Tracking the Death Star

Some call it the Death Star. Others say It is a mystery planet. Still others call it voodoo astronomy.

Few, however, disagree that some mysterious factor-something from Space has been periodically snuffing ort life on earth during the past 100 million years. Theorists believe that whatever this force it, it has blown animals and other substances on the planet high into the stratosphere, about once every 26 million years or so.

This idea has recently exploded into one of the hottest topics in science today and has raised a host of intriguing scientific and philosophical questions. It has also spawned intense controversy about astronomy, evolution, and the origins of life.

The launching pad for this new research focus is the new theory that, contrary is previous thought, large-scale extinction have periodically wiped out all forms of life at various stages in the earth's life. In this thinking, the extinction of the dinosaurs 65 million years ago was not an isolated case in history, but one of several waves of extinctions.

The justification for this new idea came from a series of startling geological studies in the late 1970s by two University of Chicago geologists. David Raup and John Sepkowski. By carefully analyzing fossil records of the evolution of marine animals, the two concluded that large-scale extinctions occur about every 26 million years.

The revelation touched off hot debate among scientists, and because most scholars reasoned that no known earthly phenomenon could explain the 26-million-year cycles astronomers quickly entered the picture.

"It was not just a bad weekend. Something out of the ordinary happened, " says University of Arizona paleontologist David Jablonski. "It may have taken hundreds of years, but there is no question that there have been mass extinctions which shaped life, in unpredictable ways."

While the causes of extinction of the dinosuars have led to endless speculation, a group at the University of California at Berkeley led by Luis and Walter Alvarez have proposed a theory gaining wide acceptance as one of the plausible explanations.

Analyzing clay collected from several parts of the world, they found samples containing 160 times as much iridium as is normally found in terrestrial rocks, Iridium, however, is a common component of many extraterrestrial objects, " such as comets, asteroids, and other planets.

The group then set about examining how the iridium spread throughout the world, says Helen Michael, one of the Berkeley team's principle researchers. They reasoned that iridium-rich asteroids bombarded the earth, kicking up so much terrestrial and asteroidal debris into the atmosphere that it blocked sunlight from the earth for as long as several months. As temperatures plummeted, plants and animals died, leaving the current fossil record.

The theory made a big splash, explains Michael Rampino, of the Goddard Space Institute of Space Studies in New York. "They touched off a whole new field with some deep-seated implications."

The hypothesis initially met plenty of opposition, and still does, Raup explains. For example, some believe there may have been a combination of factors leading to any or all of the extinctions, "It takes a lot of different types of stress such as climate changes to cause an extinction. It's a very muddy topic, "he says.

In addition, he states, scientists prefer "earthbound" explanations.

But the hypothesis was quickly put to the test as scientists from all over the world scrambled to test rocks for iridium, and to the surprise of many, they discovered abnormally high levels of material at layers corresponding to the most recent extinction.

The theory lost some ground when geologists failed to discover excesses of iridium corresponding to earlier extinctions.

At this point, Raup and Stepkowski reentered the picture. They used the observations of thousands of scientists to compile a list identifying all families of marine animals that ever existed, carefully noting where they appeared and disappeared.

The found a pattern, and at a meeting to Arizona last august, the two dropped their bombshell. The extinctions, they said, seemed separated by periods of roughly 26 million years--the most recent occurring 11 million, 37 million, and 91 million years ago, leaving the present age about midway between extinctions.

At first, scientists were highly skeptical that extinctions could occur like clock work, but those who examined the evidence quickly became convinced.

"No one has really been able to shoot the hypothesis down." said Brian Niarsden of the Siarsden of the Harvard-Smithsonian Center for Astrophysics. "And until they can prove it isn't so, the theory still holds. That's the way science goes."

Moreover, if Kaup and Sepkoski are right, the implications stretch far into the heart of evolutionary science. Most scientists believe that races and species die out primarily as a result of genetic deficiencies, but the so-called "Death Star" theory may imply that extinctions are simply cases of bad luck.

Some believe the theory strengthens Aggassiz Professor of Zoology Stephen J. Gould's and Niles Eldridge's theory of "punctuated equilibrium," which holds that evolution proceeds in bursts rather than gradual increments.

Under the Raup and Sepkoski model of cyclical changes, the world is periodically wiped clean, leaving the evolutionary process with an empty slate.

Within days of the Raup-Sepkoski announcement, word of their work quickly spread to other areas of science. Astronomers leaped into the fray with the greatest zeal, and before long, the race was on to explain one of the most intriguing theories of life on earth.

The first step in linking the theory with astonomy came when two scientists from the Goddard Institute of Space Studies in New York connected the 26 million-year extinction cycle with the 26 million-year intervals between the sun's plunges through the dusty central plane of the Milky Way.

The sun is one of about 100 billion stars revolving in the Milky Way. All stars spin toward the center of the pinwheel-shaped galaxy, and each trip to the middle takes about 250 million years.

But as the sun spins inward it also bobs up and down through an imaginary plane that intersects the galaxy. The complete up-and-down trip through the debris-filled galaxy takes about 67 million years, and the sun slips through the most crowded region every 33 million years. During this time it could be blocked by the debris. The 33 million-year period corresponds roughly to the 26-million year cycle of extinction postulated by Raup and Sepkoski and the differences between the two figures can be accounted for by uncertainties in astronomical and geological datings, say the originators of the theory, Richard Smothers and Michael Rampino.

As the solar system passes through the central plane, it may collide with one of the massive dust clouds. While the cloud itself would have little graviational effect on the earth or the sun, its gravitational pull would. The strong lure would perturb the Oort cloud--a vast spherical shell of trillions of icy particles which surround the solar system.

Although such objects are as far as 10 trillion miles away, they clearly can effect the earth. If the solar system passed through the Oort cloud, a bombardment of comets would shake loose. Even if a small fraction of them made contact with the earth, the impact would blow enough debris into the atmosphere to cut off sunlight and cause a cosmic winter, ultimately extinguishing most of the life on the planet.

After cooking up their theory, the two scientists turned to proving it, primarily by examining craters on the earth that can be geologically dated, Sure enough, after analyzing data compiled by geologist Richard Grieve of brown university, they found that the ages of craters formed over the past 250 million years were clustered in 26 million-year intervals.

But other scientists have been quick to point out a key flaw. The solar system right now is passing through the central plane of the Milky Way and quickly approaching the dust clouds. Thus we should now be experiencing the first stages of the bombardment. But since we have yet to be blown into the stratosphere with the rest of the planet, researchers say they have grown somewhat skeptical.

The last bombardment supposedly took place 11 million years ago, which means we are some 15 million years ahead of schedule.

But the Goddard scientists have an explanation: the earth, they say, is currently passing through one of the galaxy's many "empty pockets," which they describe as cloud-free safety zones. They add that other geophysical phenomena, such as reversals in the earth's magnetic field, could add more evidence to support the anomaly.