"Earth has provided a stable platform for the evolution of life over 4 billion years. But that lease is limited; we know for sure that it will expire after a few billion more. Long before that, our planet may become a place where it is no longer suitable for us to live. Increasing luminosity of the sun may gradually boil our oceans, or more sudden catastrophes may threaten our existence. If we are wise, we will have furnished our new apartments long before that time." - Robert Shapiro, Planetary Dreams, 1999