'Death star' and dinosaur-extinction theories are mostly speculation

MY favorite physics professor used to insist that scientists ''do'' science because it is fun. That seems to be the perspective in which to view ongoing speculation about a comet clobbering the dinosaurs or the possible existence of a ''death star'' called Nemesis, which sends a periodic rain of comets to cause mass extinctions on Earth.

Now that five of the scientific papers embodying these widely publicized theories have actually appeared in a recent issue of Nature, it is obvious that there is little solid evidence to support them. The authors seem to be caught up in the fun of a stimulating intellectual exercise rather than revealing any major new discovery.

The notion that a comet may have wiped out the dinosaurs 65 million years ago has been discussed for several years. The impact of such a comet hitting Earth could have raised a dust cloud that blacked out the sun for many months. This would have suppressed photosynthesis and disrupted food chains for both lower and higher animals.

Luis Alvarez, a physicist and Nobel laureate at the University of California's Lawrence Berkeley Laboratory (LBL), together with his geologist son , Walter, and other colleagues, proposed such an impact-extinction theory in 1980.

Among other things, they note an abnormal enrichment of the element iridium in some 65 million-year-old clay sediments. They consider this a ''signature'' of a comet.

Many astronomers believe that the solar system is accompanied by a cloud of comets and that occasional passing stars send some of them in among the planets. In 1982, Jack Hills of the Los Alamos Scientific Laboratory suggested that an exceptionally close star passage could send billions of comets toward the inner solar system. Earth itself would likely suffer abnormal bombardment.

These and similar suggestions have stirred a continuing scientific debate. They prepared the way for an explosion of speculation over the past six to eight months when David M. Raup and J. John Sepkoski of the University of Chicago began saying that major extinctions have occurred at intervals of 26 million years. Their ''discovery'' is based on a sophisticated statistical analysis of paleontological data.

Alerted by preprints or informal reports of the Raup and Sepkoski paper, several groups have floated new speculations about extraterrestrial causes of mass extinction. Richard Muller of LBL, Marc Davis of the University of California, and Piet Hut of the Institute for Advanced Study at Princeton have improved on Jack Hills's theory by suggesting that our sun has a companion star which periodically comes close enough to deflect a rain of comets into the inner solar system. Daniel Whitmire of the University of Southwestern Louisiana and Albert Jackson of Computer Sciences Corporation have a similar theory.

Muller and his colleagues dubbed this hypothetical ''death star'' Nemesis. It has been widely mentioned in the news media in recent months.

Other scientists have suggested that the regular motion of the solar system through the galactic plane may be involved. As the sun and planets move from below to above the galactic plane and subsequently back down again, they will encounter dust clouds. These, too, could divert comets toward Earth. Still other scientists claim to have found evidence of recurring bombardment in the geological record of ancient craters.

There is some discrepancy in the periodicities. Raup and Sepkoski identify a catastrophe every 26 million years. Others cite 28.9 million or around 30 million years. But, discrepancy or not, the notion that Earth may be regularly subjected to major outside interference has stirred a lot of interest.

As often happens in scientific speculation, it pays to wait for the actual papers before getting too excited. The Raup and Sepkoski analysis finally appeared in the Proceedings of the National Academy of Sciences in February. But the five papers speculating about death stars and cometary attacks, which now have appeared in Nature, were prepared long before that.

Reviewing all this in a companion article in Nature, geologist A. Hallam of the University of Birmingham in England wisely points out that attempts to explain periodic extinctions are futile, if those periodicities don't exist. He then shows that uncertainties in the geological record throw serious doubt on the Raup and Sepkoski analysis. He further notes that there is good evidence that mass extinctions are due to such earthly causes as climatic change or sea level changes. He concludes, tartly, that ''before astronomers indulge in further speculations about the causes of mass extinctions they would do well to learn something about the rich stratigraphical record of their own planet.''

Nature editor John Maddox notes that scientists have been so eager to promote their speculations that they fired off papers to his journal before the Raup and Sepkoski analysis itself was published. Also, thanks partly to news reports of such half-baked theorizing, Nature began receiving criticisms of these speculations even before the papers were published.

Such developments, which confuse the normal processes of scientific publication and critical review, ''constitute a kind of nonsense,'' Maddox observes.

Scientists may have their fun. But the public at large should take their speculations with a great deal of skepticism.

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