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108 been evolving on Earth for 4 billion years, implying that the interval between disasters is at least that long. Either our calculations are erroneous or we have been very lucky. To Norris, this implied that life on Earth may be unique in the galaxy.® Borrowing from an evolutionary theory developed by Stephen Jay Gould and Niles Eldredge, astronomer Milan Cirkovic proposed a kind of “punc- tuated equilibrium” for the evolution of technological intelligence in the Galaxy, a long-term balance between gradual and catastrophic effects. Technological civilizations may be eliminated periodically in the course of galactic evolution; the most important single causes of catastrophe may be gamma-ray bursters.® There may not have been enough time for civilizations significantly older than ours to arise during this part of the aero cycle. We also might face threats from interstellar wanderers. Runaway stars that may have been flung out of binary systems race through the Galaxy at high speeds; one pulsar is zipping along at 1100 kilometers (680 miles) a second. Burned-out stars and brown dwarfs may be difficult to spot until they are relatively near our solar system. Our Galaxy may contain about one billion dead, extremely dense pulsars. About a billion nonprimordial black holes may be strewn through the Galaxy; the nearest may be only 10 or 20 light-years away. Some astronomers have proposed that huge numbers of dark matter halos—about as large as our solar system—may drift through the Milky Way, with one passing through our neighborhood every few thousand years.” Simulations of planet formation in protostellar disks have shown that a high percentage of planetary-sized objects are ejected from the system or are pulled out of it by encounters with other stars. There may be many free-floating planets in interstellar space that are nearly impossible to detect, particularly because they are cool.” Even if there were no collision, Earth’s orbit could be disturbed by a close encounter with a massive body. Simulations have shown that a near miss from a rogue star less than half the size of our Sun could bounce the Earth and Mars out of their normal trajectories.” The Deadly Intruder H.G. Wells foresaw an extraterrestrial terminator in an 1897 story called “The Star.” Astronomers detect a wandering planet entering our solar system. After a merger with Neptune, this body plunges inward, appar- ently on a collision course with the Earth. Our planet’s temperature rises; snows melt, floods and violent storms begin, earthquakes and volcanic eruptions ravage the world. Millions die. Although the collision never takes place, the Earth’s climate has been changed, forcing humans to migrate toward the poles.” Probabilities: Longevity