The cosmic timebomb

The asteroid that wiped out the dinosaurs was thrown to Earth in a moment of 'planetary madness'. And scientists can now predict when the heavens will go haywire again, says Marcus Chown

There's something badly wrong with the pendulum clock in the corner of the room. Normally, it ticks rhythmically, its bob swinging back and forth with hypnotic regularity. Over time, however, the size of the swing gradually gets larger, the ticks louder and louder. And, very occasionally - in fact, so occasionally that nobody has yet ever observed it - the clock goes stark-staring mad, ticking completely erratically as the pendulum bob swings first to one side, then twice or three times as far to the other side.

There's something badly wrong with the pendulum clock in the corner of the room. Normally, it ticks rhythmically, its bob swinging back and forth with hypnotic regularity. Over time, however, the size of the swing gradually gets larger, the ticks louder and louder. And, very occasionally - in fact, so occasionally that nobody has yet ever observed it - the clock goes stark-staring mad, ticking completely erratically as the pendulum bob swings first to one side, then twice or three times as far to the other side.

Surely, there is no clock that behaves like this? According to a team of geophysicists and mathematicians, there is: the clock in the sky. "For tens of millions of years, the planets circle the Sun with the predictability of clockwork," says Michael Ghil of the Ecole Normale Supérieure in Paris and the University of California at Los Angeles (UCLA). "Then, without the slightest warning, everything goes utterly haywire."

The heavens are generally considered to be a paragon of predictability so this is a radical stuff. But it is only the beginning. Ghil and his colleagues, Ferenc Varadi and Bruce Runnegar at UCLA, believe the last time the solar system went insane was roughly 65 million years ago. "It seems too much of a coincidence," says Ghil. "We think it may have been connected with the extinction of the dinosaurs."

The kind of planetary madness Ghil and his colleagues are talking about goes by the name of "chaos". Chaos is defined as erratic motion with no sign of any regularity. Loosely speaking, chaotic systems are infinitely sensitive to initial conditions, like a hurricane in the Caribbean that was triggered by the flutter of a butterfly's wings in distant Hawaii.

In the solar system, the most important drivers of chaos are Jupiter and Saturn because they are the most massive of the planets. In their investigation of planetary chaos, it is therefore these two planets that Ghil and his colleagues have focused their attention on. The Jupiter-Saturn system is actually not inherently chaotic. However, it is known to skate close to the edge of chaos. The possibility therefore exists that, occasionally, something might cause it to teeter over the edge into planetary insanity.

Ghil and his colleagues considered the possibility that the "something" might be fluctuations in the pressure exerted on Saturn by sunlight and the wind of subatomic particles blowing from the Sun. Over tens of millions of years, their combined buffeting could have a significant effect on Saturn's orbit. The researchers guessed that solar variability might change the planet's "semi-major axis" - a measure of the length of its elliptical path round the Sun - by as much as 0.1 per cent. "We think this is perfectly plausible," says Ghil.

To see what changing Saturn's semi-major axis did to the Jupiter-Saturn system, Ghil and his colleagues used a "digital orrery". This is a purpose-built computer rigged to simulate the motion of the planets under their mutual gravity. The researchers also incorporated a novel feature of the behaviour of Jupiter and Saturn.

Jupiter orbits the Sun "about" five times for every two times Saturn goes round. If the ratio of the orbital periods was precisely 5:2, the combined effect of the gravity of two massive planets on other bodies in the solar system would be greatest every 10 years - that is, when the two planets are on the same side of the Sun and pulling together. But, because this 5:2 "resonance" is not exact, the planets are in perfect alignment on the same side of the Sun only every 1,000 to 2,000 years. "What this means is that the effect of Jupiter and Saturn on the other bodies in the solar system rises to a crescendo every 1,000-odd years," says Ghil.

Until now, researchers who have used computers to simulate the long-term future of the solar system have assumed that this effect is of no consequence, guessing that over long periods of time its effect "averages out". "We had a hunch, however, that this wasn't true," says Ghil. Using their digital orrery and taking this effect into account, Ghil and his colleagues discovered that as the semi-major axis of Saturn's orbit changes, the Jupiter-Saturn system drifts back and forth between motion which is regular and motion which is totally chaotic. "The system trips over into chaos every few tens of millions of years," says Ghil.

The team's most remarkable discovery, however, is that in a wide range of simulations in which the semi-major axis of Saturn is allowed to vary, a burst of chaos arises around 65 million years before the present. "The timing coincides strikingly with the Cretaceous-Tertiary [geological] boundary which marks the extinction event that wiped out the dinosaurs," says Ghil.

As yet, says Ghil, it is impossible to tell how long the burst of chaos persisted. Nevertheless, it is possible to investigate the effect it would have had on other bodies in the solar system - specifically, asteroids. The asteroids are thought to be the left-over rubble of a planet which was prevented from congealing out of the "proto-planetary nebula" by the disruptive effect of Jupiter. Vast numbers of asteroids - ranging in size from pebbles to rocky bodies 1,000 kilometres across - circle the Sun between the orbits of Jupiter and Mars.

Ghil and his colleagues simulated the effect on the asteroids of a burst of chaos in the Jupiter-Saturn system. They found a wealth of effects. "The most important are abrupt changes in the semi-major axis of asteroid orbits," he says. "These would lead eventually to complete ejection of bodies from the asteroid belt." Some of these could easily end on a collision course with Earth.

The sequence of events revealed by the simulations is complex. Some asteroids suffer small jumps in the size of their semi-major axis, others large jumps. Some move to smaller orbits, some to longer orbits. "A population of asteroids can drift back and forth through a succession of different orbits," says Ghil.

Crucially, bodies whose elliptical orbits become ever more elongated eventually come under the influence of the gravity of other planets and are tugged free of the asteroid belt. "They get catapulted out of the asteroid belt, some into orbits which cross the Earth's orbit," says Ghil. This is precisely what Ghil and his colleagues think might have happened 65 million years ago. "A burst of chaos in the Jupiter-Saturn system caused a flurry of Earth-crossing asteroids," says Ghil. "Among them was one which struck the Earth off the coast of Central America, providing the killer blow which finished off the dinosaurs."

If Ghil and his colleagues are right, the demise of the dinosaurs cannot be attributed to an entirely random event. As the dinosaurs grazed unawares, the great clock of the solar system went temporarily out of kilter. The dinosaurs may have been victims of an event hardwired into the dynamics of the solar system. "And they may not have been the only victims," says Ghil.

The team's simulations reveal that another burst of planetary chaos occurred about 250 million years ago. This seems to correspond precisely with another major mass extinction at the Permian-Triassic boundary. "As yet, however, we aren't totally confident about this," says Ghil.

The new paradigm which seems to be emerging is of a solar system which evolves quietly for tens of millions of years but which goes through occasional periods of madness. And what has happened in the past will happen again. The simulations show another burst of chaos is due in the future. "I wouldn't lose sleep over it," says Ghil. "The due date is AD30 million, so there's plenty of time to evacuate the Earth!"

Marcus Chown is author of 'The Universe Next Door' (Headline)