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around the time of the birth of Jesus Christ.
    “This tree saw the first light from the supernova that made the Crab Nebula, right about here,” Drake said, touching a point midway between the stump’s center and perimeter. Light from the supernova washed over the Earth in 1054, just as Western Europe was emerging from its Dark Ages. Sweeping his hand halfway farther out toward theperimeter, he brushed over the Age of Discovery, past rings recording the years when Europeans first explored and colonized the Americas. His hand kept moving until it slid from the stump’s edge.
    Over the course of the tree’s 2,000-year existence, the Milky Way had fallen nearly five trillion miles closer to its nearest neighboring spiral galaxy, Andromeda, yet the distance between the two galaxies remained so great that a collision would not occur until perhaps 3 billion years in the future. In 2,000 years, the Sun had scarcely budged in its 250-million-year orbit about the galactic center, and, considering its life span of billions of years, hadn’t aged a day. Since their formation 4.6 billion years ago, our Sun and its planets have made perhaps eighteen galactic orbits—our solar system is eighteen “galactic years” old. When it was seventeen, redwood trees did not yet exist on Earth. When it was sixteen, simple organisms were taking their first tentative excursions from the sea to colonize the land. In fact, fossil evidence testified that for about fifteen of its eighteen galactic years, our planet had played host to little more than unicellular microbes and multicellular bacterial colonies, and was utterly devoid of anything so complicated as grass, trees, or animals, let alone beings capable of solving differential equations, building rockets, painting landscapes, writing symphonies, or feeling love.
    By its twenty-second galactic birthday, some thousand million years hence, our planet may well return to its former barren state. Astrophysical and climatological models suggest that by then the Sun, steadily brightening as it ages, should increase in luminosity by about 10 percent—a seemingly minor change, but enough to render Earth’s climate too hot and its atmosphere too anemic to support complex multicellular life. Around that time, the oceans will begin evaporating, and most of Earth’s water will rapidly cook off into space. The loss of oceans a billion years from now marks the most likely expiration date for all life on Earth’s surface, though the omnipresent microbial biosphere might endure for billions of years further, shielded deep within the planet’s parched crust. Somewhere in the neighborhood of fivebillion years from now, the Sun will exhaust its supply of hydrogen and begin fusing its more energy-rich helium, gradually ballooning 250 times its current size to become a red giant star. Astronomers debate whether the Earth will be submerged within the hot outer layers of the swollen red Sun or whether it will escape relatively unscathed and only suffer its crust being melted back to magma. Either way, at that late date the life of our planet will be brought to a decisive conclusion.
    Considering the long concatenation of astrophysical events that led to our habitable planet, and the unknown synergies of technology and geology that could shape its fate, the distinction between chance and necessity blurs. Given a few hundred million years, would life arise on any rocky, wet, warm world? Would intelligence and technology emerge only on worlds with histories that mirrored our own, replete with the equivalents of Earth’s Moon, mobile crust, and blue sky? Or was a focus on these features merely a failure of our Earth-bound imaginations? Was our planet and its history a useful template or a stumbling block in the search for alien life and intelligence? Would we even recognize our own planet as “Earth-like” if we glimpsed it a half billion years in its past or in its future? Answers to questions like these would