, in Regis, p. 35)
Meanwhile, Mayr believes that the evolutionary convergence of eyes throughout the spectrum of life on earth simply demonstrates a feature that evolves whenever it is of selective advantage. And as we are the only species possessing ‘genuine intelligence’ (along with eyes), it then appears that such genuine intelligence is nowhere near as necessary as eyes in order for a species to survive.
Raup brings up a good point about our definition of intelligence, a point that I believe hides deeper philosophical issues. Our above liberal definition of intelligence qualified it as essentially a problem-solving activity. We can immediately see that this definition is much too broad, as many species here on earth would exhibit this quality of intelligence. Raup gives some nice examples to illustrate his point:
Protective mimicry is a common phenomenon. A butterfly, for example, may achieve immunity from predators by evolving a color pattern which mimics the appearance of a poisonous species known and recognized by predators. The predator avoids all butterflies with that particular color pattern, regardless of species. Mimicry evolves over a long series of generations by selecting those chance mutations that make the nonpoisonous species look more like the poisonous ones. In the process, many butterflies are eaten by predators but the result is the enhanced survival of the species. Exactly the same result could have been achieved by an intelligent organism. (Raup, in Regis, p. 39).
Though the actions of the butterfly (and any other organism that invokes camouflaging) may appear, in a post-hoc sort of way, as signs of intelligence, we surely don’t, however, maintain that the butterfly is intelligent. That is, we don’t maintain that a butterfly — through an act of reasoning — consciously decides action X is in its best interest. Raup hence concludes that:
The problem of protection can be solved either by intelligence or by standard Darwinian adaptation… The manifestations we ascribe to an intelligent being, and which are crucial to the SETI strategy, can be produced by an unintelligent organism and the mechanism for accomplishing this is the ubiquitous process of adaptation. (Raup, in Regis, pp. 41, 42)
Interestingly enough, Raup interprets his position as improving the chances of SETI’s success, whereas Mayr believes the SETI program to be a waste of taxpayers’ money. Mayr, for example, writes:
[It is] interesting and rather charactistic that almost all of the promoters of the thesis of ETI are physical scientists…Why are those biologists who have the greatest expertise on evolutionary probabilities, so almost unanimously skeptical of the probability of ETI? It seems to me that this is to a large extent due to the tendency of physical scientists to think deterministically, while organismic biologists know how opportunistic and unpredictable evolution is. (Mayr, in Regis, p. 24)
This comment by Mayr may indicate a slight prejudice against the physical sciences in favor of the paradigm of his own field, but it also signifies that a comprehensive appraisal of SETI’s potential must incorporate the views of many different disciplines.
As the biological sciences increasingly apply physical principles to biology, we see evidence of a decisive limiting, in effect, of the various possible designs and structures a life form can take: the shape of a fiberwound cylinder, for example, the most common skeletal unit on the planet, allows lateral bending while resisting longtidinal compression, and appears in both lower and higher animal forms.
Not only do higher level biological categories, such as skeletons, have limited numbers of designs but, oftentimes, identical mathematical ratios of bone length to physical stess, etc.
We see structural restrictions in bilateral symmetries around tubal forms (e.g., arms and legs positioned around food-input and output orifices) and the actual number of arms and legs a successful land-roaming organism is likely to take. (There is a mathematics that dictates a brain-dependent preference for lower numbers of limbs. Some of the tentacles of an octopus are left to unconscious robotic movement; the six legs of an insect are controlled as two sets of threes by their brains, etc.)
Ah, what future generations will discover…
