Is Our Planet So Special?

"Are We So Special?"

Could we be alone in our part of the galaxy, or more dramatic still, could we be the only technological society in the universe? A New York Times (2/8/00) science article has reported the opinion that complex life might be decidedly uncommon. The article discusses a new book - Rare Earth - in which Peter Ward and Donald Brownlee (University of Washington) point out the many ways in which our solar system may be optimized for our existence. If that's the case, then the other solar systems of the cosmos might sport life, but little of it would be more sophisticated than "shower scum."

EarthSETI researchers have long thought about this subject. The idea that intelligent life might be widespread has always been controversial. Many biologists, including Ernst Mayr and Stephen Jay Gould, have argued that humankind's recent appearance on this planet was contingent upon an unlikely chain of events. For example, had the dinosaurs and half of all other species not been wiped out 65 million years ago, we wouldn't be here, stacking their bones in our museums. Similarly, it has been suggested many times that circumstances that seem peculiar to Earth - a large moon, plate tectonics, a salubrious atmosphere, a placid sun, or even the presence of beefy planetary neighbors such as Jupiter and Saturn - were all essential to the evolution of terrestrial intelligence.

But were such things truly "essential," or merely a help? When we tote up a laundry list of Earth's astronomical properties it may seem to imply that our existence is remarkable, and possibly even unique. That could be true, of course. But one should always be leery of probabilities calculated after the fact. A mutt off the street could have a spot on one ear, two eyes of different color, and a limp. In other words, it would be a mutt unlike most other dogs (and if examined carefully enough, a mutt unlike all other dogs). But this hound could also have fleas. Would we be correct in concluding that dog, so obviously special, is the only one likely to have fleas?

The question is not how idiosyncratic is Earth, but whether our world enjoys circumstances that are simultaneously rare and essential to complex life. As far as we know, this isn't the case.

sunConsider our stellar host. Is the Sun a rarity in the galaxy? Sol is a G-type star of medium brightness, endowed with long life and a tranquil disposition. Sure, it spews a moderate amount of ultraviolet our way, but most of this dangerous radiation is screened from Earth's surface by atmospheric ozone. Nonetheless, we know that most other stars are, indeed, unlike the Sun. Some are bigger and brighter, and a great deal more are runty and dim.

The brighter stars die young, typically in only a few dozen million years, hardly sufficient to incubate complex life. The plentiful dimmer stars are both more volatile (they routinely flare) and more likely to trap habitable planets in a synchronous rotation, with one side always turned toward their stellar masters. Unless such a planet has a thick mantle of air, this could wreck the weather (although some recent research indicates that even a somewhat thinner atmosphere laced with greenhouse gasses would make these tidally tugged worlds habitable). However, if we wish to be conservative, we might opine that dim stars are poor bets for supporting sophisticated biology.

Large stars are non-starters, and small stars are problematic. But sun-like stars are not rare. Approximately one star in ten is similar to Sol, and that means that tens of billions of them inhabit our galaxy. Indeed, even stars somewhat brighter than the Sun, which because of their heightened ultraviolet production might be thought dangerous, could be the loci of life. Jim Kasting has shown that the enhanced ultraviolet radiation from F-type stars would produce so much atmospheric ozone that planetary surfaces could be very well shielded.

star formationRare Earth's authors suggest that the composition of our solar system, including the materials necessary for making rocky planets, might be unusual. The chemical composition of stars certainly varies, and some are poorly endowed with heavy elements (everything more massive than helium), essential ingredients for the formation of both planets and protoplasm. This is described by astronomers as a variation in "metallicity." Globular clusters, composed of the galaxy's oldest (and most heavy-element poor) stars, have metallicities of 0.1% or more, or about an order of magnitude less than the Sun (this sprinkling of heavy elements is larded into the interstellar medium by supernova explosions). But models of planet formation don't rule out the formation of new worlds even in these depleted systems. And there's still plenty of raw material. Note that the mass of the Earth is only about 0.0003% that of the Sun, so even in the metal-deprived neighborhoods of globular clusters there is more than enough suitable material for constructing Earth-like planets.

In addition, the overwhelming majority of luminous stars are in the galaxy's disk, not in the globulars. Since star formation has remained relatively constant since the Milky Way's birth, roughly one-half of all stars have metallicities comparable to, or even greater than, that of the Sun. The composition of our solar system will be similar to that of billions of other sun-centered solar systems. Roughly half of these will be old enough to have incubated intelligent beings.

But what about such possible terrestrial peculiarities as plate tectonics or the companionship of a large moon? Plate tectonics are useful for cycling carbon between the atmosphere, the ocean, carbonate rocks, and living things. This happens because the slip-slide of the oceans pushes carbonate rocks on the sea bottom (such as limestone) under the continental margins. The carbonates melt and are blown back into the atmosphere by volcanoes. But tectonic activity needn't be either rare or short-lived. It is driven by internal heat produced by radioactive materials in Earth's center. Other planets of similar composition and of a size at least that of Earth will also enjoy a dynamic surface. Indeed, if the positions of Venus and Mars had been interchanged at birth, then Venus might be both tectonically and biologically active today. There is nothing miraculous about tectonic activity, and in fact there is some evidence that it has occurred on both Mars and Venus.

moonDoes sophisticated life require a large moon? Some have argued that it does, for the moon helps stabilize Earth's rotation axis and keeps it from possibly dangerous tilts. Other Earth-like planets might not be blessed with such a hefty moon, as it seems that our own natural satellite was produced in an accidental collision between Earth and a Mars-sized or larger asteroid more than 4 billion years ago. But computations have shown that if the moon had not been formed, our planet would spin faster - fast enough, in fact, to stabilize it against major tipping. In addition, even if an Earth-like planet occasionally does a spin flip, it will spend 10 million years or more doing so. Life can probably adapt to such slow changes. Indeed, it already has, during episodes of polar wander on Earth.

alien planetFinally, there's the matter of biological evolution to intelligence. Is there some good reason why evolution should occasionally produce sentient beings? The answer is, and remains, controversial. However, the fossil record tells us that the maximum degree of encephalization - which (crudely speaking) measures the ratio of brain to body mass - has increased considerably in complex animals for the last 100 million years or so. The dinosaurs were bulky but not bright. Even the most cerebral of these lumbering lizards had less brain power, as judged by encephalization, than an ostrich. The mutual stimulus to increased mental capability inherent in predator-prey activity has led to increased encephalization for some segments of the animal kingdom. Consequently, many of today's animals would handily outscore their Mesozoic predecessors on any IQ test. Human-level sophistication could be a common outcome of this ratcheting up of neural capability, although it is the uncertainty of this conclusion that drives us to look for intelligence elsewhere.

The bottom line is this: our solar system and our Earth have "personality" - they exhibit properties that might be found only occasionally in other star systems. But there are no show stoppers that mandate against the evolution of sophisticated life elsewhere. Our solar system has no properties that are obviously essential to complex or intelligent life that other worlds would never have. We may even have been cheated out of some helpful phenomena that could have sped evolution on Earth. We might be less lucky than we recognize, and creatures on other worlds may regard with disappointment the nature of our planet.

Possibly we have failed to appreciate the importance of some aspect of our situation, and the fact that we are functionally very special - that intelligence and complexity are a rare exception to the cosmic rule. But while no experiment can prove we're alone, SETI experiments could show that we're not. Consequently, arguments about whether intelligence is a rare event in the universe - while instructive and interesting - may be akin to discussing with Columbus whether there's any chance his voyage will uncover a new continent. Discussion does not settle such arguments. Only experimentation can.

We should continue to sail the seas of discovery, for only by doing so do we have a chance of revealing our place in the cosmos. As Giuseppi Cocconi and Philip Morrison stated four decades ago, "the probability of success is difficult to estimate, but if we never search, the chance of success is zero."

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