Image: Illustration of of comic impact
Don Davis
An artist's conception shows the cosmic impact that scientists believe touched off mass extinctions of many species, including dinosaurs.
updated 5/13/2008 12:59:31 PM ET 2008-05-13T16:59:31

The sun bounces up and down as it roams the Milky Way, and such wavering might have hurled showers of comets Earth's way that caused mass extinctions, including the one that killed the dinosaurs, a new study claims.

Such cosmic impacts might also have spread life to alien worlds, scientists speculate.

However, doubts have been raised about these suggestions.

To arrive at the comet showers idea, astronomers calculated the path of our solar system across the Milky Way as it circles the galactic core. As we pass through the densest part of the galactic disk, the gravitational pull of the surrounding gas and dust clouds dislodges comets in the Oort Cloud in the outer solar system, causing these icy goliaths to plunge toward the sun, the researchers said.

The sun passes through this galactic zone every 35 million to 40 million years, raising the chances of comets hurtling inward tenfold, according to calculations. This cycle seems to coincide with evidence of craters and mass extinctions on Earth, which suggest we suffer more collisions roughly every 36 million years.

"It's a beautiful match between what we see on the ground and what is expected from the galactic record," said researcher William Napier, an astronomer at the Cardiff Center for Astrobiology in Wales. He and his colleague Janaki Wickramasinghe will detail their findings in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society.

Although the solar bounce cycle may have been killer news for non-avian dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and the many plant and invertebrate species that went extinct 65 million years ago, it may also have helped life spread to other planets. The comets may have blasted microbes off Earth that can withstand the interstellar void.

Deadly dinos"Microorganisms thrown into space from this barrage can pass straight into star- and planet-forming regions within the nebula, without being sterilized en route by cosmic rays," Napier told "This opens the door to the exciting idea that life may spread not just within the solar system, but may be pan-galactic."

However, there is skepticism about these calculations.

"The whole concept is wrong," said NASA Jet Propulsion Laboratory astronomer Paul Weissman, who did not participate in this study. "As you cross the galactic plane, it's not like you go over a speed bump. The variation in any cratering rate would be very gentle, not in sudden pulses."

In the past, some researchers had suggested that mass extinctions on Earth happened roughly 30 million years apart, and that cosmic impacts also recurred at about that frequency, and that both were linked. But Weissman noted there is "considerable doubt" as to whether mass extinctions or cosmic impacts do occur at such regular intervals.

Other suggested causes for these supposed comet showers have included a red dwarf star or a tenth planet in a distant orbit around the sun that dipped into the Oort cloud.

"Many researchers, including myself, responded to these ideas, pointing out faults in the logic. The distant red dwarf star was in a highly unstable orbit that would escape to interstellar space after about 10 orbits on average," Weissman said. The tenth planet idea never gained any traction because it was difficult to create an orbit with the right characteristics, he added, "and there is no evidence for a large planet beyond Neptune, despite numerous searches."

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