Habitable Zones and Times in the Universe

Because our limitations of the Universe deal with time, we must pose our question in a temporal sense: Are there times that are habitable in the Universe? As we will see in the next chapters, life (at least life as we know it) requires many elements that had to be created after the Big Bang (the advent of the Universe, some 15 billion years ago). Twenty-six elements (including carbon, oxygen, nitrogen, phosphorus, potassium, sodium, iron, and copper) play a major role in the building blocks of advanced life, and many others (including the heavy radioactive elements such as uranium) play an important secondary role by creating, deep within Earth, heat indirectly necessary for life. All of 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. 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.

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. The geological activity on Earth that is so important in controlling the atmospheric temperature via the CO2-rock cycle is driven by the heat liberated by the radioactive decay of uranium, thorium, and potassium atoms. These elements are produced by supernovae explosions, and their rate of formation is declining with time. 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.

Our definition of a universe habitable zone is based on time, and though intriguing, it is still a bit unsatisfying. Is there some geographic, rather than temporal, component of the Universe that favors or is poisonous to life? If we could map the Universe, would we find favorable and unfavorable regions, just as we do in stellar systems and in our galaxy? In other words, is life uniformly distributed throughout the Universe, or are there regions where it will exist and others where it will not? We cannot yet answer questions such as this, but some remarkable new discoveries have enabled us at least to address them.

For 10 days in December 1995, the Hubble Space Telescope in orbit around Earth focused its large mirror on a small region in space. A total of 342

exposures were made in the vicinity of Ursa Major, the Big Dipper. The area of space examined is tiny: from our perspective, only //30 the size of the full Moon. The target area in this small region—now known as the Hubble Deep Field—was a sprinkling of galaxies. The Hubble Deep Field appears to be one of the richest windows into distant galaxies known in the sky.

The results of these 10 days of photography have been nothing short of spectacular—and in a sense revolutionary. The photographs revealed galaxies 3 to 15 times fainter—and thus proportionally more distant—than any previously observed. More than 1500 individual galaxies can be identified in the photos. The light from these faint objects has come to us from the deep past—from periods long before our own galaxy formed, and our own sun. The most distant galaxies visible in these 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. We thus may be seeing images of the prebiotic Universe.

Another insight gleaned from the Hubble Deep Field is that older galaxies seem to have more irregular shapes than newer galaxies. From 30% to 40% of the most distant galaxies (and hence of the the oldest galaxies) are unusual or deformed compared to those nearest our own galaxy. The galaxies of the early Universe are quite different from newer galaxies. Does galaxy morphology affect habitability? And has habitability changed through time?

An even more surprising result was the finding that the various distances from Earth of the many galaxies seen in these photos cluster around a few values. Galaxies appear to be concentrated in great bubble-like or sheet-like structures with vast voids between. We might ask whether regions along these great sheets of galaxies have higher or lower hospitality to life. A key to habitability in various galaxies may be their abundance of heavy elements. Planets that form around metal-poor stars may be too small to retain oceans, atmosphere and plate tectonics. Metal-poor planets may not be able to support or maintain animal life, for reasons that we will detail in later chapters. It is known that entire galaxies are metal-poor and hence likely devoid of animal life.

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