by Richard Muller

from Muller'sGroup Website


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 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 (labeled Total) is for Sepkoski's entire data set of 17,500 genera, whereas 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 written in book form: "Nemesis," by Richard Muller (Weidenfeld & Nicolson, 1988). See Chapter 1 - Cosmic Terrorist.

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 extinctions.

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. For more details, see below insert:


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 contacted 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.



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.

Fortunately, several all-sky surveys are underway that should find Nemesis in the next few years, if it is there, and rule out Nemesis if they don't. (Nemesis could hide if it were a black hole, but that is not very plausible.) These surveys include Pan-Starrs and the LSST.