This is part one of a four part essay. In this section, I will introduce the concept of the Copernican Principle (aka, the Principle of Mediocrity) as well as recent challenges to it in the form of the Rare Earth Hypothesis.
Once upon a time, not all that very long ago, many people sincerely believed in fairies (sometimes called elves, leprechauns or what not). It was believed that fairies lived an otherworldly realm that would only occasionally intersect with the human one. They were believed to have great powers and intelligence. It was thought that would occasionally take people into their realm for purposes that were sometimes benevolent, sometimes malevolent and often incomprehensible.
In cultural terms, aliens have come to occupy much of the mythological space that fairies once inhabited. Popular culture sees aliens as mysterious, intelligent beings (sometimes good, sometimes evil, often incomprehensible) that intersect our world at will, appear when and where they want to whom they want and, all to often, taking hapless souls into their ships for the purposes of study and experimentation.
I, for one, think that popular accounts of aliens are just that – modern day fairy tales. Be that as it may, I am hardly alone in sometimes looking up at the sky and wondering whether or not someone else, up there, is looking outwards too. The very idea of alien cultures and civilizations is invigorating. I grew up on the science fiction perspective that aliens were inevitable and that it was only a matter of time before we would come into contact with them (for good or ill). The Roddenberryesque notion that, one day, we would become explorers finding new worlds with new peoples on a weekly basis was thrilling and invigorating.
But are they, in fact, there? When we look up at the sky are we seeing civilizations without end or are we, merely, seeing nothing but bright embers and dust?
A little over five hundred years ago, Nicolas Copernicus fired the first salvo that would displace us from the center of the universe with the publication of De Revolutionibus Orbium Caelestium (On the Revolution of the Celestial Orbs). Up until that point in time, the dominant cosmological model was the Ptolemaic model which positioned the Earth at the center of the universe. Contrary to certain popularizations of this, Copernicus' contention was not met with immediate outrage and censure. The general reaction, even among scholars, was to simply ignore it.
It wasn't until Galileo's observations of the moons of Jupiter (which demonstrated the existence of a sort of miniature solar system) did the issue become heated. Everyone is familiar with Galileo's trial and the fact that he was forced to publicly recant the Copernican view. By that point, however, the Ptolemaic theory was already a lost cause.
Over the intervening centuries it became clear that the Earth wasn't only not at the center of the solar system but that the solar system, itself, was not at the center of the universe. In fact, our sun turned out to be a rather ordinary seeming start in a rather ordinary position in a rather ordinary galaxy that was, in turn, one of hundreds of billions of galaxies that were visible to us.
If I may be allowed a very brief tangent, science and philosophy have a special relationship. Too often they are viewed as alternatives to one another but the truth is that science and philosophy are deeply intertwined. In point of fact, science was originally known as Natural Philosophy (to distinguish it from metaphysical philosophy). Over time, science has become its own field and many scientists have come to see philosophy as nothing more than a distraction (a compliment that many philosophers are all too happy to return) but the fact is that philosophical questions impinge on science (bioethics, to give an easy example) and scientific questions impinge upon philosophy.
As our place in the universe got knocked from one peg down to the next, a new philosophical principle started to take hold. It's known as either the Copernican Principle or the Principle of Mediocrity.
The Principle of Mediocrity states that we should assume that any aspect of our local environment (including the existence of life and intelligence) should be assumed to be a common aspect of the universe as a whole (in more formal terms, it is the proposition that there are no privileged observers in the universe, but let's stick with the lay formulation, for now). It should be stressed that the Principle is assumed to have a certain "resolution" or granularity. We would not expect, for instance, to find that Starbucks coffee shops are common in the universe (no matter how tempting the thought) nor would be expect to find human beings out there. We should, however, expect to find life and intelligence (perhaps even humanoid life, but that's a much more controversial proposition). When we gaze up at the stars so, too, should others be gazing out from them at us.
Most science fiction stories implicitly accept that the Principle holds (at least with respect to the existence of intelligent life). Astrobiologists have also tended to favor a Copernican interpretation. Until very recently, there was little reason to suppose that the Principle wouldn't continue to hold with respect to the question of life and intelligence. Recently, however, there have been counter-salvoes that have challenged this assumption.
In the last decade, we have finally found conclusive evidence for the existence of extra-solar planets. In fact, we've found dozens of worlds just in the local neighborhood. At first glance, this should seem like a vindication of the Principle. There's one problem: the solar systems that we've discovered don't tend to bear much resemblance to our own.
The most obvious difference is that the worlds we are finding have tended to fall into the "super-Jovian" classification of planets. A super-Jovian planet is one that has more mass (typically a lot more mass) than the mass of Jupiter which is, of course, the most massive planet in our own solar system. It must be understood that the primary reason for this is that our methods of detection only allow us to find massive worlds. It is as though we conducted a census that only included people who weighed more than 300 pounds (about 135 kilograms). Naturally such a census would imply that everyone in the world was obese.
There's a problem with this observation, however. There are too many super-Jovians. If we sent out our selective census taker, we'd expect him to report that most houses were apparently empty (because most houses wouldn't have 300 pound residents). If, however, he came back and reported that nearly every house had at least one 300+ pound resident then we would have good cause to suspect that a disturbing number of people were, in fact, quite heavy. That's what we've been finding with our planetary census. Almost everyone we've been looking, we've been finding planets – extremely massive planets.
This, in itself, might only suggest that our solar system is merely unusual in lacking a super-Jovian or two but there's more. Solar systems aren't like the clockwork orrerys that we learned about in school. Solar systems are dynamic systems. Over time, planetary orbits can (and do) change. In our own solar system, the orbits of the planets have been remarkably stable over the last four and a half billion years, or so. Likewise, the orbits of our worlds are almost perfectly circular (the elliptical deviation of the Earth, for instance, is so slight that it would be invisible to the naked eye if it were rendered to scale). When we look at other solar systems, however, we tend to find things aren't at all like they are in our solar system.
A disturbing number of the super-Jovians we are discovering fall into two categories. The first category of planets are what are known as "hot Jupiters". These are massive planets with extremely close orbits to their stars. Most of these worlds make complete orbits of their parent stars in a matter of days. Many others fall into a category that is prosaically called "eccentrics". An eccentric planet is one that has an extremely pronounced elliptical orbit. The more eccentric a planet's orbit, the more likely it would be to cross the orbits of other worlds.
Worlds do collide. It is believed that our own world was impacted by another world the size of Mars about four and a half billion years ago (leading to the formation of our anomalous moon). Planetary collisions, however, are expected to come to end relatively early in a solar systems history for the simple reason that any bodies that are in collision producing orbits would, in fact, collide within a few tens of millions of years. Collisions, however, are relatively rare events even in a young solar system. A bigger concern is gravitational interactions.
Massive planets have, naturally enough, massive gravitational fields. When a big planet passes near a smaller world, even if they don't collide, they will experience gravitational interaction. Like an uneven game of tug of war between an adult and a child, the smaller world will bear the brunt of this, being yanked well out of its existing orbit. It is entirely possible for a world to be either tossed into its star or to be ejected from its solar system entirely. Eccentric Jovians and super-Jovians would tend to have this effect on any terrestrial worlds (smaller, rocky worlds like the earth). Hot Jupiters are thought to be the end result of such ejection events as the super-Jovians eventually end up in hyper-close orbits around their parents.
The fact that we're finding so many solar systems like this is a blow to the notion that our solar system is pretty much like any other one. It would seem that we are, in fact, somewhat special (and not merely in the naïve "Goldilocks" proposition that our solar system is "just right" for us -- much as an intelligent puddle might observe that the hole that contains it is perfectly contoured to it). If Hot Jupiters and Eccentrics are, in fact, the norm it would follow that our own solar system is, literally, abnormal. In other words, we get to move up a peg on the Copernican scale. There is, after all, something that is privileged about our place in the universe (at least relatively speaking).
Just how many pegs up do we get to go, though? Two scientists by the name of Peter Ward and Don Brownlee have recently published a book called Rare Earth which advances the proposition that complex life is, in fact, very rare (they would contend that it may even be unique although I'm going to dispute that further on). In addition to the extra-solar planetary surveys that I've already discussed, they add additional arguments that include speculations regarding the moon's role in the advancement of life, the role of having "good" Jovians in proper orbits to sweep up asteroids and comets, the critical role of plate tectonics, and so forth. Their ultimate conclusion is that there are additional factors that must be added to the famous Drake Equation and that, when those factors are accounted for, the expected conclusion is that worlds with complex life should be exceedingly rare.
The Rare Earth hypothesis is not without controversy and not without its detractors. A number of their proposed factors rely of either preliminary or speculative data and it is not entirely clear what values should be plugged into each of their proposed factors. Be that as it may, there is one observation that seems to make Rare Earth much more credible: the Great Silence.
As I've noted in my previous essay on the Fermi Paradox, one of the most puzzling aspects of the search for extra-terrestrial life is that we haven't already found it. If intelligent life were to arise within a galaxy, it should be able to colonize every available world in the galaxy within an astronomically insignificant interval. Not only should the galaxy be screaming with communications between colonies but extra-terrestrials should be here and have already been here for hundreds (if not billions) of years. The profound silence of the universe and the utter lack of credible evidence for extraterrestrial visitation is a puzzle that demands consideration. Of all the proposed solutions to the Fermi Paradox, the very simplest is that there isn't anyone out there. The Rare Earth hypothesis provides a theoretical and (partially) empirical framework that supports this conclusion.
Is this, then, the doom of the Principle of Mediocrity? Not quite. As noted, the Rare Earth Hypothesis is not universally embraced. Even with the damning evidence that our solar system might not be common, we don't know how uncommon terrestrial worlds are. There are projects in the works that should allow us to resolve Earth-sized worlds by the end of the decade. We may even be able to determine what sort of atmospheres such worlds would have. If we find the presence of free oxygen, we would have strong cause to conclude that extraterrestrial life of some type does exist (although it may be as simple as bacterial life) since atmospheric oxygen, without the replenishing effects of life, is chemically reactive and would be absorbed into the crust of a planet within a few million years, otherwise.
Even so, finding evidence of life is not the same as finding evidence of intelligence or, even, evidence of complex life. We must face the very real possibility that the Rare Earth Hypothesis is an accurate rebuttal to the Copernican Hypothesis. Intelligence may be rare. The question then becomes how rare.
Curiously enough, cosmology gives us a reason to suppose that no matter how rare it is, we should expect that there are an immense number of intelligences in existence.
This concludes part one of this essay. Part two will be published next Sunday.
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