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Note on Sources |  | | | |  Earth-like planets are common.  Metrodorus of Chios quoted by Joel Achenbach. "[Life Beyond Earth](http://www.nationalgeographic.org/ngm/0001/fngm/)," *National Geographic*, Jan 2000. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 7-11.  Earth is rare.  Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xi. Geoff Marcy. E-mail with NOVA producer Julia Cort. 7 June 2004. Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153.  Life flourishes in hostile places.  David Darling. *Life Everywhere: The Maverick Science of Astrobiology*, (New York: Basic Books, 2001), xi. Robert Ballard. *Explorations: My Quest for Adventure and Discovery Under the Sea*, (New York: Hyperion, 1995).  Life flourishes in hostile places, but not *complex* life.  Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153. Ian Crawford, "[Where Are They?](http://www.sciam.com/article.cfm?articleID=0009CDEA-33FC-1C74-9B81809EC588EF21)" *Scientific American* (July 2000): 43. Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xxvi.  Intelligence is likely.  David Darling. *Life Everywhere: The Maverick Science of Astrobiology*, (New York: Basic Books, 2001), 140-141 Seth Shostak, "[Intelligent Aliens?](http://www.space.com/searchforlife/shostak_intelligent_020530.html)" *Space.com*, 30 May 2002, (9 June 2004). Roger Hanlon, interview by NOVA producer Julia Cort. *Origins: Where Are the Aliens?,* NOVA: Public Broadcasting System, 29 September 2004.  Intelligence is unlikely.  Stephen Jay Gould, *Wonderful Life: The Burgess Shale and the Nature of History*, (New York: W. W. Norton, 1989). Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 154. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 163. Peter Ward, NOVA interview by Neil Tyson. 2004.  Alien civilizations are likely to be advanced.  Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 48. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 96.  Alien civilizations are likely to be gone.  Joel Achenbach. "[Life Beyond Earth](http://www.nationalgeographic.org/ngm/0001/fngm/)," *National Geographic*, Jan 2000. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 163. Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life Is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xxvii.  Aliens haven't visited because they can't or don't want to.  David E. Fisher and Marshall Jon Fisher. *Strangers in the Night* (Washington, D.C.: Counterpoint, 1998), 257. Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 72. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 5.  If aliens existed, they would have visited.  Sebastian von Hoerner, "The Likelihood of Interstellar Colonization" *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 30. Ian Crawford, "[Where Are They?](http://www.sciam.com/article.cfm?articleID=0009CDEA-33FC-1C74-9B81809EC588EF21)" *Scientific American* (July 2000): 42.  SETI hasn't succeeded because the search has just begun.  David E. Fisher and Marshall Jon Fisher. *Strangers in the Night* (Washington, D.C.: Counterpoint, 1998), 260-261 Frank Drake and Dava Sobel. *Is Anyone Out There?* (New York: Delacorte Press, 1992), 235-236. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 12.  SETI's quest is futile  George W. Swenson, Jr., "[Intergalactically Speaking](http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=86432992-17B0-4B7B-A538-C63722AEEB7&ARTICLEID_CHAR=0E010E81-DDC5-4C0E-9306-0FDFF4F81CC&sc=I100322)," *Scientific American*: (July 2000), 47. Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 71. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 164. 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##  Earth-like planets are common.  Metrodorus of Chios quoted by Joel Achenbach. "[Life Beyond Earth](http://www.nationalgeographic.org/ngm/0001/fngm/)," *National Geographic*, Jan 2000. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 7-11. ##  Earth is rare.  Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xi. Geoff Marcy. E-mail with NOVA producer Julia Cort. 7 June 2004. Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153. ##  Life flourishes in hostile places.  David Darling. *Life Everywhere: The Maverick Science of Astrobiology*, (New York: Basic Books, 2001), xi. Robert Ballard. *Explorations: My Quest for Adventure and Discovery Under the Sea*, (New York: Hyperion, 1995). ##  Life flourishes in hostile places, but not *complex* life.  Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153. Ian Crawford, "[Where Are They?](http://www.sciam.com/article.cfm?articleID=0009CDEA-33FC-1C74-9B81809EC588EF21)" *Scientific American* (July 2000): 43. Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xxvi. ##  Intelligence is likely.  David Darling. *Life Everywhere: The Maverick Science of Astrobiology*, (New York: Basic Books, 2001), 140-141 Seth Shostak, "[Intelligent Aliens?](http://www.space.com/searchforlife/shostak_intelligent_020530.html)" *Space.com*, 30 May 2002, (9 June 2004). Roger Hanlon, interview by NOVA producer Julia Cort. *Origins: Where Are the Aliens?,* NOVA: Public Broadcasting System, 29 September 2004. ##  Intelligence is unlikely.  Stephen Jay Gould, *Wonderful Life: The Burgess Shale and the Nature of History*, (New York: W. W. Norton, 1989). Mayr, Ernst. "The Search for Extraterrestrial Intelligence." *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 154. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 163. Peter Ward, NOVA interview by Neil Tyson. 2004. ##  Alien civilizations are likely to be advanced.  Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 48. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 96. ##  Alien civilizations are likely to be gone.  Joel Achenbach. "[Life Beyond Earth](http://www.nationalgeographic.org/ngm/0001/fngm/)," *National Geographic*, Jan 2000. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 163. Donald Brownlee and Peter D. Ward. *Rare Earth: Why Complex Life Is Uncommon in the Universe*, (New York: Copernicus Books, 2000), xxvii. ##  Aliens haven't visited because they can't or don't want to.  David E. Fisher and Marshall Jon Fisher. *Strangers in the Night* (Washington, D.C.: Counterpoint, 1998), 257. Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 72. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 5. ##  If aliens existed, they would have visited.  Sebastian von Hoerner, "The Likelihood of Interstellar Colonization" *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 30. Ian Crawford, "[Where Are They?](http://www.sciam.com/article.cfm?articleID=0009CDEA-33FC-1C74-9B81809EC588EF21)" *Scientific American* (July 2000): 42. ##  SETI hasn't succeeded because the search has just begun.  David E. Fisher and Marshall Jon Fisher. *Strangers in the Night* (Washington, D.C.: Counterpoint, 1998), 260-261 Frank Drake and Dava Sobel. *Is Anyone Out There?* (New York: Delacorte Press, 1992), 235-236. Seth Shostak. *Sharing the Universe: Perspectives on Extraterrestrial Life,* (Berkeley: Berkeley Hills Books, 1998), 12. ##  SETI's quest is futile  George W. Swenson, Jr., "[Intergalactically Speaking](http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=86432992-17B0-4B7B-A538-C63722AEEB7&ARTICLEID_CHAR=0E010E81-DDC5-4C0E-9306-0FDFF4F81CC&sc=I100322)," *Scientific American*: (July 2000), 47. Paul Davies. *Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life*. (New York: BasicBooks, 1995), 71. Jared Diamond. "Alone in a Crowded Universe," *Extraterrestrials: Where are They? 2nd ed.,* Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 164. [](https://www.pbs.org/wgbh/nova/origins/alie-sources.html#fea_top)