OUR solar system has a bounce in its step, which may regularly send streams of comets hurtling into the Earth’s neighbourhood.
The solar system orbits the Milky Way’s centre, but it does not travel exactly on the galactic plane. As a result, the gravity of the plane pulls the solar system towards it, through it, and back again at regular intervals – we pass through the plane every 35 to 40 million years.
William Napier and Janaki Wickramasinghe at Cardiff University, UK, built a computer model of this motion. They found that when the oscillation brings us closer to the galactic plane, its gravitational tug can perturb comets in the Oort cloud, which surrounds the solar system. Similarly, passing through the galactic plane may bring the solar system into collision with the massive gas nebulas that reside there, which can also dislodge comets. The upshot, the researchers say, is that the number of comets entering the inner solar system can increase tenfold – with clear implications for Earth.
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Napier and Wickramasinghe think this may explain controversial geological evidence of periodic meteor showers on Earth, which some claim strike us every 36 million years or so. “It’s a beautiful match between what we see on the ground and what is expected from the galactic record,” says Napier. .
Richard Stothers at NASA’s Goddard Institute for Space Studies in New York thinks the model is persuasive. “I don’t think that there can be much doubt any longer about this source of comet flux variation,” he says. But Paul Weismann of NASA’s Jet Propulsion Lab in Pasadena, California, doubts that the galactic plane’s pull would be strong enough to knock many comets out of place. “It is not a bump that you go over that suddenly causes comet showers,” he notes.
