Hey, have you heard the news? We've finally found other planets out there, orbiting distant stars. Lots of them. This discovery is recent and surely profound. But what does it mean for our place in the grand scheme?
Is Earth the best of all possible worlds, just an average world, or the only world there is? In the last four hundred years science has narrowed down the options, dispatching the third one to the dustbin of history. We know that our world is one among many. But is it only one of many, or is it something more as well? Does Earth belong in the special and gifted class of planets? As befits a species stumbling through adolescence, we are wondering where we fit in, seeking our peers, looking for answers.
Expectations about more distant planets have been, of necessity, based on the local crew. If our planetary system were somehow typical, it would be a gift to science.* It would make our job so much easier, because if we can safely generalize from our own solar system, then we already know what goes on out there in the rest of the galaxy. If our planetary family is ordinary, then it may not be all that unusual for one or more terrestrial planets to end up in the habitable zone where liquid water sloshes onto ripe shores. Some of these watery worlds will become biospheres, perhaps following the same biochemical pathways that life has found here, creating oxygen-rich atmospheres, ozone layers, forested continents, animal crackers, and Internets. If it happened once, it can happen again, right?
Today, we all know that we are nothing. We learned in grade school that we inhabit an ordinary planet circling an average star floating inconspicuously and, until recently, innocently in a backwater arm of a galaxy containing hundreds of billions of other stars, a humdrum galaxy that is itself undistinguished among more galaxies than you could shake a stick at in a zillion and a half lifetimes.
Jeez. Maybe there is no reason to think that our planet is special in any way at all. This is an extreme application of a general principle in cosmology, known as the principle of mediocrity, which says that the universe is basically the same everywhere. Wherever you go, there you are. There is nothing unusual about our position, and the view from anywhere else should look more or less the same as ours. This thought is comforting to scientists because we like to believe that our conclusions do not depend on where in the cosmos we happen to be born. We are not looking for regional truth. What we seek transcends location, and that's a much easier trick if location isn't important.
The principle of mediocrity is a close relative of the arguments by analogy that have made people believe in a plurality of inhabited worlds for hundreds of years, at least. Can we use it to deduce the existence of habitable planets and life out there? From the remarkable uniformity in all the qualities we can observe in distant realms, should we deduce a sameness in the features we cannot yet observe? It has worked before. We assumed, by analogy, that planets were around other stars. Now we know we were right. Do you suppose that life might be another of those features found more or less everywhere? As Fontenelle, in 1686, said about the view of nearby Saint-Denis from the towers of Notre Dame,
*Not to look a gift universe in the mouth, but if we were too typical, it would be highly suspicious.
everything one can see there resembles Paris: steeples, houses, and so forth. So we conclude that Saint-Denis, like Paris, is inhabited. If the universe tends to repeat itself, in gross architecture and building materials, does that mean that it is fitted with the same fine trim and landscaping elsewhere, achieving an economy of detail that we just can't quite make out yet?
During our first decades of exploring our solar system, we learned that planets evolve in ways that largely depend on accidents of birth, and chance occurrences. As a result, planetary fates cannot be predicted from their initial states. If planets were like electrons or atoms, it would be different, because if you've seen one electron, you've seen them all. Atoms are somewhat more complex than electrons. They are not all the same, but they can be grouped into a hundred or so types (the elements) that are easily classified (in the periodic table) and understood. If planets were like atoms, by studying a small sample we could know them all.
Will we ever be able to assemble all the planets we know into something like a periodic table of planetary types? No. Because planets, and apparently entire solar systems (by which I mean stars with retinues of orbiting planets), are more like people. Each is a complex individual, shaped by the unique experiences of a long life in a chaotic, changing environment. In the details, no two will ever be alike.
Imagine if you were trying to understand human beings as a general phenomenon but you only had intimate knowledge of one person.* If from studying your subject you deduced that everyone needs to sleep, everyone experiences puberty, and that people respond to music, you'd be right. But, depending on the peculiarities of your randomly chosen subject, you might also conclude that everyone is way into heavy metal, that Falun Gong exercises at lunchtime are a daily ritual for all humanity, or that everybody must get stoned. In generalizing from your random sample of one, you would draw some correct inferences and be led astray with others.
Let's face it. We need more planets.
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