Refuges for Life in a Hostile Universe; October 2001; Scientific American Magazine; by Guillermo Gonzalez, Donald Brownlee and Peter D. Ward; 8 Page(s)
In science-fiction stories, interstellar travelers visit exotic locales in the Milky Way and meet with interesting aliens. You name the place, and someone has put a civilization there: the galactic center, a globular cluster, a star-forming region, a binary star system, a red dwarf star. Part of the reason that sci-fi writers have to be so inventive is that scientists keep spoiling the fun. It used to be quite respectable to speculate about intelligent beings on the moon, Mars, Venus, Jupiter or even the sun, but nowadays canal-building Martians and cool oases inside the sun are merely quaint notions. As writers go ever farther afield to situate their characters, scientists are not far behind. Researchers are now casting a skeptical eye on musings about the prevalence of intelligent life throughout the Milky Way. Just as most of the solar system is hostile to multicellular organisms, the same may be true of much of the galaxy.
Within a given planetary system, astronomers describe the optimal locations for life in terms of the circumstellar habitable zone (CHZ). Although its definition has varied, the CHZ is generally considered to be the region around a star where liquid water can persist on the surface of a terrestrial, or Earth-like, planet for at least a few billion years. The zone is ring-shaped [see illustration on opposite page]. Its inner boundary is the closest that a planet can orbit its host star without losing its oceans to space. In the most extreme case, a runaway greenhouse effect might take hold and boil off the oceans (as happened on Venus). The outer boundary is the farthest a planet can roam before its oceans freeze over. From basic stellar theory, astronomers can estimate the size of the CHZ for a star of any mass [see "How Climate Evolved on the Terrestrial Planets," by James F. Kasting, Owen B. Toon and James B. Pollack; Scientific American, February 1988].