Explore "The Deep" by Isaac Asimov on Textopian. Read with annotations and highlights.
In the end, any particular planet must die. It may be a quick death as its sun explodes. It may be a slow death, as its sun sinks into decay and its oceans lock in ice. In the latter case, at least, intelligent life has a chance of survival.
The direction of survival may be outward into space, to a planet closer to the cooling sun or to a planet of another sun altogether. This particular avenue is closed if the planet is unfortunate enough to be the only significant body rotating about its primary and if, at the time, no other star is within half a thousand light-years.