A few notes on the “Shiva” Hypothesis.
The Shiva Hypothesis, named after the Hindu goddess of destruction, was put forth by two astronomers from New York University named Michael Rampino and Bruce Haggerty, publishing a paper on the subject of periodic mass extinctions caused by a rise in the frequency of Comet strikes during certain epochs. The main thesis of their work was that every thirty or forty million years the Earth seems to go through mass extinctions, or rather suffers a significantly higher frequency of mass extinctions. They argued that the reason could have something to do with the Solar System aligning itself with the center of the galaxy at those times. When the plane of the Solar System aligns itself with the center of the galaxy there will ensue a greater gravitational pull on both the Kuiper belt and the Oort clouds where these comets are located. The increased attraction of the galactic core alignment will then to pull more comets in towards the Sun and a higher number of them will by chance strike the Earth causing the mass extinctions.
Whether the gravitational pull of the galactic core alignment with the Solar System plane and Kuiper belt alone is actually strong enough to have such an effect is questionable. However, we do know that objects in the Kuiper belt and especially in the Oort cloud are so far away from the Sun that they are in effect not all that stable since their tangential velocity is going to be relatively slower than those bodies closer to the Sun. Because of this lack of orbital velocity there is not all that much holding them in a stable orbit. Thus even a small tug might just be enough to send them plunging into a lower orbit, and potentially towards the Sun where they will gain speed as they get closer.
However, it’s also true that if the giant planets themselves should align with the galactic core alignment to the plane of the Solar System then there may actually be a significant pull to draw comets from the Kuiper belt especially-since it is relatively nearer than the Oort cloud- into the lower Solar orbit without question. For example each time Jupiter and Saturn should align with the Sun, and the galactic core, there may indeed be enough gravitational pull to cause many of these icy bodies to plunge headlong into the Sun; and on their way, possibly into us. This possibility also exists with Uranus which is much nearer to the Oort cloud and Kuiper belt than either Jupiter or Saturn(though also smaller) but might be greatest with Neptune which is the closest of all planets to the Kuiper belt.
Should these planets align with one another and with the galactic core there might be a significant increase in extra gravitational pull which would cause a major rise in the tendency of these bodies to veer off towards the Sun and Earth causing the increase in mass extinctions as a few of these will eventually hit the Earth.
The last near syzgy as this multi planet alignment is called, might have happened about twenty years ago when Neptune and Uranus aligned on the near side of the Sun relative to the galactic core. This would mean those Comets on the opposite side of the Sun(the far side relative to the galactic core) might have experienced a slightly stronger pull and might have been drawn in to lower orbits. It would take a comet quite awhile to reach us even if that did happen, and we might experience the results in the coming years or decades if that indeed happened.
The other possible alignment might have happened eleven years ago when both Jupiter and Saturn where in the same area, this time in a near perfect line up with the Sun and this time on the far side of the Sun relative to the galactic core and adjacent to the Kuiper belt. That kind of line up would probably have a notably increased effect unquestionably since the two planets, Jupiter and Saturn are quite large and have powerful gravitational fields, and aligning with both the Sun and galactic core are almost certainly going to produce some increase in gravitational pull on the comets. But the direct alignment of the galactic core may not have at that time taken place at full strength, and so even in that case there might not have been a maximum pull. Still this event may explain the recent incidences of asteroid near misses as this would also tend to draw some of the asteroids on the far side of the Sun into close Earth orbit. As for comets however, we would again have to wait quite awhile before seeing the effects. Yet the two events we are familiar with, the Shoemaker Levy strike on Jupiter, and the impending near miss or possibly even direct strike of comet Spring Siding on Mars may well have something to do with either, or both of the two alignment events mentioned. There does seem to be an increase in the frequency of close approaches of late.
There is no doubt that planetary alignments do cause gravitational perturbations. Of this we are certain, but the fact that these icy comets are so very far away leaves some room for question. Yet the flip side of it is that these objects indeed have very little gravitational pull(or curvature if you prefer) to actually hold them in a steady orbit. Thus even a slight change in gravitational intensity may be enough to alter their orbits towards the Sun. We must be aware that if the core of the galaxy does have a notable effect on these, when it aligns with the giant planets and Sun, it will certainly be amplified. So even if the Shiva Hypothesis is not quite convincing to some on its own, when we add the phenomenon of planetary alignment we get a very real possibility of an increase in the stochastic frequency of near Earth visits by comets, just as Rampino and Haggerty assert. It might also be true that asteroids will become more frequent in the inner Solar System as their orbits too will deviate if Jupiter happens to be far enough away, or on the other side of the increase in gravitational pull.
One thing is sure, these phenomena should not be dismissed. It is no big challenge to put a few numbers into Newton’s equations and see just how much of an effect these phenomena can have on the distant comets. Of course we would have to determine certain parameters to within some confidence, such as for example the mass of the galactic core, the actual size and dimensions, the distance from the Kuiper belt and Oort cloud, the tangential velocity of the comets, and the relative density of population of such objects in both the Kuiper belt and the Oort cloud. Whatever difference there would be from their normal orbit, and alignment would of course tend to accelerate them into our neck of the woods. But also we should keep in mind that any movement by a comet towards the sun, especially if it encounters one of the giant planets on the way will almost certainly increase the chances of it crossing Earth orbit and at very high velocity. A very simple computer simulation would easily demonstrate the possibility of increased comet activity as soon as the parameters above are filled in with confidence in their accuracy. In the end there is little doubt that as the giant planets line up with the Sun, there will always be a better chance for these distant visitors to make an appearance even if that may be hundreds of years in the future at times.
For this reason we should really keep an eye out for these things. It may well be just a matter of chance, but maybe our luck is running a little thin after four and a half billion years. In that time we have not probably encountered a very large comet or asteroid at or above 30 kilometers in diameter for if we did, the odds are good we, and all other life would not be here today. The asteroid that killed off the Dinos was estimated to be no larger than approximately 15 kilometers at best. As for the worst that can happen, one only has to look at Mars to see what may have been. Of all the possible scenarios that did Mars in as a living planet, if ever it was, none stands as more probable than a large comet or asteroid collision. The large black gash on Mars’ mountainous side does not look much like something that geological forces could manage. If anything that certainly looks like a huge collision with something very large and massive. As it happens, Mars is due for another near miss of a comet in October 2014.
So all in all we have something to really think about, and perhaps not take our civilization, or our beautiful, truly blessed planet for granted. There are in reality no guarantees for anything. On any given day the whole of the world as we know it can come to a cosmic end with not even a footnote left of us in the vast desolation of space.
Yet, there may be some hope for us, if we can act together in time.