Science: Did Comets Kill the Dinosaurs?

A bold new theory about mass extinctions

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In the evening around the Leuschner Observatory in Lafayette, Calif., a few enterprising rattlesnakes slither out to toast themselves on the asphalt parking lot, which retains the warmth of the sun long after the air has cooled. Inside, a 30-in. telescope begins a laborious computer-controlled search of the heavens, covering only a tiny patch of sky during the next six hours of darkness. And the following day, at the nearby University of California campus in Berkeley, Physicist Richard Muller, like a seer divining entrails, scrutinizes the new batch of video recordings from Lafayette. He seeks a sign of a dim star that many scientists think does not exist: Nemesis, the death star, a possible companion to the sun.

Some 400 miles to the southeast, atop snow-covered Mount Palomar, Eugene Shoemaker, a geologist on leave from the U.S. Geological Survey, and his wife Carolyn, an asteroid astronomer, scurry around the unheated dome of the 18- in. Schmidt telescope. They photograph the sky in four-minute exposures, hunting for fast-moving objects against the background of the fixed stars. So far their Palomar study has identified 25 asteroids that cross the earth's orbit, bringing the known total to 60. Asteroids like this, they think, have occasionally crashed into the earth with catastrophic consequences, and they strive to calculate how frequently these inopportune meetings occur.

As the Shoemakers chart the asteroids' travels, astronomers at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., hunt for another elusive creature of the night: the legendary tenth planet, or Planet X. They comb through the tangled statistics and images transmitted in 1983 from the now defunct orbiting Infrared Astronomical Satellite (I.R.A.S.), struggling to find a single pinpoint source of radiation that over a six-month period has shifted in a particular pattern among the fixed stars, as only a nearby planet can do. Says Daniel Whitmire, a University of Southwestern Louisiana astrophysicist who is involved in the search: "There's a chance it's already been recorded and is awaiting discovery right now."

These quests are all part of the controversy and ferment that have been bubbling through the scientific community since the rise of a spectacular new theory that attempts to explain the mass extinctions--most notably the one in which the dinosaurs perished--that have punctuated the history of life on this planet. Every 26 million years or so, the theory holds, a rain of comets that lasts hundreds or thousands of centuries bombards the earth. The impact of some of the larger comets spews enough debris into the atmosphere to block the sun for months. As the skies darken, temperatures on the ground plummet and the majority of existing plant and animal species perish.

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