(10 of 10)
To meet all the other requirements imposed on it by the Louisiana scientists, Planet X would have an orbit that is elongated and highly inclined and a mass one to five times that of the earth. In other words, their Planet X is remarkably similar to the one that could account for the irregularities in the orbit of Uranus. The beauty of the theory, in Whitmire's view, is that it relies on a planet originally proposed for reasons that have nothing to do with mass extinctions. Still, he admits, the proof of the pudding "is going to come in the observing."
Most scientists are waiting for that pudding to be served before they commit themselves to the idea of periodicity, let alone a particular model. Says Cornell Astronomer Carl Sagan: "None of the explanations is anything like fully satisfying." Yet all but a few diehards acknowledge the brilliance of the Alvarez work. They believe the iridium layer and subsequent discoveries indicate that impacts of extraterrestrial objects may have played a significant role in certain extinctions, either directly or by delivering a final coup de grace to species already debilitated by climatic changes.
Whether these catastrophic impacts are random or cyclic remains to be seen. But if they occur at all, they could shake the foundations of evolutionary biology and call into question the current concept of natural selection. Should the Alvarez theory be correct, says Harvard Paleontologist Stephen Jay Gould, the importance of competition between species diminishes. If every so often a megablast opens up a broad array of ecological niches, then new creatures can flourish without having to crowd out the old. "If you ask the question, 'Why are we here?' " says Gould, "the answer is, 'Because the dinosaurs disappeared, not because the mammals outcompeted them.' "
Astronomers probing the skies for signs of extraterrestrial intelligence can take heart from the new catastrophe theories as well. For example, the double- star systems that make up the majority of stars in the galaxy were long thought to be too unstable to support planets that could settle into regular orbits and give rise to life. But if the sun is part of a celestial duet, says Raup, then "the whole evolutionary process may thrive on this kind of disturbance."
Or could it be that the permutations of life on earth are governed not by comets or death stars but by something more old-fashioned--like the fates? Although Shoemaker and Walter Alvarez do not consider themselves superstitious, they recently found reason to rethink their beliefs. During a visit to Berkeley, Shoemaker had roundly blasted the Nemesis idea, so Alvarez took him to a Chinese restaurant for a further discussion of Muller's model. After dinner, Alvarez cracked open his fortune cookie, pulled out the paper strip, glanced at it and, suppressing a laugh, handed it to Shoemaker. It read: "The star of riches is shining on you."