PLANET X, some scientists theorize, may have been implicated in the rapid disappearance of the dinosaurs and the phenomenon known as the iridium anomaly. Iridium is a scarce metal. Yet it occurs in remarkably high concentrations at two levels of the earth's geological strata: one formed about 65 million years ago, the other about 34 million. One explanation offered is that Earth was struck by a meteor containing large amounts of the metal. Another is that it arrived in a comet shower caused by something which disturbed the supply of comets in the inner Oort cloud, a large cloud of small bodies that revolves around the sun at a distance of about one light-year.

The distribution of comets in the cosmos is not understood. Some scientists believe they occur in clusters or rings. What is known is that their origin is beyond the outer edge of the solar system. Why they enter the inner solar system is uncertain, and there is no way to know if there was once a much larger number of them in the inner solar system.

If, however, a shower of comets struck Earth, it could have formed the anomalous iridium deposits as well as effecting drastic changes in the atmosphere -- and in the process creating a dust cloud dense enough to reduce or destroy vegetation and thereby cause the extinction of the dinosaurs.