Astronomers have long thought that the best place for life as we know it is a planet in the "habitable zone" - the range of orbits that leads to planets with liquid water - surrounding a main sequence star like our sun. Science fiction writers know better - and now astronomers are rethinking past work.
American astrophysicist William Danchi, and French colleagues Bruno Lopez and Jean Scheider, argue that the search for planets should not be limited to main sequence stars like our sun. The main sequence is only the first stable period of our sun's life; when it begins to burn its hydrogen around a growing helium core it offers another period of several billion years of stability. Finally, stars that have the right mass eventually become red giants; the temperature of the star's core increases as it shrinks, but the outer layers expand and cool. The "habitable zone" of a red giant (like the sun will be) extends from about 630 million miles to 2 billion miles.
Danchi, Lopez and Schneider argue that about 150 sub-giant and red giant stars are situated within 100 light years of Earth (compared to about 1,000 main sequence stars). NASA's Terrestrial Planet Finder space mission will focus only on main sequence stars. These stars will have habitable planets that are further from their suns, and will therefore be easier to find in the glare of the parent stars.
Science fiction authors have long used red giant stars as a convenient location for alien civilizations; often, it is used to denote the planet of a very ancient and wise civilization. In his 1953 novel Childhood's End, Arthur C. Clarke describes the planet of the Overlords who came to help Earth through a difficult developmental stage:
This was the supreme moment of his life: now he was to be the first human being ever to look upon a world lit by another sun...
It was cold, though not uncomfortably so. The light from the great red sun low down on the horizon was quite ample for human eyes, but Jan wondered how long it would be before he yearned for greens and blues. Then he saw that enormous, wafer-thin crescent reaching up the sky like a great bow placed beside the sun. He stared at it for a long time before he realized that his journey was not yet altogether ended. That was the world of the Overlords. This must be its satellite, merely the base from which their vessels operated.