Several years ago I came across a remarkable book entitled Rare Earth, written by two scientists: Peter Ward and Donald Brownlee. In this book these two scientists challenge a widely-held belief that the universe is filled with a large number of civilizations, many of which may be far more advanced than ours. The scientist Carl Sagan once posited that there were probably hundreds of thousands of civilizations within our own galaxy, the Milky Way.
But Ward and Brown believe that human life may be rarer than many scientists have imagined. They point, for example, to the distance of the planet earth from the sun. If the earth were closer to the sun, the heat would be so intense that life would not be possible. In the same manner, if the earth were a greater distance from the sun, it would be so cold that the emergence of life would not be a possibility.
Ward and Brownlee also point to another remarkable aspect of our solar system. The planet Jupiter is an extremely large planet. In many w