Mass Extinction, a New Explanation

Apparent Periodicity of Mass Extinctions

Scientific evidence solidly indicates that Earth's ancient history has apparently involved a number of mass extinctions, most notably the one at 65 million years ago where the final dinosaurs died out along with many other creatures.   A number of rather speculative "theories" have been publicized as trying to explain them, and especially regarding an apparent 26 million year repetitiveness of mass extinctions.   The logic seems especially weak in the majority of such explanations.

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There now seems to be another possible explanation, which seems extremely logical and is even supported by solid facts.   It is a natural consequence of the fact that due to normal gravitational effects, while the Sun is revolving around our Milky Way Galaxy, the Sun must slowly weave back and forth across the width of the Spiral Arm of the Galaxy we are in.   We happen to currently be near the inner edge of the Arm we are in, so there are a lot of nearby stars all gravitationally pulling us toward them toward the centerline of the Arm.   After we should pass through that centerline region (with considerable velocity) we will continue outward toward the outer edge of the Arm.   Eventually, the total attraction of all the Arm stars (then behind us) will slow us down.   We will stop moving outward and again drop back toward the centerline of the Arm.   The very existence of all those nearby stars, and the fact that gravitation is extremely dependent on distance (called the inverse square law) requires this action to occur and to repeat forever.   All of the stars within our Arm must be doing the same weaving dance, back and forth across the Arm.   Many only move a smaller distance before returning to the centerline. But the Sun apparently has about as great a lateral movement as is possible, as we currently are very close to the very inner edge of this Arm.

Each time the Sun (and planets) passes through the cluttered neighborhood of the centerline of the Arm, at relatively high speeds, there are many foreign objects in its way.   Violent collisions seem certain.

Shape of a Spiral Arm

Consider this simplified representation of a top-view of the flattened, somewhat cone-shaped (tapering) Spiral Arm (straightened for simplicity here), with random stars within it.   There are actually many billions of stars that should be in this picture.   The Arm actually continues to the right, to join the Core of the Galaxy.

Shape of a Spiral Arm, showing the Sun

Now see that we have marked the Sun, which is known not to currently be on the central axis of the Arm.   This is the situation that applies to most stars. All the individual stars in the Arm are gravitationally attracting the Sun.   We know that the Sun is currently near the "inner edge" (bottom of our drawing) of the Arm, as shown.   Do you see that very large numbers of stars are attracting the Sun upward in the drawing, toward the centerline of the Arm?   When you add up the gravitational effects of all those billions of stars, there is a net gravitational attraction (within the Arm) pulling the Sun toward the centerline of the Arm (upward in our drawing).

Sun is weaving across the Spiral Arm

The effect of this collective attraction is to cause the Sun (and everything else in the Arm) to weave back and forth across the width of the Arm. From where we are right now, the Sun will "fall" toward the centerline for millions of years, building up speed all the way. By the time the Sun crosses the Arm's centerline, it will be traveling rather quickly. Violent collisions with objects in that congested area, of all sizes, seem likely to be common. Standard gravitational calculations suggest that this full cycle takes around 52 million years to complete, so that centerline passages occurs about every 26 million years, twice in each cycle. This excellently matches the paleontological evidence of periodicity of mass extinctions.

This premise is compatible with the currently popular theory where a large meteorite struck the earth. That theory believes that a house-sized object crashed into the Earth (possibly in the current Gulf of Mexico) and the resulting impact filled the earth's atmosphere with a variety of types of dust and dirt particles. That theory suggests that so many particles were put into the air that they blocked out sunlight, for possibly several years. Very small dust particles are known to stay suspended in the atmosphere for many months, as after major recent volcanic eruptions when dark dust clouds have then darkened skies over much of the earth for months after the eruption. With sunlight blocked, plants might not be able to grow, and so they might die, and so would animals that depended on eating them for food. Also, the temperature might have gotten either too warm or too cool for some plants and creatures to survive. Overall, a seemingly reasonable theory.

There seems to be additional solid evidence for this concept. For centuries, scientists have been mystified by the fact that the southern hemisphere of the Moon has an enormous number of craters while the northern hemisphere has far fewer and is covered by a lot of smooth areas that are believed to be much younger. The Surveyor spacecraft mapped Mars and found the same patterns, many craters in the southern hemisphere and more young smooth areas in the northern hemisphere. Scientists seem to either think this is a remarkable coincidence or simply ignore it! But this premise provides a wonderful explanation for these findings. If, as the Sun and Earth last passed through the Galaxy Arm centerline region 13 million years ago, the southern hemispheres of all solar system objects were leading (the Solar System was essentially going downward through the centerline region of the Arm), then many ferocious impacts would have occurred primarily in that southern hemisphere. This reasoning would then suggest that the heat of the impacts would have caused extensive melting and then lava flows which would have created the smooth areas that are fairly young. Actually, it predicts that the craters are all about 13 million years old!

Notice that is in total disagreement with the usual ASSUMPTION that the craters we see on the Moon are all 4.5 billion years old, from when the Moon was first formed!

In addition, this current premise offers an interesting new possibility for also explaining the "bursts of evolutionary activity" that are apparently recorded in the geological record.

The Milky Way Galaxy

The Sun and the Solar System, including the Earth, are in a Spiral Arm of the Milky Way Galaxy. A Spiral Arm is a very flattened, tapering cone-shaped collection of stars. Each of the billions of stars in a Spiral Arm move independently to each other, but they share a common motion around the Center of the Galaxy. In addition, all of those billions of objects have gravitational attraction to every other object, according to the well-known Newton's Laws.

The Sun and Earth happen to currently be near the "inner edge" of an Arm segment called the Orion Arm. There are virtually no nearby stars farther inward, but several billion of them that are nearer the center-line of the Arm are (currently) gravitationally pulling the Sun and earth "outward", toward that center-line of the Arm. Collectively, this gravitational attraction creates a relative net acceleration, ACROSS THE ARM, toward the center-line of the Arm.

Over a very long period of time, this crossways acceleration creates a crossways velocity, and the Sun will certainly move across the width of the Arm! Once it gets near the other side, all of those attracting stars in the Arm are behind it, which act to slow it down. It will eventually stop moving laterally outward and again "fall" back toward the center-line of the Arm, eventually getting back to where we are today, near the inner edge. This "weaving" action will continuously repeat for the Sun and every other star and other object within the Arm. The Sun therefore "weaves" continuously back and forth across the width of the Arm!

Given reasonable estimates of the dimensions of the Arm, and how many star masses are in it, and the distribution of those masses, it is possible to estimate this collective acceleration, by using Newton's standard equation for the gravitational attraction between ANY two bodies. The summed value of this relative acceleration of the Sun is currently approximately 2.7 * 10-11 meters/second2. As the Sun "falls" toward the centerline, it will get closer to more of the massive stars attracting it, and around 1/3 of the way to the centerline, it will have already doubled that acceleration. As it nears the centerline, the acceleration will become smaller in magnitude, as the Sun "passes" many of those massive stars and they start attracting it in the opposite direction. By the time the Sun has gotten to the centerline, it will have achieved its highest "Arm-lateral" velocity but have no additional acceleration. This is standard Physics! The momentum of the Sun will then carry it outward in the Arm, gradually slowing due to the rearward acceleration of stars that it has passed. Eventually, the Sun will be near the OUTER edge of this Arm, and it will start to fall back inward toward the centerline again. The cycle continues until the Sun again arrives near the inner edge, where we are right now.

This sequence therefore represents a cycle that will repeat, with the Sun (and all other component stars of the Arm) weaving back and forth across the width of the Arm, in an incredibly complex "dance". This "inter-weaving" is prominent because of the massiveness of the component parts of the Arm.

Why is this significant?

First of all, preliminary computer analysis of the many gravitational forces acting on the Sun, and its resultant acceleration variations, suggest that the Sun will complete one whole cycle across the Arm and back, in roughly 52 million years. Second, we are currently near the inner edge of the Arm, where there are relatively few stars and other objects, and there is relatively low relative velocities between various objects out here. But near the centerline, there are two complicating circumstances. Any logical distribution suggests that there are at least several times as many stars and other smaller objects in that area. But worse, realize that the Sun (and Earth) enters that area with a substantial lateral velocity, around 23.8 km/sec (around 51,000 mph), and that there are OTHER stars and objects (half a cycle out of phase with the Sun) that are moving in the opposite direction at around the same velocity! These are tremendous differential velocities! Head-on collisions, even with relatively small objects, could have ferocious impacts!


The Sun and Earth are currently near the inner edge of our Arm. That implies that around 1/4 of a cycle, or 13 million years ago, the Sun and Earth passed through the cluttered and dangerous area of the centerline of the Arm. Also, 39 million years ago, we would have passed through the same area (going the other direction, outward) and 65 million years ago, and 91 million years ago, etc.

Interestingly, the geologic record suggests that mass extinctions occurred on Earth at roughly those eras. A 26 million year pattern of repetition of mass extinctions was noted during the 1980s. A number of speculative theories were presented to try to explain the periodicity. Currently, the most popular one is that our Sun is actually a "double star" with the other star called "Nemesis", which allegedly is in a 26 million year long very elliptic orbit and only nears the Sun at such intervals, to disrupt a speculated "cloud" of comets (called the Oort Cloud), causing some of them to approach the inner solar system and do damage to the Earth and its creatures.

It is amazing that this idea has gathered the backing of many scientists! There is NO evidence whatever for the Sun being a double-star (other than the fact that many other stars are double), or for ANY observed gravitational effect or light from any object that could actually be such a star. In addition, there is actually no evidence that there even is an Oort Cloud of dormant comets. Further, the instigators of that concept must not have looked through the appropriate calculations!

In an orbit that would have a 26 million year period, the semi-major axis would be around 800 AU, and the premise offered is that it spends most of its time at the outer end of a highly elliptic orbit, which means it is usually about 1,600 AU from the Sun, about 50 times as far out as Neptune, essentially out alone in space. All of science knows that the Sun is traveling about 20 km/sec toward the direction of the Constellation of Hercules. Many of the relatively distant stars of Orion are therefore about 600 light years "behind" us. Simple calculation shows that the Sun was in that vicinity, with all those stars that are presently 600 light years away, only 10 million years ago. The point is, the Sun's motion (and that of other stars) causes us to pass a LOT of stars in any 26 million year period, many of which are massive enough to disrupt the orbit of a "Nemesis" star that is so far out. To believe that such a Nemesis could persist in a stable orbit for hundreds of millions of years is beyond comprehension! Even if it actually existed, its orbital period would certainly regularly be altered by near passes of other massive stars.

Finally, there is no compelling evidence that a comet interacting with or colliding with the Earth would necessarily do much damage. In 1910, the Earth passed through a tail of Halley's Comet, with many people fearful, but nothing happened. In my education as a Physicist, I was taught that a worthy theory must be based on actual evidence, and there doesn't seem to be any regarding the very popular claims for the Nemesis Death Star. (The recent "Chain of Pearls" sequence of collisions involving the planet Jupiter involved a much more massive object than a normal comet.)

The present essay suggests a premise that seems to be solidly based on facts. Newton's Laws are accepted as valid, and research has developed various estimates for the dimensions and populations of our Spiral Arm. Very traditional Physics, and standard computer simulations of the gravitational interactions result in the conclusions here. More than that, it is intuitively logical. Other researchers somehow seem to assume that the Sun is fixed in its position within the Arm, where it is obvious that it MUST oscillate across the width of the Arm. (The computer simulations show that it also oscillates in a "vertical" direction, at a different, more rapid rate, but that short and slow motion seems to have minimal consequence.)


Since the Sun and Earth must necessarily pass through the area near the centerline of the Arm every 26 million years or so, it seems reasonable that a moderate sized (say the size of a house) stray object going in the opposite direction, would cause ferocious damage to the Earth and its environment. The impact would be a head-on crash between two very high-speed objects, with the resultant relative velocity being far higher (probably more than triple) than that possible regarding any comet or other solar system object. And triple the velocity of impact means around NINE TIMES as much (kinetic) energy of impact would be released, plenty enough to do major long-term damage to our atmosphere.

This premise implies an "approximate" 26 million year periodicity, but allows quite a bit of leeway. The alleged Nemesis Death Star would require extremely precise timing of the extinctions, and the evidence is that the extinction dates are just not precise enough to be explainable by such a double star. In my premise, the Sun spends two or three million years in the environment described above, where many more non-Solar-System objects are present, and many of those are on head-on collision courses with the Solar System. That means that the possibility of catastrophic collisions is very high for two or three million years around the time of actual Arm centerline crossing. This allows for substantial variation in the overall repetition pattern of extinctions, but it presents a 26 million year general periodicity on possible causes of mass extinctions.

Punctuated Equilibrium Evolution

Beginning in 1972 (Gould, Eldredge), scientists have been considering a radical difference from the previously assumed continual, slow, methodical evolution of living creatures. This new approach suggests "bursts" of evolutionary development, with extended periods of "stasis" (minimal evolution) between them. The scientific community is attempting to study and understand this possibility.

There is substantial evidence that periods of bursts of evolutionary development have occurred at very similar times to the mass extinctions. There may be a simple and obvious explanation for this. If a large proportion of dominant life forms have just died off in a mass extinction, there can be a vacuum of sorts, for existing surviving life forms to adapt into roles of greater dominance. Nothing external would be needed for such a situation.

However, in connection with the situation described above, there might be some interesting new possibilities. At roughly the same times as the mass extinctions, the Sun and the planets are passing through a very congested area of stars and miscellaneous other materials near the center-line of the Arm. Astrophysics has discovered quite an assortment of simple organic molecules out in distant open space. What if, as the Earth was passing through that congested area in the Spiral Arm, our atmosphere picked up any of a wide assortment of such organic molecules?

This speculation has several possible results. The "new" organic molecules could be incorporated into existing creatures or plants. That could result in materially modified DNA and distinctly different living organisms, essentially almost instant evolution. Given normal statistics, probably half of the changes might be positive and the other half negative. This would result in some "new" species having competitive advantages and quickly multiplying, while others would be degraded and soon die off. That could explain both the mass extinctions and evolutionary bursts, even without violent collisions. Another possibility would be that the molecules picked up from space might actually already be (dormant) simple life forms, like viruses or bacteria. Once in the Earth atmosphere, the effects could again be rapid and extensive, from a great devastation of an Ebola virus to some selective advantage of a digestion-aiding bacteria. Finally, such molecules might just somehow upset the existing balance among Earthly organisms. Thousands of such possibilities could exist, such as a self-replicating organic material that stayed at the ocean's surface and was impermeable to light, killing all the plants and animals under it, causing the water to absorb more heat and therefore evaporate more into the atmosphere, etc.

The general premise suggested here just implies that the Sun and Earth would pass through such congested areas at roughly 26 million year intervals. These speculations regarding potential "contamination" by extra-Solar molecules, are just that, speculation. Maybe some other researcher can figure out some way of either confirming or denying it.

This fact-based, gravitational-law-based, concept of the Sun oscillating back and forth across the width of our Spiral Arm of the Milky Way Galaxy seems a lot more logical and solid than speculation about a hypothetical double-star for the Sun. It does not require ANY 'creative' physics or speculations, because it is completely just a simple consequence of Newton's Law of Gravitation.

Interesting enough, these results were not the initial goal of my research. An attempt at explaining the persistence of the existence of Spiral Arms in galaxies, based purely on Newton's Laws, was progressing with computer simulations that were meant for determining if the Spiral Arm had structural integrity (it does!) and whether its shape and edge delineation were stable (they are!). Standard applications of Newton's Law of Gravitation were used, with no exotic assumptions involved. During those researches, in tracking the location of the Sun in the computer simulations, it was noted that the Sun was oscillating back and forth across the width of the Arm, much like the tone arm on an old phonograph after a song had played.

The appearance of a 26 million year periodicity of passing through the Arm's centerline, and our known current position near the inner edge of the Arm we are in, seemed like amazing coincidences, in presenting seemingly very obvious explanations for the known mass extinctions. This essay is presented with that intent, as a possible explanation for some events that are difficult to otherwise scientifically explain. No assumption of any exotic form was involved in arriving at these results.

The calculations and equations are available, but they are just the standard application of Newton's Law of gravitational attraction, and any researcher could quickly duplicate my results. Some variation on the interval of periodicity is possible, depending on: (1) the density distribution assumed across the breadth of the Arm; (2) the total number of Solar masses that the Arm contains; and (3) the width of the Arm at our location (and not just of a local strand). There are NO other significant variables that can affect the results!

There could also be a very traditional explanation for repetitive mass extinctions. Please see Mass Extinction, an Old Explanation An Old Explanation for Apparent Periodicity of Mass Extinctions.

The Galaxy Spiral Arms Stability and Dynamics, from which these results initially sprang, is also presented on the Internet.

First Proposed, May 1998;
First Presented on the Internet (as part of a Spiral Arm essay), June 1998;
First Presented as a separate essay, August, 2001.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago