these elements were created within the centers of stars—often in exploding stars, or supernovae—rather than in the Big Bang itself, so they were not present in sufficient abundance for perhaps the first 2 billion years or more of the Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p29-30] Then, the zone” of the Universe, in the sense of time, began only “habitable after its first 2 billion years. The early history of the Universe was also dominated by objects known as quasars, which would have been very dangerous [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30] The early Universe must have been lifeless or at least empty of advanced life, and quite remarkably, there are also limits on the time during which the Universe can exhibit Earth-like planets that provide adequate life support for advanced life. [Peter Ward & Donald Brownlee: Rare Earth, Copernicus Books, 2000, Why Complex Life Is Uncommon in the Universe, p30] In our galaxy, stars that form at present have less of these radioisotopes than the sun did when it formed 4.6 billion years ago. It is entirely possible that any true Earth clones now forming around other stars would not have enough radioactive heat to drive plate tectonics, a key process that helps stabilize Earth’s surface temperature. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30] The most distant galaxies visible in photos probably date to some time during the first few billion years after the start of the Universe, and hence they may antedate life anywhere. It is unlikely that any of the stars in these galaxies could have Earth-like planets because the heavy elements to build them were not yet abundantly available. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p32] The end of planetary habitability In other planetary systems, primitive life might flourish but never advance
these elements were created within the centers of stars—often in exploding stars, or supernovae—rather than in the Big Bang itself, so they were not present in sufficient abundance for perhaps the first 2 billion years or more of the Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p29-30] Then, the “habitable zone” of the Universe, in the sense of time, began only after its first 2 billion years. The early history of the Universe was also dominated by objects known as quasars, which would have been very dangerous [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30] The early Universe must have been lifeless or at least empty of advanced life, and quite remarkably, there are also limits on the time during which the Universe can exhibit Earth-like planets that provide adequate life support for advanced life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30] In our galaxy, stars that form at present have less of these radioisotopes than the sun did when it formed 4.6 billion years ago. It is entirely possible that any true Earth clones now forming around other stars would not have enough radioactive heat to drive plate tectonics, a key process that helps stabilize Earth’s surface temperature. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30] The most distant galaxies visible in photos probably date to some time during the first few billion years after the start of the Universe, and hence they may antedate life anywhere. It is unlikely that any of the stars in these galaxies could have Earth-like planets because the heavy elements to build them were not yet abundantly available. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p32] The end of planetary habitability In other planetary systems, primitive life might flourish but never advance