After about 10 billion years, the surface of the white dwarf Sun cools to around 3000–4000 Celsius. At these temperatures the object looks distinctly red (even though it is still called a white dwarf), and is tens of thousands of times dimmer than the main-sequence star it used to be. By now the cooling rate, which slows drastically with age, is incredibly slow. But though it takes an unfathomable amount of time – longer even than the current age of the Universe – the white dwarf Sun one day vanishes totally from the optical window in the electromagnetic spectrum through which we humans today admire the Universe. Too cold to emit any signs of optical radiation at all, the dead Sun ceases to shine. It becomes a black dwarf.

At long last, perhaps 100 billion years from now, maybe even longer, the light will go out in the Solar System. The battered planets still remain, their orbits being stable, huddled around a star that they can no longer ‘see’. But overall, the scale of the planetary realm is almost twice as large as it is today. At only half a solar mass, the dark star's gravity clings quite feebly to its retinue of charred worlds, and they each orbit about 1.85 times further out than they do today. Meanwhile, those planets look little, if anything, like the worlds we know today. Over the tens of billions of years, facing a steadily declining heat source, the terrestrial planets have cooled down to just a few degrees above the coldest temperature possible, absolute zero.