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The "Nemesis Theory" was an outgrowth of the discovery of Alvarez et al. , that the impact of a large (10 km diameter) comet or asteroid was responsible for the great mass extinction that took place 65 million years ago. Studies of the fossil record by Dave Raup and Jack Sepkoski shows that this was not an isolated event, but one of several mass extinctions that appear to occur on a regular 26 million year cycle. Their original paper analyzed marine fossil families, and was published in the Proceedings of the National Academy of Science USA, vol 81, pages 801 - 805 (1984). The original extinction data of Raup and Sepkoski are replotted in the following figure. The vertical axis shows the "extinction rate. " This was taken from the values given by Raup and Sepkoski for the percent family extinctions at each geologic boundary. In order to take into account the uncertainty in the boundary ages, each data point was plotted as a Gaussian, with width equal to the uncertainty, and area equal to the extinction rate.
This plot thus represents a statistical estimate of the extinction rate vs. time. The individual Gaussian's for each stage boundary are shown as dotted lines. The extinction 65 million years ago is indicated with the little dinosaur icon. The peak near 11 Ma is real, but exaggerated by the requirement that the plot go to zero at the present. Arrows are plotted every 26 million years.
Note that many of these are close to the peaks in the extinction rate. This is the apparent 26 million year periodicity discovered by Raup and Sepkoski. There have been many statistical studies of these data. Although several studies indicate the periodicity is significant, not everyone agrees. I suggest that you decide for yourself. If you decide that the extinctions are not statistically significant, then there is no need for the Nemesis theory.
Additional work by Sepkoski shows that the periodicity is also present for fossil genera. His results were published in the Journal of the Geological Society of London, vol 146, pp 7 - 19 (1989). Figure 2 from this paper is shown below. Please note that the time axis has been reversed compared to that of the previous figure. Plotted is the per-genus extinction rate (in units of extinctions / genus /Myr) for 49 sampling intervals. The upper time series (labelled Total) is for Sepkoski's entire data set of 17, 500 genera, where the lower "filtered" time series is for a subset of 11, 000 from which genera confined to single stratigraphic intervals have been excluded.
The vertical lines are plotted at 26 Myr intervals. The Nemesis theory was devised to account for this regularity in the timing of the mass extinctions reported by Raup and Sepkoski. According to this model, a companion star orbiting the Sun perturbs the Oort comet cloud every 26 Myr causing comet showers in the inner solar system. One or more of these comets strike the Earth causing a mass extinction. The Nemesis theory was originally published in Nature by Davis, Hut, and Muller (vol 308, pp 715 - 717, 1984). A longer description of the work leading up to the theory was writen in book form: "Nemesis, " by Richard Muller (Weidenfeld & Nicolson, 1988).
You can read Chapter 1 Cosmic Terrorist. here. This book is out of print, but I have some extra copies. Contact me if you need a copy.
There is a great deal of confusion among astronomers about the stability of the Nemesis orbit. Even many theorists who should know better believe that the orbit is unstable, and that the original Nemesis paper was in error. However detailed calculations by Piet Hut at the Institute for Advanced Study in Princeton show that the original estimate about the orbit were correct. Hut's results were published in Nature, vol 311, pp. 636 - 640 (1984). In our original paper we had stated that the orbit presently has a stability time constant of approximately one billion years. Many people naively assumed that this was incompatible with the 4. 5 billion-year age of the solar system.
But unlike the lifetime of a radioactive element, the lifetime of the Nemesis orbit is not predicted to be constant with time. In fact, Hut has shown that the lifetime decreases linearly, not exponentially, with age. The expected orbit lifetime when the solar system was formed was (presumably) about 5. 5 billion years. When nearby stars pass the solar system, the orbit of Nemesis is given slight boosts in energy. The Nemesis orbit becomes larger and less stable. At present, the Nemesis orbit has a semi-major axis of about 1. 5 light-years, and the orbit is expected to remain bound to the sun for only another billion years.
Note that the Nemesis theory predicts that the periodicity should not be precise. Perturbations from passing stars are not sufficient to disrupt the orbit, but they are sufficient to cause a slight (a few Myr) jitter in the interval between extinction. Why do so many people think the orbit is unstable? The basic answer is that scientists often don't have time to read the literature, so they depend on the summaries of others. Click here for more details.
Nemesis is most likely a red dwarf star, magnitude between 7 and 12. Virtually all such stars have been catalogued, but very few of them have had their distance measured. It is likely that Nemesis, if it exists, will be visible with binoculars or a small telescope. We don't need a large telescope to find Nemesis. We need a small or medium telescope, and enough time to look at and analyze 3000 candidate stars. A series of images taken throughout the year should allow us to measure the large parallax of this star.
We are also eliminating the stars measured by the Hipparcos satellite. We began the search for Nemesis using the automated telescope at Leuschner Observatory. However this telescope was not designed for the heavy use it was receiving from this search and from our automated search for nearby supernova. So we are in the process of constructing a new telescope. It is now undergoing final testing, and it soon will be moved to a mountaintop near Monterey. Its primary use will be for the Hands on Universe and Automated Supernova Search projects, but we only need it part time to find Nemesis.
LUIS ALVAREZ walked into my office looking like he was ready for a fight. "Rich, I just got a crazy paper from Raup and Sepkoski. They say that great catastrophes occur on the Earth every 26 million years, like clockwork. It's ridiculous. " I recognized the names of the two respected paleontologists. Their claim did sound absurd. It was either that, or revolutionary, and one recent revolution had been enough. Four years earlier, in 1979, Alvarez had discovered what had killed the dinosaurs.
Working with his son Walter, a geologist, and Frank Award and Helen Michel, two nuclear chemists, he had shown that the extinction had been triggered 65 million years ago by an asteroid crashing into the Earth. Many paleontologists had initially paid no attention to this work, and one had publicly dismissed Alvarez as a "nut, " regardless of his Nobel Prize in physics. Now, it seemed, the nuts were sending their theories to Alvarez. "I've written them a letter pointing out their mistakes, " Alvarez continued. "Would you look it over before I mail it?" It sounded like a modest request, but I knew better. Alvarez expected a lot.
He wanted me to study the "crazy paper, " understand it in detail, and then do the same with his letter. He wanted each of his calculations redone from scratch. It would be a time-consuming and tedious task, but I couldn't turn him down. He and I depended on each other for this kind of work. We knew we could trust each other to do a thorough job.
Moreover, we had enough mutual respect so that we didn't mind looking foolish to each other, although neither of us liked looking foolish to the outside world. So I reluctantly accepted the task, as I had many times before. The Alvarez theory had slowly been gaining acceptance in the scientific world. The astronomers had been the most receptive, perhaps because their photographs often showed large asteroids and comets floating around in space in orbits that crossed the path of the Earth. They knew that disastrous impacts must have taken place frequently in the past.
Many geologists had likewise been won over. But a majority of paleontologists still seemed opposed to the theory, which was disruptive to their standard models of evolution. Alvarez took pride in the fact that some of the most respected paleontologists nevertheless liked his theory, including Stephen Jay Gould, Dale Russell, David Raup, and J. John Sepkoski. I began my assignment by reading the paper by Raup and Sepkoski.
They had collected a vast amount of data on family extinctions in the oceans, far more than had previously been assembled. That fact disturbed me; I hate to dismiss the conclusions of experts, especially conclusions based on such minute study. Their analysis showed that there were intense periods of extinctions every 26 million years. It wasn't surprising that there should be extinctions this often, but it was surprising that they should be so regularly spaced. Alvarez's work had shown that at least two of these extinctions were caused by asteroid impacts, the one that killed the dinosaurs at the end of the Cretaceous period, 65 million years ago, and the one that killed many land mammals at the end of the Eocene, 3539 million years ago. (The age was uncertain because of the difficulty of dating old rocks. ) Astrophysics was a field I thought I knew; my work in it had earned me a professorship in physics at Berkeley and three prestigious national awards.
But the paper beggared my understanding. I found it incredible that an asteroid would hit precisely every 26 million years. In the vastness of space, even the Earth is a very small target. An asteroid passing close to the sun has only slightly better than one chance in a billion of hitting our planet. The impacts that do occur should be randomly spaced, not evenly strung out in time. What could make them hit on a regular schedule?
Perhaps some cosmic terrorist was taking aim with an asteroid gun. Ludicrous results require ludicrous theories. I hurried to the end of their paper, like a reader cheating on a mystery novel, to see how Raup and Sepkoski would explain the periodicity. I was disappointed to find that they had no theory, only facts.
Physicists have a wry saying: "If it happens, then it must be possible. " Many discoveries had been missed because scientists ignored data that didn't fit into their established mode of thinking, their paradigm, and I didn't want to fall into that trap. Maybe it would be best to review their data, I thought, and try to judge them independently of theory. On a chart, they had plotted the varying extinction rate for the last 250 million years. The big peaks in the rate were spaced 26 million years apart. Next I turned to Alvarez's letter. He thought there were several mistakes in the way that Raup and Sepkoski had analyzed their data.
Several of the apparent peaks, he argued, should be removed from the analysis because of their low statistical certainty. Likewise, both the Cretaceous and Eocene extinctions should not be considered as part of a periodic pattern, since they were due to asteroid impacts and therefore must be random in time. This had been as obvious to Alvarez as to me. With these extinctions removed, the remaining ones were so widely separated that it looked like all evidence for periodicity had vanished.
Alvarez's approach was convincing, but was it right? It was my job to be the devil's advocate, to defend the conclusions of Raup and Sepkoski. I went back to their paper and looked at the chart again. I mustn't be too skeptical, I thought. I replotted their data, substituting the conventions of physicists for those of paleontologists. I gave each extinction an uncertainty in age as well as in intensity.
The new chart looked more impressive than I had expected. It was a rough version of the one shown on page 6. I had placed the arrows at the regular 26 -million-year intervals. Eight of them pointed right at extinction peaks; only two missed. The peaks certainly seemed to be evenly spaced.
Maybe they were right. I realized I had better reexamine Al...
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