Our Rare Earth

Let us recap why we think Earth is rare. Our planet coalesced out of the debris from previous cosmic events at a position within a galaxy highly appropriate for the eventual evolution of animal life, around a star also highly appropriate—a star rich in metal, a star found in a safe region of a spiral galaxy, a star moving very slowly on its galactic pinwheel. Not in the center of the galaxy, not in a metal-poor galaxy, not in a globular cluster, not near an active gamma ray source, not in a multiple-star system, not even in a binary, or near a pulsar, or near stars too small, too large, or soon to go supernova. We became a planet where global temperatures have allowed liquid water to exist for more than 4 billion years—and for that, our planet had to have a nearly circular orbit at a distance from a star itself emitting a nearly constant energy output for a long period of time. Our planet received a volume of water sufficient to cover most—but not all—of the planetary surface. Asteroids and comets hit us but not excessively so, thanks to the presence of giant gas planets such as Jupiter beyond us. In the time since animals evolved over 600 million years ago, we have not been punched out, although the means of our destruction by catastrophic impact is certainly there. Earth received the right range of building materials—and had the correct amount of internal heat—to allow plate tectonics to work on the planet, shaping the continents required and keeping global temperatures within a narrow range for several billion years. Even as the Sun grew brighter and atmosphere composition changed, the Earth's remarkable thermostatic regulating process successfully kept the surface temperature within livable range. Alone among terrestrial planets we have a large moon, and this single fact, which sets us apart from Mercury, Venus, and Mars, may have been crucial to the rise and continued existence of animal life on Earth. The continued marginalization of Earth and its place in the Universe perhaps should be reassessed. We are not the center of the Universe, and we never will be. But we are not so ordinary as Western science has made us out to be for two millennia. Our global inferiority complex may be unwarranted. What if Earth is extremely rare because of its animals (or, to put it another way, because of its animal habitability)?

The possibility that animal life may be very rare in the Universe also heightens the tragedy of the current rate of extinction on our planet. Earlier, we suggested that the rise of an intelligent species on any planet might be a common source of mass extinction. That certainly seems to be the case on Earth. And if animals are as rare in the Universe as we suspect, it puts species extinction in a whole new light. Are we eliminating species not only from our planet but also from a quadrant of the galaxy as a whole?

To understand the rates of extinction on Earth today, one has only to examine the plight of tropical rainforests. Forests have been a part of this planet for more than 300 million years, and although the nature of species has changed over that long period, the nature of the forests has changed little. The forests are the great Noah's ark of species on this planet. Although the land surface of our globe is only one-third that of the oceans, it appears that 80% to 90% of the total animal and plant biodiversity of the planet inhabits the land, and most of that diversity is found in tropical forests. As we destroy these forests, we destroy species. It has been estimated that between 5 and 30 million species of animals live in the tropical rainforests and that only about 5% of these are known to science. The fossil record tells us quite clearly that the world has attained the highest level of biological diversity ever in its history. There are also disturbing and unmistakable signs that this plateau in the number of species on Earth has been crested and the biodiversity of Earth is diminishing.

There appear to be several forces driving a reduction of biodiversity—a destruction of biodiversity, to be less delicate. The most important seems to be the rapid increase in human population. Ten thousand years ago there may have been at most 2 to 3 million humans scattered around the globe. There were no cities, no great population centers. There were fewer people on the globe than are now found in virtually any large American city. Two thousand years ago the number had swelled to perhaps 130 to 200 million people. Our first billion was reached in the year 1800. If we take the time of origin of our species as about 100,000 years ago, it seems that it took our species 100,000 years to reach the billion-person population plateau. Then things sped up considerably. We reached 2 billion people in 1930, about 1000 times faster than it took to reach the first billion. But the rate of increase kept accelerating. By 1950, only 20 years later, we had reached 2.5 billion souls. In 1999, we hit 6 billion. There will be approximately 7 billion people by 2020 and perhaps 11 billion by 2050 to 2100.

Rainforest conversion, which changes forest to fields, and then (usually) to overgrazed, eroded, and infertile land within a generation, is perhaps the most direct executioner of biodiversity. It appears that 25% of the world's top-

soil has been lost since 1945. One-third of the world's forest area has disappeared in the same interval. The result is species extinction. A thousand years from now, when humanity reflects on the world that was, and looks out at the desert surrounding the rare and notably less diverse animals that remain, whom will it hold responsible?

President Theodore Roosevelt closed off the Yellowstone region to development in forming the first national park in the United States. Wouldn't it be ironic if some alien equivalent had done the same thing for our planet? As-trobiologists have suggested this—it is known as the Zoo Hypothesis. The joke would be on us: We are somebody's national park, our rare planet Earth stocked with animals for safekeeping. Perhaps that is why we have yet to hear any signals from space. A big fence surrounds our solar system: "Earth Inter-galactic Park. Posted: No trespassing or tampering. The only planet with animals for the next 5000 light-years."

Twenty thousand years ago, Earth was locked in the glacial grip of the last ice age. Wooly mammoths and great mastodons, ground sloths, camels, and saber-toothed tigers roamed North America; people didn't. Humans were still thousands of years from crossing the land bridge from what would someday be called Siberia to the place we now know as Alaska. Humans were still 10,000 years from mastering agriculture. On some given yet forever anonymous day in that long ago time of 20,000 B.C., a distant neutron star in the constellation Aquila, part of the Summer Triangle so familiar to star watchers in the Northern hemisphere, underwent a violent cataclysm of some type and belched hard radiation into space, hurling an expanding sphere of poison at the speed of light in all directions. For 20,000 years it sped through space. It hit Earth over the Pacific Ocean on the evening of August 27, 1998, as it continued ever onward, its energy diminishing with each mile it traveled from its original source.

For 5 minutes on that late summer day, Earth was bombarded by gamma rays and X-rays, the lethal twins generated by thermonuclear bombs as well as by the interiors of stars. Even after traveling 20,000 light-years, the energy was sufficient to send radiation sensors on seven Earth satellites to maximum reading or off scale. Two of these satellites were shut down to save their instruments from burnout. The radiation penetrated to within 30 miles of Earth's surface and then was dissipated by the lower regions of our planet's atmosphere. This event was the first time that such high energy from outside the solar system was detected to have had a measurable effect on the atmosphere. But in all probability, it was not the first time Earth has been buffeted by energy from interstellar space. Perhaps a closer neutron star, or some other stellar demon not yet known to us, caused one or more of the mass extinctions in Earth's past. Perhaps we have only begun to see the demons surrounding us as we take our first tentative peeks through our planetary bedroom window into space.

Astronomers believe that the 1998 event was caused by the surface disruption of a kind of star that had only been theorized to exist: a magnetar. A type of neutron star, a magnetar is perhaps 20 miles in diameter but is more massive than our sun. It is estimated that a thimbleful of its material would weigh 100 million tons. It is matter compressed far beyond the point of human comprehension. The star has a surface of iron, but iron of a type never found in our solar system. The star spins, as all neutron stars spin, and the result is the formation of an intensely powerful magnetic field. For reasons we can only guess at, the surface of this star—20,000 years ago—underwent a massive disruption, sending energy into space as a consequence.

Energy dissipates with distance. Had the magnetar in question been only 10,000 light-years away, the energy reaching Earth would have been four times stronger—perhaps strong enough to damage the ozone layer. Did this particular event sterilize worlds within a light-year or less? Were there civilizations existing on worlds that were seared out of existence by gamma rays and a magnetic field pulse sufficient to tear the very molecules of living matter apart? Was another Earth sterilized? Perhaps life can flourish only in neighborhoods far from magnetars. Have magnetars—as well as so much else we have seen in the pages of this book—made animal life rare in the Universe? And what else is out there, lurking in the dark?

The discovery of a phenomenon such as the magnetar is an object lesson that suggests a great deal more than life's rarity: There is still so much more to learn about the heavens surrounding us. We humans are like 2-year-olds, just beginning to comprehend the immensity, wonder, and hazards of the wide world. So too with our understanding of astrobiology. It is clearly just beginning.

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