A bizarre wobble 65 million years ago, perturbing the orbits of Mars, Earth and Mercury, may have caused the demise of the dinosaurs, new models of the Solar System suggest.

The celestial upset could have disrupted the trajectories of asteroids - normally safely confined to asteroid belts -sending one or more careering into the Earth. The proposed wobble coincides with the Cretaceous-Tertiary (K-T) boundary, when an asteroid or comet is thought to have smashed into Earth, exterminating the dinosaurs.

"The ultimate cause of the K-T impact - and the demise of the dinosaurs - may have been a chaos-induced change in Solar System dynamics," astrobiologist Bruce Runnegar of the University of California in Los Angeles told the Earth Systems Processes Global Meeting in Edinburgh, Scotland, yesterday.

Runnegar's models suggest that the subtle shifts in orbits - which would be too small to measure if they occurred today - emerged from the amplification of tiny irregularities over time. "The chaotic nature of the system causes the event," says Runnegar.

Looking at ancient ocean sediments, Runnegar and his colleagues had previously identified evidence of a 400,000-year cycle in the Earth's climate that corresponded closely to natural fluctuations in its orbit.

To understand how this cycle may have influenced Earth's climate during the past 100 million years, the team constructed computer models based on natural variations in planetary orbits, their proximity to the Sun and their gravitational effects on one another.

In each of the models, they found that the fluctuations of the Solar System's dynamics remained constant going back to 65 million years ago. To their surprise, the frequency of the orbits of the inner planets then suddenly changed.

"If the orbits of Mercury, Earth and Mars were being shaken up at this time, maybe asteroids were being shaken up too," says Runnegard. This might possibly send a previously benign asteroid hurtling towards Earth.

The team's prime suspects are the Hungarias asteroids, whose behaviour is governed by Mercury. Being one of the inner planets, Mercury's orbit would have been affected.

The researchers have put the known positions of Hungarias into their models and are running them again to see if one asteroid gets flung to Earth. The models can take six months to run, so the team are still waiting to see what happens.

"Last week we went back to the Triassic," says Runnegar. "We should know what happened at the K-T boundary by Friday."

"It's a beautiful theory," says Paul Olsen, who studies Solar System-induced changes in Earth's climate at Columbia University in New York. "But I don't know what to think about it."

Runnegar's model, says Olsen, may be reflecting the "nitty-gritty" of the complex mathematics used to extrapolate so far back in time rather than what may have actually occurred.

But by looking elsewhere for older geological evidence of fluctuations in climate that may be due to the Solar System effects, it should be possible to corroborate or kill the model. "There are very good tests for the hypothesis," says Olsen.