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Deadly wobble.
The solar system's rollercoaster-like path around the galaxy periodically makes it vulnerable to lethal radiation from intergalactic space.

Researchers may have uncovered the reason why Earth's biodiversity mysteriously plummets periodically. They have found that a rollercoaster-like wobble in the sun's orbit around the center of the Milky Way galaxy regularly moves Earth closer to a source of dangerous intergalactic cosmic rays.

Over the last 500 million years or so, the number of species on Earth has tended to dip regularly about every 62 million years. The last time this happened, about 55 million years ago--or about 10 million years after the great K-T extinction event that wiped out the dinosaurs--biodiversity sank by about 10%; around 115 million years ago, it dropped by a similar amount. So far, evolutionary biologists have only been able to establish that the phenomenon seems cyclical, but they haven't isolated a cause.