Any large, permanent, self-sustaining Human colony needs a world similar to our Earth in many respects. Such a world must have a breathable atmosphere. It must contain no poisonous gases, like chlorine or fluorine. Its partial pressure of oxygen must be at least 0.17 bars, the minimum with which Human lungs can function. Pure oxygen doesn’t do either, since it causes rapid burning that destroys our carbon-based life. The inert portion of the planet’s atmosphere must contain substantial nitrogen, for without this element, proteins, the essential building blocks of our bodies, cannot be formed. Yet if the partial pressure of nitrogen is too high, it reacts with our body chemistry to cause sickness and death. Carbon dioxide is also needed for the plants that comprise our foods, but too much carbon dioxide makes a planet’s atmosphere a “greenhouse” that would be unbearably hot. A habitable planet must have large oceans, since water is the primary component of our bodies and the foods we eat. Studies of many planets have led scientists to believe that unless 40 percent of a world’s surface is ocean, no portion of it will have enough rainfall to support our form of life.
Table 3.1 and Figure 3.1 illustrates other important similarities of habitable worlds. Each of the Human colony planets orbits about the same distance from its sun as does the Earth from its sun, and their suns are the same size as the Earth’s sun. Though it’s not immediately obvious, the reason suns of all habitable planets must be about the same size relates to the formation of a breathable atmosphere. The oxygen-nitrogen atmosphere needed for Human life never occurs on totally lifeless worlds. Such an atmosphere is a by-product of a long evolutionary process. Though the intricacies of this process remain a mystery, scientists know with certainty that it takes billions of years. The giant stars of our galaxy do not live that long. For reasons explained by complex physics, large stars consume their fuel quickly and burn out, while small stars, like Earth’s sun, remain the same size and brightness for billions of years. In fact, the smaller a star is, the longer it will live. Stars can be too small for habitable planets as well. To be warm enough for Human life, a planet must orbit its sun closely enough to receive the right amount of energy. If Earth’s sun had 30 percent less mass than it has, then the Earth would have to orbit it so closely that the sun’s gravitational attraction would stop the Earth’s rotation with respect to the sun, just as the Earth has stopped the relative rotation of its moon. This would make one side of the planet too hot for life, while the other side would be too cold. The requirement that stars be neither too large, nor too small, has allowed GAILE scientists to rule out all but 12 percent of the stars in the heavens as possessing habitable planets. In addition to the relationship between star life and size, there is also a relationship between star temperature and size.
Though notable exceptions exist, large stars tend to be hotter than small stars. It is therefore quite natural that most of the stars of a suitable size for habitable planets also have about the same temperature and emit the same color of light. Stars are typed or classified by letter according to temperature, and virtually all suns that might have habitable planets fall into three of the seven major types— F, G, and K.
Since nature has dictated that stars with habitable planets must be the same size and temperature, then the planets habitable by Humans must orbit at about the same distance as the Earth orbits the sun. Human beings and the life forms we consume for food can exist only within a very narrow range of temperatures. Above 65° C. or below -60° C. we must wear environmental suits to avoid perishing within minutes. Our food crops are confined to an even narrower range. For practical purposes, no place can be considered habitable whose temperature falls consistently above 45° C. or below 0° C