In a June 2004 note about the Search for Extraterrestrial Intelligence (SETI), I wrote: "Life – an entity capable of reproduction and of acquiring, organizing, and using material and energy for its own ends – exists elsewhere in the universe. No scientifically trained person doubts this, unless mythologies cloud his mind. The odds in favor are just too overwhelming."
Does life exist elsewhere in space? There's not much doubt about it.
This assertion deserves follow-up and support.
Let's start with this: We know that life exists on one planet, the Earth. That's not trivial; it's an important piece of information. We must conclude from this information that physical conditions that permitted life to arise have existed, and conditions that permit life to continue do exist in the universe. That much we can conclude, but we must be careful about further guesses based on our existence alone.
When we ask such questions, and when we use our position on the Earth as a sample or example in considering the likelihood of other similar developments elsewhere in the Universe, we can easily be misled by what's called an "observation selection effect". That is, the fact that we ourselves are life on the Earth can of course be used as proof that life can arise and that it does exist in the Universe, but cannot be used directly to determine the likelihood of such life elsewhere. That's because, no matter how unlikely the development of life is, even if it has only appeared at one place and time, the question we are asking could only be asked at a place where life developed. That is, if we are the only life in the Universe we are in fact here and we are the only ones who can ask the question. So there's nothing surprising in the fact that we are here; it's a necessity since we are asking the question (Descartes' "Cogito, ergo sum" makes the same point), and the observation that we are here can't be used directly to argue for further life in the Universe. This principle is now often known as the "anthropic cosmological principle".
Unfortunately, standard statistics are of little use to us in these considerations, since our sample size is exactly "one"; that is, we have made one observation of a planet supporting life, and based on that sample no conclusion can be drawn regarding the population that our one observation belongs to. Statistically, the population of planets with life may as easily be one as one billion.
Nevertheless, although it's not sufficient to rely on "common sense" or limited data to draw conclusions about a scientific question, these can be used to generate plausible hypotheses. Our experience suggests that unique objects or events are virtually unknown in the physical world. This is both our "common sense" experience from everyday life as well as the experience of the natural sciences to date. It's close to being a principle of science that every physical condition or event belongs to a class. If we find, for example, a grain of sand, we can conclude that the conditions conducive to production of grains of sand have existed, and we reasonably expect that there may be additional grains of sand. The same with a tree, a molecule of water, a star, a galaxy, or a black hole. In other words, it's our experience that if physical conditions are known to exist which have been observed to produce a result, this result will not be unique. Most of our experience with this phenomenon comes from observations on the Earth, but the principle has so far held true also in the astronomical realm. Whatever we have found in space, we have found to be a member of a class of similar objects. We see the mass of stars, which we classify in various classes with countless billions in each class, each one created by consistent and repetitive physical processes. We find countless galaxies, their formation again a response to repeated physical conditions. Quasars, gas clouds, black holes ... whatever we have found, we have found in large numbers. We have never found anything unique. We have recently begun to find planets around other solar systems, mostly in the near-space environment which we are able to explore at this stage. Again, the more we look, the more planets we find. It will be a while before we're able to survey any substantial portion of our galaxy for planetary systems, but no one can doubt that we will continue to find more, the more we search.
I've seen no one advance any argument for why the occurrence of life should deviate from this principle that appears to cover all our scientific findings to date. Our expectation should therefore be that it does not deviate, and that we should expect to find life elsewhere, responding to a repetition of the factors that have brought it about here on Earth. (Incidentally, our expectation about universes – yes, plural – should be exactly the same, but that's another tale.)
But saying that we can reasonably expect to find life elsewhere in space does not justify the assertion that it does exist, which is the argument of this note. Since we don't yet have direct evidence, we'll rely on the indirect evidence to suggest the likelihood of life existing elsewhere.
We begin with what we know about the conditions for life as we know it.
What are these conditions? Life somewhat resembling the Earth's would apparently require:
Those are the known essentials. From our experience on Earth we've found that the following factors are also helpful (some perhaps essential) in sustaining life on our planet:
- A terrestrial planet orbiting an appropriate star.
- The planet is at an appropriate distance from the star to receive an appropriate rate of life-sustaining energy flow (light & heat).
- Sufficiency of required chemical elements on the planet; probably carbon, nitrogen, oxygen, hydrogen, and a halide (chlorine, fluorine, etc), and perhaps phosphorus, potassium, sodium, and others.
We don't yet know very much about the initial appearance of life on Earth, and it's certain that a great number of other factors are important, such as weather and the nature of the available energy (i.e., chemical, heat, light). But the essential point is that a number of physical factors appeared together in the case of the Earth, and resulted in (or permitted) the appearance of life. (This doesn't discount the possibility that other combinations of factors may just as well permit the development of life.) The key questions become, "How often has such a group of physical factors come together in the Universe?" and "What's the likelihood of life emerging when they do come together?"
- A liquid iron core which produces a magnetic field that shields us from our star's deadly magnetic storms – which could otherwise rip our atmosphere away.
- A giant fellow planet (Jupiter) in an outer orbit, whose gravity can deflect incoming comets from collisions with our planet.
- Presence of liquid water.
- Presence of a gaseous atmosphere.
The clearest statement of these questions was presented by Frank Drake at the Green Bank astronomical conference in 1961. Although the "Drake equation" is now over 40 years old, it is still considered valid, though debate will continue for a long time over the values of the various factors in the equation. The complete Drake equation (or formula) estimates the number of technological civilizations in our galaxy, using seven multiplicative factors. However, after a mere four factors the equation incidentally estimates the number of planets in the galaxy where life – even the simplest form of life, which is what we're discussing here – would develop. This short form of the Drake equation (where we can also ignore the rate of formation of new stars) then becomes:
N = R · fp · ne · fl
Here, R is the number of "suitable" stars in the galaxy (or in whatever portion of the universe is being investigated); fp is the fraction of those stars that have a planetary system; ne is the number of planets per planetary system that are suitable for life; and fl is the fraction of suitable planets on which life actually will develop. Multiplying these factors then gives an estimate of N, the expected number of planets sustaining life in the galaxy.
It's obvious that there will be dispute about the magnitudes of these factors, and therefore about the final estimate of the number of planets sustaining life. So it's no surprise that widely different estimates have been arrived at. But the exact number is not germane to our discussion; since we're asking, "Does life exist elsewhere in space?", we're only interested in knowing whether N is greater than "one". Of course we know that N is at least equal to "one", since the Earth is that one "living" planet in our sample. Therefore we know that none of the factors in the equation are equal to zero. In fact, the first two factors (R and fp) are now known to be large. The key factor is of course the last one: How frequently will life develop on Earth-like planets that have the physical ability to sustain life? This seems to be begging the question: it's in a way what we set out to discover. But we'll find that we can solve our portion of the Drake equation for this term (fl) by setting N equal to "one":
Of course the estimates above don't support a level of precision beyond order-of-magnitude, and probably not even that. But that's adequate for our discussion. Our result so far is that we know that the value of fl is not less than about 3 x 10-8 . So it is either just that value (or whatever the correct value is that gives N=1) – in which case there may be no life outside the Earth – or it is higher, in which case there will be any number of planets supporting life in our galaxy. Now, exobiologists (those folks who specialize in studying the likelihood of life forms outside the Earth – and yes, they do all believe it exists) have generally estimated values of fl ranging from its theoretical maximum of 1.0 – that is, life springs up wherever it has the opportunity – down to 0.01, in which case life would only appear on 1 percent of suitable worlds. Our calculated value of fl for the case where the Earth is the only site of life is 300,000 times lower than that conservative estimate. No serious exobiologist would support such a vanishingly small likelihood of life (perhaps because in that case it would be difficult to remain a serious exobiologist.) These considerations would seem to make life elsewhere in our galaxy a virtual certainty. (I will add, parenthetically, that if these calculations are expanded to the entire known universe, the probabilities we're talking about will increase by at least one hundred billion, and the credulity required to believe that the Earth holds the universe's only life would increase to values that could only be sustained by faith.)
- R: The number of stars in our Milky Way galaxy has been variously estimated from 200 billion to 400 billion. We can use 300 billion as a mean estimate, and take one-tenth of these, 30,000,000,000, as a very conservative estimate of the number of "suitable" stars.
- fp: A broadly accepted estimate of the fraction of stars that supports a planetary system is 0.5. For a conservative estimate we can again use 0.1, so that the product R · fp gives us 3,000,000,000 suitable planetary systems in the galaxy.
- ne: Estimates vary widely as to how frequently terrestrial planets that are candidates for development of life will be found in planetary systems. One-tenth of planetary systems is probably a conservative estimate. We'll be a good deal more conservative and choose 0.01, meaning that we'll expect to find a suitable terrestrial planet only once in a hundred otherwise suitable planetary systems. Then our equation to this point, R · fp · ne , indicates that there could be 30,000,000 terrestrial planets suitable for life in the galaxy.
- fl: Since we know that N, the number of planets supporting life, equals at least 1.0 (the Earth is that "one"), we can solve the equation for fl by setting N equal to 1.0: Simply 1/30,000,000 gives 0.0000000333 (or 3.33 x 10-8) for this factor. In other words, given the conservative estimates above, we know that the success rate of life developing on suitable planets is at least one per thirty million such planets in our galaxy. The question is whether the actual rate is higher than that. If it is, we can expect that there is life elsewhere.
Biologists, including exobiologists, base their theories of the development of life in large part on studies of the early history of the Earth, such as we know it. There is no reason to think that the Earth's physical development differs significantly from other terrestrial planets. Our observations of the other terrestrial planets in our solar system confirm this. Planetary formation is necessarily a violent event, and such results as a hot core of heavy metals and other molten minerals, a cooled and mobile crust, and an atmosphere of gases must be typical, not anomalous. So also with magnetic and electrical fields and storms. In 1953, Stanley Miller (my Physical Chemistry professor at UC San Diego in the '70s) conducted experiments with discharging electricity – simulating lightning – through a surrogate primaeval atmosphere (nitrogen, ammonia, hydrogen, water vapor, phosphates, sulfates, etc., but no organic compounds or atomic oxygen) and showed that amino acids were generated. This classic result has been replicated and expanded many times since, and shows conclusively that the building blocks of life, the essential elements of proteins, could have been synthesized in such a way on the primitive Earth. Of course we know that they were in fact synthesized, and that the resulting complex molecules self-organized over billions of years into what eventually became simple life forms.
We have touched on several lines of inquiry, all of which lead to the same conclusion: That the likelihood of the Earth being the only site of life in our galaxy (or in the universe) is vanishingly small. This is the reason scientists are quite certain about life existing on other planets, and most scientists have little doubt that we will in time detect such life. Our best and most immediate opportunity may be on Mars, which has been shown to have had liquid water in the past. But it's not certain that other requirements for life have been present on Mars, and in any case life may only arise on a fraction of suitable planets. So if no trace of former life were to be found on Mars, this would have no impact on our conclusion. On the other hand, the discovery of signs of life anywhere outside the Earth, when it comes, will be a watershed find, surely one of the most important in the history of science. But it will not surprise scientists a bit. What effect it may have on those who base their theories of life on ancient mythologies is another matter; it should be interesting to observe.
© 2005 H. Paul Lillebo