How will the solar system end?

We live in uninteresting times. Since the ructions that created the planets in the solar system's first 100 million years (see "How was the solar system built?") - and apart from an early migration of the giant planets and the odd colliding comet not swept safely aside by Jupiter - nothing much has really been happening. The planets circle like clockwork, the sun burns steadily, and even delicate life has survived on at least one world.

It cannot last. Something unpleasant is bound to shatter this comfortable calm.

Our sun will die, of course, about six billion years from now. But things could get ugly long before that. The steady gyrations of the solar system today may conceal the seeds of chaos. Even the tiniest of irregularities can build up over time, gradually altering the paths of the planets. Between now and final sundown, it has been calculated, there is a roughly 2 per cent chance of catastrophe. Mars might drift too close to Jupiter and be thrown out of the solar system. If we're very unlucky, hot-headed Mercury could run wild and smash into Earth.

Meanwhile, the sun will slowly get brighter. Within 2 billion years, its heat will probably kill off life on Earth's surface. Mars, on the other hand - if it is still there - should gain a cosier climate. Even if it is dead today, it could one day come to life.

But again, not forever. When the sun's core burns up the last of its hydrogen fuel, the whole structure of the star will radically rearrange. It will slowly bloat to more than a million times its present volume, becoming a red giant. That giant will swallow Mercury and Venus and, according to the latest simulations, probably Earth too.

Baked by the sky-filling sun, and stained redder than ever, Mars will now definitively be dead. The icy moons of Saturn and Jupiter might in turn become hospitable. Saturn's giant moon Titan is particularly promising, as it already holds a rich soup of organic molecules. The red giant's heat could leave once-icy Titan with a global bath of water and ammonia where those organic molecules could form life.

Any creatures that bob to the surface of these outer moons would look up at a rather different sky. By that time, the Milky Way will probably have collided with our neighbouring galaxy Andromeda to form a unified "Milkomeda", where violent bursts of star formation - the nurseries of a new generation of solar systems - will light up the heavens for a time.

Any late flowering of life in our solar system, if it happens at all, will not last long. After its brief escapade as a red giant, the sun's inner furnace will finally fail, and it will cast off its outer layers and shrink into a tiny white dwarf. The briefly balmy Titan will freeze over once more. Its host planet Saturn, together with the other denizens of the outer solar system, will orbit on for tens of billions of years more, until treachery from within or marauders from without do for them, too. Jupiter or Saturn could eject their lighter comrades, Uranus and Neptune, or passing stars could strip away any planet, even massive Jupiter.

The future is never certain, though, and alternative endings can be written. There is a slim chance that the whole solar system, sun and all, might be thrown out of Milkomeda intact. Out in the emptiness of intergalactic space, the planets would be safe from marauders. There they could continue to circle our darkening star until their energy is eventually sapped and they spiral inwards. One by one as they hit the black-dwarf sun, a few final flares will rage against the dying of the light.