Astronomy Pix of the Fortnight, XLIV

Excellent book review of Alone in the Universe: Why Our Planet is Unique by John Gribben.  (Excerpt below the jump.)  It supports one of the plausible explanations for The Eerie Silence:  the universe is so old, surely there are civilizations scores of thousands of years more advanced than our own who have mastered space travel and/or communication – so why haven’t we heard from them, or at least overheard some  background noise (e.g. radio transmissions)?

It’s the “inverse” of the more popular (and far less interesting) idea that the universe is so big surely there is life elsewhere.  It just may be that Monty Python had it right in the Galaxy Song:  “How incredibly unlikely is your birth.”  Or as this author puts it:

The creation of Eden is far more complex than you may have heardthe product of a profoundly improbable sequence of cosmic, geologic and climatic events — some thoroughly documented, some inferable from fragmentary evidence—that allowed our planet to become a unique refuge where life could develop to its full potential.

That improbable sequence includes a “suicidal” collision with a Mars-sized object very early in the solar system’s formation that accomplished at least two fantastic things:

  1. Created our magnetic core&cocoon , which protect us from solar particles.  (And creates all those lovely auroras.)
  2. Gave us a moon that protects us from a tug-of-war between the gravities of the Sun and Jupiter – a tug of war that would wobble us like Mars.

Also in that highly improbable sequence: the thinning of our rocky crust to give us a “lively tectonic existence” that kept the pot stirring – most notably with water vapor.

Now for the regularly scheduled programming:

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(Scroll a little further down for the book review excerpts – John)

Recent discoveries might seem to boost the likelihood of life elsewhere in the galaxy. We have confirmed the stunning ubiquity of extrasolar planets in other star systems, the latest a possible Earth-analog orbiting right in the habitable sweet spot—not too close, not too far—from its central sun. Biologists have encountered bacteria underneath a mile of Antarctic ice and nestled within rocks in a Yellowstone geyser; it’s only a modest stretch to imagine that the next generation of robotic spacecraft might find simple biota in equally hostile havens on Mars or on one of Jupiter’s moons.

But as John Gribbin points out in his grimly plausible book, “Alone in the Universe,” there is a world of difference between habitable planets and inhabited planets. Mr. Gribbin’s narrative reduces the vision of Disney’s documentary into the counterfactual fever-dream it really is. The author’s conclusion: Earth is the sole abode of intelligent life in the galaxy, the product of a profoundly improbable sequence of cosmic, geologic and climatic events—some thoroughly documented, some inferable from fragmentary evidence—that allowed our planet to become a unique refuge where life could develop to its full potential.

Chief among these, paradoxically, was a near-cataclysmic planetary collision during Earth’s infancy, which gave birth to the moon. Such encounters were relatively common in the harum-scarum chaos of an early solar system that teemed with veering planets and asteroids. In its suicidal blow against our world, the Mars-size impactor generated enough heat to liquefy both itself and Earth’s exterior. Its dense, metallic core plunged inward to join our planet’s existing metallic center, while the rest swept up part of the fiery terrestrial shell to form the moon.

One consequence of Earth’s tumultuous youth was the thinning of its rocky crust. This has provided the planet with a lively tectonic existence, complete with vapor-spewing volcanoes, continents that divide and drift, and an ecologically advantageous global-temperature-regulation system. Earth’s swollen metallic core remained liquid; its constant churning gives rise to electrical currents that generate a far-flung magnetic cocoon that shields us from dangerous solar particles. (The creation of Eden is far more complex than one might have heard.)

Another fortuitous coincidence on Mr. Gribbin’s checklist is the moon’s large size relative to Earth, a ratio unique in the solar system. Without such a gravitational partner to restrain the disrupting tugs of the sun and Jupiter, our planet might suffer paroxysms of axis-tilting. (Try to run a civilization when your once-temperate hemisphere suddenly heels over to an Arctic orientation.) Mr. Gribbin outlines how a series of climate-altering Ice Ages and tectonic shifts benefited human ancestors roaming the grasslands of East Africa…

Mr. Gribbin admits the possibility —even probability—that elementary life forms have arisen elsewhere in the galaxy. But the object of his scientific and statistical scrutiny is intelligent extraterrestrial life. While he cannot prove a galaxy-wide absence of other civilizations, he presents an array of modern, research-based evidence that renders that conclusion eminently reasonable. He even suggests a decades-long survey of infrared emissions around stars (possibly arising from planetary atmospheres, even water vapor). This would yield the true number of “wet-Earth” planets in the galaxy—in his estimation, zero.

One leg of Mr. Gribbin’s argument rests on the theorized life expectancy of advanced civilizations, which he claims is much more fleeting, on a cosmic timescale, than we care to admit. Our species has inhabited this planet for about one hundred-thousandth the age of the galaxy, and it was merely a century ago that we began to transmit radio waves. If technological civilizations did arise before ours, they might have succumbed to war or environmental degradation well before our primate ancestors stood upright.

The rosy alternative—a long-surviving society—seems even less plausible. With millions of years of technological advancement, why haven’t they migrated throughout the galaxy by now? Or why haven’t we picked up the least shred of their radio-wave chatter? Of course, Mr. Gribbin dismisses such questions: These purported civilizations never existed.

Our civilization’s own halting steps into outer space so far suggests an uncertain future for the exploration or colonization of extrasolar worlds. The idea that we—or our robotic avatars—might be the first species to traverse the galaxy presumes a fundamental change in space propulsion, which at present (except in Hollywood) is unsuited to cosmic distances. Looming environmental disaster might yet provide the impetus to send aloft a select segment of the population in a one-way space ark. But an escape-pod scenario is a far cry from true interstellar migration.

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