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Nemesis

Origin of the theory

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 Gaussians 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 apparant 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, wheras 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 [email protected] if you need a copy.

Stability of the Nemesis orbit

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 exinctions.

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.

The Search for Nemesis

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.

NEMESIS

Chapter 1: Cosmic Terrorist

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 Asaro 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 Alvarez's case, and see if it was flawed. This job was turning out to be more fun than I had expected.

On my second reading of Alvarez's letter, I found it particularly dubious that the Cretaceous and Eocene extinctions should be excluded. How do we know that asteroids do not hit the Earth periodically? I asked. Maybe our failure to arrive at a theory just meant that we hadn't been clever enough. Not finding something is not the same as proving it is not there. I decided to reserve judgment.

A few minutes later Alvarez stopped by to see if I had finished, and I told him that I had found a mistake in his logic. It had been improper to exclude the Cretaceous and Eocene mass extinctions, I said. I presented my case like a lawyer, interested in proving my client innocent, even though I wasn't totally convinced myself.

Alvarez rejoined strongly, like a lawyer himself. "To keep those extinctions in the analysis would be cheating," he said. His belligerent offense threw me momentarily off balance. "You're taking a no-think approach," he continued. "A scientist is not allowed to ignore something he knows to be true, and we know those events were due to asteroid impacts."

I knew Alvarez far too well to acquiesce in his onslaught. My approach was not no-think, I said. It was proper to ignore certain "prior knowledge" in testing a hypothesis. He had no right to assume that the Cretaceous and Eocene extinctions could not be a part of a larger periodic pattern. Maybe if we were clever enough to find the right explanation, we would see that asteroid impacts can be periodic.

Alvarez repeated his previous argument, with a little more emphasis on the phrase "no-think." His body language seemed to say, "Why doesn't Rich understand me? How can he be so dumb?" I repeated my old arguments. We were talking right past each other. He knew he was right. I knew I was right. We weren't getting anywhere. This was not a question of politics or religion or opinion. It was a question of data analysis, something all physicists should be able to agree on. Certainly Alvarez and I should be able to agree, after nearly two decades of working together.

I tried again. "Suppose someday we found a way to make an asteroid hit the Earth every 26 million years. Then wouldn't you have to admit that you were wrong, and that all the data should have been used?"

"What is your model?" he demanded. I thought he was evading my question.

"It doesn't matter! It's the possibility of such a model that makes your logic wrong, not the existence of any particular model."

There was a slight quiver in Alvarez's voice. He, too, seemed to be getting angry. "Look, Rich," he retorted, "I've been in the data-analysis business a long time, and most people consider me quite an expert. You just can't take a no-think approach and ignore something you know."

He was claiming authority! Scientists aren't allowed to do that. Hold your temper, Rich, I said to myself. Don't show him you're getting annoyed.

"The burden of proof is on you," I continued, in an artificially calm voice. "I don't have to come up with a model. Unless you can demonstrate that no such models are possible, your logic is wrong."

"How could asteroids hit the Earth periodically? What is your model?" he demanded again. My frustration brought me close to the breaking point. Why couldn't Alvarez understand what I was saying? He was my scientific hero. How could he be so stupid?

Damn it! I thought. If I have to, I'll win this argument on his terms. I'll invent a model. Now my adrenaline was flowing. After another moment's thought, I said: "Suppose there is a companion star that orbits the sun. Every 26 million years it comes close to the Earth and does something, I'm not sure what, but it makes asteroids hit the Earth. Maybe it brings the asteroids with it."

I was surprised by Alvarez's thoughtful silence. He seemed to be taking the idea seriously and mentally checking to see if there was anything wrong with it. His anger had disappeared.

Finally he said, "You surprised me, Rich. I was sure you would come up with a model that brought in dust or rocks from outside the solar system, and then I was going to hit you with a fact I bet you didn't know, that the iridium layer associated with the disappearance of the dinosaurs came from within our own solar system. The rhenium- 187/rhenium-185 ratio in the boundary clay is the same as in the Earth's crust. I figured that you didn't know this. But your companion star was presumably born along with the sun, and so it would have the same isotope ratios as the sun. The argument I was holding in reserve is no good. Nice going."

Alvarez paused. He had been trying to think a step ahead of me, anticipating my moves, like a chess master. He had guessed what my criticism would be and had his answer ready-but I had made a different move. He seemed pleased that his former student could surprise him. He finally said, "I think that your orbit would be too big. The companion would be pulled away by the gravity of other nearby stars."

I hadn't expected the argument to cool down so suddenly. We were back to discussing physics, not authority or logic. I hadn't meant my model to be taken that seriously, although I had felt that my point would be made if the model could withstand assault for at least a few minutes. He was right that I was ignorant of the rhenium discovery. Alvarez's son Walter, a geologist, had found a clay layer that had been deposited in the oceans precisely at the time the dinosaurs were destroyed. This clay layer, the elder Alvarez hypothesized, had been created by the impact of an extraterrestrial body (such as a comet or an asteroid) on the Earth. Rhenium comes in several forms-among others, rhenium-185, which is stable, and rhenium-187, which is radioactive and disappears with a half-life of 40 billion years. In the 4.5 billion years since the formation of the solar system, approximately 8% of the rhenium- 187 should have disintegrated. And, in fact, roughly that amount had. Unless the rhenium in the clay had been produced at the same time as the rhenium in the Earth (i.e., at the formation of the solar system), the ratios were very unlikely to be so nearly identical. In other words, the extraterrestrial body would appear to have been born at the same time as the sun.

Now I took the initiative. "Let's see if you are right that the star would be pulled away from the sun. We can calculate how big the orbit would be." I wrote Kepler's laws of gravitational motion on the blackboard. The major diameter of an elliptical orbit is the period of the orbit, in this case 26 million years, raised to the 2/3 power, and multiplied by 2. My Hewlett-Packard 11C pocket calculator quickly yielded the answer: 176,000 astronomical units, i.e., 176,000 times as far as the distance from the Earth to the sun, about 2.8 light-years. (A light-year is the distance that light travels in one year.) That put the companion star close enough to the sun so it would not get pulled away by other stars. Alvarez nodded. The theory had survived five minutes, so far.

"It looks good to me. I won't mail my letter." Alvarez's turnaround was as abrupt as his argument had been fierce. He had switched sides so quickly that I couldn't tell whether I had won the argument or not. It was my turn to say something nice to him, but he spoke first. "Let's call Raup and Sepkoski and tell them that you found a model that explains their data."

So was born the Nemesis hypothesis, though I had no idea at the time where this would lead me.

Nemesis for Nemesis?

The issue of the theoretical stability of the Nemesis orbit has been settled, but most astronomers don't know the answer. Actually, they think they know the answer, but they are incorrect. As the 19th century humorist Josh Billings said, "The trouble with most folks isn't so much their ignorance. It's know'n so many things that ain't so." I can guide you to the origin of the confusion.

Look at Nature Vol 311, Oct 18, 1984. You will find a host of articles on the stability of the Nemesis orbit. In addition, you will find an editorial comment by Mark Bailey (on page 602), entitled "Nemesis for Nemesis."

The articles are as follows:

1. J. G. Hills (page 636) analyzes the stability of the Nemesis orbit. He supports the Nemesis hypothesis and calculates some details. He speculates that Nemesis may be responsible for the eccentric orbit of Pluto. (Hills was the theorist who originally recognized the possibility of comet showers.)

2. Piet Hut (page 638) does the most complete and definitive analysis of the Nemesis orbit. He concludes that the results given in the original Nemesis paper are verified: the orbit has a stability time constant of about one billion (10^9) years. This means that the remaining life of the orbit is a billion years. When the solar system was created 4.5 billion years ago, the Nemesis lifetime would have been about 5.5 billion years, and we have used up 4.5 of those. The 10^9 year stability implies that the present orbit is not perfectly periodic, and this is verified by a careful examination of the extinction data. Hut shows that the Nemesis orbit is stable only if it is near the plane of the Milky Way. (Hut is now a fellow at the Institute for Advanced Study at Princeton.)

3. Torbett and Smoluchowski (page 641) conclude that passing giant molecular clouds would make the Nemesis orbit unstable. However they neglect the fact that these massive clouds are very diffuse; later work (D. Morris and R. Muller, Icarus v. 65, p. 1-12) show that these clouds actually have no effect on the orbit stability.

4. Mark Bailey wrote an editorial review (page 602) entitled "Nemesis for Nemesis," in which he says, "the Nemesis proposal is extended and shown, in fact, to be quite incapable of producing the strictly periodic sequence for which is was originally designed." This is a misinterpretation of the original Nemesis paper (Nature vol 308 pp 715-717, 1984). We never expected a perfectly periodic signal in an orbit that had only a 10^9 year lifetime. Bailey goes on to characterize Hut's paper as "a near retraction"!!!! Hut considered his paper to be a vindication of the original Nemesis paper. He ed Bailey to find out how Bailey could be so wrong in his understanding, and Bailey told Hut that he never wrote those words! "Near retraction" had been inserted by the editor at Nature!

Bailey also refers to a paper by Clube and Napier, in which they show that the Nemesis orbit has a stability of 10^9 years. But Clube and Napier then conclude that this rules out the Nemesis theory, rather than realizing that this stability is exactly what we had said in our original paper. Apparently they never realized (as did Hut) that the expected lifetime of Nemesis is linear, not exponential, so that that the present stability is not the same as the stability 4.5 billion years ago.

But now for the fascinating sociology of science. I have talked to many astronomers since 1984, and the majority of them believe that the Nemesis theory was ruled out, because the orbit turned out to be unstable. In most of these cases I could track down the origin of their opinion. Frequently the opinion had been obtained from someone else -- often the local planetary scientist. But in every case, the ultimate origin was the altered article by Mark Bailey in Nature.

Why is this? Because Bailey summarized the three articles -- there was no need for a busy scientist to read the actual papers. I never found an expert (i.e. someone that others depended on for their opinion) that had actually read the Hut article. Why bother, when it amounts to a "virtual retraction"?

The trouble with most folks, isn't so much their ignorance ....

At the time, Piet and I thought we would find Nemesis soon, so he decided not to write a letter to the editor complaining about the error in the Bailey summary.

That's half the story of why Nemesis is not believed. The other half is that we predicted we would find it within a few years, and we haven't. So most people think our search found no such star. In fact, the search stalled soon after it started. There is no reason to believe that Nemesis is not the solution to the mystery of the periodic extinctions, and there is no alternative theory that has survived scrutiny.

Word Count: 4287

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