OUR NEMESIS?
It started as a discussion in 1983 between two experimental physicists, Richard Muller and Luis Alvarez, regarding a paper they had received. The paper came from David Raup and John Sepkoski, two respected paleontologists, and they were making the remarkable claim that great catastrophes occur on the Earth every 26 million years, like clockwork. It was only 4 years earlier in 1979 that Alvarez had proposed that the extinction of the dinosaurs had been triggered 65 million years ago by an asteroid crashing into the Earth. Many paleontologists had initially paid no regard to this theory, and one had publicly dismissed Alvarez as a 'nut', regardless of his Nobel Prize in physics. But David Raup and John Sepkoski had both liked Alvarez's asteroid theory and now were sending their own theory to Alvarez, or rather their findings, as they offered no explanation. Muller and Alvarez agreed to research their bizarre claim that great catastrophes occur on the Earth every 26 million years.
Raup and Sepkoski had collected a vast amount of data on family extinctions in the oceans, far more than had previously been assembled, and 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 already 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 one that killed many land mammals at the end of the Eocene, 35-39 million years ago. But these new findings beggared belief, what could be the cause of such regular events? Was it credible that an asteroid would hit the Earth every 26 million years? 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 hitting us a precise intervals of every 26 million years. What could make them hit on such a regular schedule? It was ludicrous, but physicists have a wry saying: "If it happens, then it must be possible."
Muller replotted the data using the conventions of physicists rather than paleontologists, giving each extinction an uncertainty in age as well as in intensity. He then placed arrows at regular 26 million year intervals. Eight of them pointed right at the extinction peaks, only two missed. The new chart was more impressive than Muller had expected.
The figures looked impressive, there WERE mass extinctions every 26 million years, two of them were known to be caused as a result of asteroid impacts, but could they ALL be? What could cause it? What model could they come up with to explain it?
Alvarez challenged Muller to come up with a model, and Muller duly obliged: "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." Alvarez agreed it was possible, and they carried out calculations to see if the orbit of a companion star was possible without being so big that it would be carried away by the gravity of other nearby stars. 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 two. Muller quickly showed this to be about 2.8 light years. That put the companion star close enough to the sun so it would not get pulled away by other stars. Alvarez agreed, the model was holding up. The hypothetical star was named "Nemesis". It was proposed that Nemesis, in passing through the Oort cloud, (a comet belt that orbits the outer reaches of the solar system) would perturb the orbit of some of the comets and send them towards the inner parts of the solar system and towards our planet. It is believed to be a dark star, a large mass, much larger than a planet, but not large enough to form a bright star, probably a red or brown dwarf,
Nemesis has not yet been discovered, if it exists it is currently as lost as a needle in a hay stack. among a million brighter stars. If we knew which one it was we could see it through binoculars. With a small telescope its distance from the sun could easily be measured , once we knew where it was.
What do we know for sure? We know that an extraterrestrial object, either a comet or an asteroid, hit the Earth 65 million years ago and brought to an end the great Cretaceous period of the dinosaurs. Other than that we believe that the Earth is subject to periodic storms of comets and asteroids. The important discovery of periodic mass extinctions by Dave Raup and John Sepkoski lies on firm and careful analysis of the data. The periodic extinctions, and the periodic cratering that goes along with them, appear firmly established. The Nemesis theory is consistent with everything we know about physics, astronomy, geology, and paleontology. But it is circumstantial and requires verification. We need to find Nemesis.
When Walter Alvarez became interested in "an inconspicuous layer of clay in the Apennines." Luis Alvarez had suggested that trace analysis of iridium could be used to measure sedimentation rate, but to everyone's surprise it demonstrated that an extraterrestrial impact had taken place. The clay layer was found worldwide, and analysis showed it was about 10% asteroid or comet material, the rest coming from the vaporized rock thrown up by the impact. Five mass extinctions are now known to have iridium signals. The studies of the effects on climate of dust thrown into the air by the impacts led to the discovery of the nuclear winter. The connection between mass extinctions and nuclear winters was made when Edward Anders found soot in the boundary clay layer suggesting that vast firestorms had been set by the impact. The attempts of Raup and Sepkoski to show that mass extinctions occur frequently led them to a surprising, and at first totally inexplicable, conclusion: that mass extinctions take place on a nearly regular 26 -30 million year schedule. Based on this discovery Muller, Marc Davis, and Piet Hut, proposed Nemesis, a companion star to the sun that triggers comet storms, a theory simultaneously proposed by David Whitmire and Albert Jackson. The theory immediately led Muller and Alvarez to the discovery that impacts on the Earth follow the same schedule as the mass extinctions, a correlation found independently by Michael Rampino and Richard Stothers. The concept of storms of comets proved to be more general, and testable, than the Nemesis theory itself, and led to the discovery that the mass extinctions were punctuated during the several million years of their duration. A belief in the comet storms led Donald Morris and Muller to a method of explaining some of the geomagnetic reversal. All this from "an inconspicuous layer of clay in the Apennines."
We have looked at the rocks beneath our feet and predicted a star. Now we need direct evidence, a smoking gun, a star.
What do I think?
I think its a beautiful theory. It fits all the known facts, dovetailing perfectly with our knowledge of regular mass extinctions, the discovered soot from the fire storms, nuclear winters, the iridium signals in the boundary clay layers, and impact cratering around the world, You could almost say its perfect, except for one thing. No proof, we still haven't found Nemesis. However, this one missing piece of evidence is not in itself justification for dismissing such an elegant theory. Nemesis could be discovered tomorrow, or in 20 years time. However, the fact that Nemesis has not yet been discovered, is for me, a little worrying, The Hipparcos satellite was launched in 1989 and was operational to 1993. Its mission was to seek out stars for a new and very detailed star catalogue of great accuracy. Nemesis was not found, at least, if it was it has not yet been noticed.
We do not yet have definite proof, and other theories are equally strong contenders.
I think Nemesis is a definite maybe. One thing is certain, we do get hit by massive asteroids or comets every 26-30 million years, and they do cause mass extinctions. Next time it could be us.
When is the next time? Don't worry, not for another 15 million years or so. But don't think you're safe, we get hit by the odd stray asteroid on a much more regular frequency, and any one of them could produce the same deadly result. See Can we survive the next major impact?
Further web sites on Nemesis.
Nemesis, the Sun's companion star
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