The Martian Way and Other Stories (Page 26)
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.
But planetary warmth dies, too, with time. Burrows must be dug deeper and deeper until the planet is dead through and through.
On the surface of the planet, wisps of neon blew listlessly, barely able to stir the pools of oxygen that collected in the lowlands. Occasionally, during the long day, the crusted sun would flare briefly into a dull red glow and the oxygen pools would bubble a little.
Eight hundred miles below the surface, a last bubble of warmth and life existed.