The Deadly Thing at 2.4 Kiloparsecs: Are we sharing the galaxy with something large, dangerous and periodic?

Never before online, this highly speculative piece was published in Analog Magazine way back in May of 1984. It won the Analog Award for Best Fact Article for the year. Some of the research may be dated...but the concepts still intrigue...

Note: This article mentions  the work of NYU's Michael Rampino who later on realized that Earth's major extinctions appeared to recur in cycles of roughly 26 to 35 million years.  He has an improved theory for that!  Indeed, despite involving Dark Matter, it is likely better and more plausible than the one I raised in this old Analog piece. Still, my hypothesis was unprecedented and certainly fits the observed facts... a deadly thing may indeed, still be out there, lapping us every... 200 million... or even thirty million or so! Certainly the concept deserves to be posted somewhere and available on the Inter-Tube.

Okay, then. To the way-back machine!

======  Dialing back to 1984... and here we go... =======

Mass extinctions are much in the news these days. Like a scandal long buried and only just being uncovered, the demise of the dinosaurs now seems to be only the uppermost layer of something far more regular – and deadly.

Several recent events have spurred this renewed interest in the ecological holocausts of the past. The most significant of these has been progress in the arcane art of reading the fossil records in ancient sedimentary rocks.

Paleontologists such as Dr. James Valentine of the University of California at Santa Barbara have been reconstructing the family tree of Earth’s living organisms, sorting which orders or phyla ended in extinction, and which branches evolved into new, competitive forms.

Some of the pieces of the puzzle seem, at last, to be falling into place. We now know, for instance, that the fall of the great reptiles – and the associated extinction of many marine forms – was not a unique event. Valentine and others report that there have been at least four, and as many as ten suspected mass dieoffs, in which large portions of the Earth’s biota – whole families, orders, and phyla – declined and then dropped completely from sight. In three of these cases, the evidence is statistically indisputable. These extinctions were indeed catastrophes which enveloped the entire Earthly ecosystem when they occurred.

• At the end of the Cretaceous Period, approximately 65 million years ago
•  At the conclusion of the Permian Period, about 185 million years before that
•  And at the terminus of the Ordovician Period, approximately 210 million years further back in time.

The Great Die-offs

The Earth was far different in appearance each time it happened. Where the Cretaceous featured great reptiles and pre-placental mammals, the Permian was a time of tremendous fern forests and advanced amphibian forms. The Ordovician, on the other hand, featured hardly any life on land at all. But in each case the die-off was sharp and easily distinguished in the geological record. Suddenly, a large fraction of all the species at the time were wiped out.

Now (1984) Andrew Knoll of Harvard and Gonzalo Vidal of Lund University in Sweden report a fourth great extinction, at a time, 650 million years ago, when the highest forms of life were colonies of algae. This is about 200 million years before the Devonian event.

(Take note of the intervals between these major occurrences: 185, 210, and 200 million years. We’ll come back to them shortly.)

The paleontologists aren’t the only ones working on the problem of the past extinctions. A second discovery has received a lot of attention lately, adding another piece to our puzzle.

Led by Louis Alvarez of the University of California at Berkeley, a number of scientists have pointed out that some of the mass deaths are associated with unusual layers of clay – and that the layer representing the catastrophic end of the Cretaceous Period features astonishingly high abundances of certain rare isotopes.

Their conjecture is that a great meteorite struck the Earth, kicking up huge dark clouds and cutting off the sunlight. This supposedly then led to the ecological disaster observed in the fossil layers. Dust contributed by the vaporized meteorite supplied the unusual isotopes Alvarez and his team found in the Cretaceous-Tertiary boundary layer.

But the clay layer by itself is weak evidence for the falling rock conjecture. There are other ways to account for it. The abnormal isotope profile within that narrow layer is what the impact proponents rely upon most heavily.

But there may be another way to explain it.

Theories for Cycles

Veron 2008

Let’s go back to Fact One, the episodic occurrence of ecological disasters in Earth’s history. Would it not be interesting if there were some periodicity to these mass extinctions? If there were some pattern, then we might be able once and for all to assign a culprit … and incidentally know what to watch out for.

Recently two University of Chicago researchers, David Raup and John Sepkowski, have claimed that the four major and six lesser extinctions observed in the sediments seem to be part of a larger pattern that repeats at a rough average interval of 26 million years. They draw the implication that there is some repetitive process which puts the ecosystem of the Earth under stress in a regular pattern.

But even if the pattern they see is real, what sort of process could operate over such vast time scales, repeating reliably at 20 to 30 million year intervals?

Raup and Sepowski are not sure. Along with England’s Martin Whyte, they guess that the culprit may have to do with the interval workings of the Earth itself – with cyclic changes in the planet’s moment of inertia, its magnetic field, or the rate of transfer of heat to the mantle and crust.

It is an intriguing proposal, and it merits further investigation. However, there is a problem. No one can assign a clear-cut mechanism. Nor can anyone explain the dramatic difference between the six lesser and the four great extinctions.

One other potential periodic mechanism, that I discussed in the May 1983 issue of Analog, is the possibility that waves of settlement by starfaring civilizations might be responsible for episodes of extinction, followed by long periods in which the galaxy is empty of intelligent life. The theoretical time scales – 10 to 100 million years – seem to put this idea in the right range to be considered as a candidate, however it still remains pretty vague and hard to pin down. All we can do is catalog the hypothesis and move on.

The Major Extinctions

For the sake of argument, let us look at the four great die-offs alone … the four for which there is no dispute. Remember – 65, 185, 210, and 200 million years? Recall that these are fairly rough numbers. Nonetheless, one quickly sees the outlines of a pattern. If we assume we’re 65 million years into the latest phase of a repeating cycle, we might be tempted to guess that the greater die-offs occur at intervals of approximately –

197 Myr ± 12 Myr. (Myr = one million years.)

The uncertainty of 12 Myr is soft, but it is small enough to leave us encouraged that we may be onto something. It certainly looks like a pattern.

Could something periodic be causing this?

Not many natural processes occur with such regularity at such vast intervals. Only one cycle comes to mind with a periodicity similar to this. It is the revolution of the sun around the center of the galaxy … an orbit that astronomers now estimate to take approximately 238 million years.

 Might we be sharing the Milky Way with something deadly? Something that reaches out to “touch” our planet as we pass near it, roughly every galactic year?

Let’s pause and think about galaxies for a moment.

A spiral galaxy like the Milky Way does not rotate like a solid disk. Instead it is composed of many parts.

The galactic “halo,” like the core, consists of older, metal-poor, possibly planet-less stars of the first generation. In the halo the long, lazy orbits of solitary stars and globular clusters take them far out into the nearly empty territory above and below the spiral plane.

At the opposite extreme, in the galactic core, the crowded stars jostle and occasionally collide. They may even merge into super-compact bodies, giving rise to strange happenings. We shall speak more of these later.

Still, most of the really interesting things seem to be going on in the great, complicated disk of the galactic plane. Here the stars and gas and dust clouds rotate in their nearly circular paths, the inner zones finishing their orbits more quickly than those further out. This “differential rotation” is one of the things that drives the spiral design of our type of galaxy, helping to create the shock fronts where new stars are formed.

The shock fronts, along the concave faces of the spiral arms, are where clouds of gas and dust are compressed into new stellar systems. Some believe that life could not exist without these alternating zones of compression and release around the galactic rim. The sun’s orbit appears to meet one of the galaxy’s great spiral arms about every 110 million years or so. It takes about 10 million years to pass through one, about a million years alone to pass the shock front at the leading edge. We’re emerging from an encounter with the shock front of the galaxy’s Orion Arm right now.

Can one use these spiral fronts to explain the cyclical pattern of the mass extinctions? There are several theories which do make the attempt.

Source: NASA

W.H. McCrea contends that when the solar system moves into a shock region a sudden influx of gas and dust is absorbed by the sun, causing a dramatic increase in luminosity. That, according to the English astronomers Hoyle and Littleton, should increase precipitation on Earth, lowering sea levels and setting off a series of ice ages. 

The history of the last million years – featuring a series of ice ages only recently ended – lends the hypothesis some support. A related idea, by Napier and Clube, is that the galactic shock fronts are crowded with “planetesimals” like asteroids and comets, and that the sun regularly picks up a swarm of these every hundred million years or so, causing the Earth to regularly get “pasted.”

Or maybe the abundance of young, hot stars in the shock-front regions creates an area with a high incidence of supernovae (which would certainly wreak havoc on the Earth if one occurred close enough!)

All three mechanisms sound plausible, at least. Could the solar system’s periodic encounters with the spiral arm shock fronts then explain the major extinctions that have befallen life on Earth?

(2015 aside: Back in 1984 we didn't know the galaxy is "pleated" and that our solar system would rise and then dip through these pleats, several times during every 240 million year galactic orbit.)

Alas, the timing is all wrong.

Our encounter with the Orion Arm may indeed have triggered the ice ages of the ice ages of the Pleistocene, but the cycle of entering and leaving spiral arms clearly doesn’t fit the truly great die-offs of the Pre-Cambrian, Ordovician, Permian, and Cretaceous. The hundred and ten million year interval is over forty percent below the figure we calculated earlier – apparently way too low to apply to the major ecological holocausts of the past.

The Deadly Thing

If we re-examine the numbers just one more time, there does appear to be one more possibility – one more periodicity that no one seems to have covered yet. Our galactic orbital period.

We seem to be hit by something deadly every 195 million years or so. That’s similar to the 230 Myr solar orbit around the galactic center, but it’s clearly not the same. The 15% difference is enough to bother even the most impulsive pop theorist.

Until one realizes that anything truly dangerous floating about in our galaxy would itself have to be in orbit around the galactic center! With differential rotation, every distance from the center has its own unique orbital period, the sun’s happening to be 230 to 250 million years. There may be some “thing” co-orbiting with us – a little further out or closer in – the inner object “catching up” with the outer one at a period a little more rapid than one solar-galactic “year.”

It’s a problem that can be solved – roughly --  using the back of an envelope and a book of astronomical tables.

If the sun has, say, a period of 230 Myr, and we encounter “a thing” about every 197 Myr, then “Its” orbital period is solved by taking the difference of the two reciprocals (orbital frequencies) and dividing one more time.

If we do this, “It” turns out to have an orbital period of about 107 Myr.

We then go to the some of the tables of galactic rotation rates (laboriously collected by diligent astronomers, and published for the benefit of sleuths such as ourselves). The angular frequency versus radial function given in the literature is a little complicated, but when used carefully it gives a pretty clear result.

“It” has to orbit the center of the galaxy at a distance of approximately 2.4 kilo-parsecs, or seven point seven thousand light years. Our system, orbiting at about 10 kilo-parsecs, then has its nearest passage to the thing every 197 million years, as expected.

(2015 aside: note that if the extinctions cycle around 30 million years, that only shifts the orbit of the deadly thing inward, closer to the center of the Galaxy.)

Geological Astronomy

This is “geological astronomy” with a vengeance. We have just used the Earth as a great observatory, reading the sedimentary rocks like ancient photographic plates. Have we deciphered the clues correctly? Is there a Thing out there, which periodically catches up to use and does deadly mischief on our ecosystem with each near passage?

(Like many scientific discoveries or conjectures, this one has a haunting premonition in science fiction. In Poul Anderson’s Brain Wave, the die-off of the dinosaurs was caused by a beam which suppressed brain activity in the entire sector of the galaxy.)

If our Thing exists, it has to be pretty powerful, for according to our calculations we never pass closer to it than seven thousand parsecs. This means that it must somehow be selective, or act over a narrow angle.

The strongest possibility among known or modeled phenomena seems to be a rotating black hole which is emitting a powerful jet of sub-atomic particles.

The central cores of some spiral galaxies are extremely busy places, emitting awesome, energetic beams. SF author Gregory Benford (who is also professor of high-energy physics at the University of California at Irvine) has studied cases in which narrow, self-focused streamers of charged particles seem to be shooting narrowly across tens of thousands of parsecs, carrying as much energy as is being emitted from all the rest of the galaxy!

Clearly nothing like these monsters exists in the Milky Way today. But recent radio surveys have discovered an intriguing object, albeit much, much smaller – perhaps a fair to moderate black hole – very close to our galactic center. Radio-maps indicate a pair of jets several light years in length, spurting outward from the object.

(2015 note: this object has been confirmed to be our galaxy's central black hole, containing more than a million solar masses, and yet quiescent, at present, having long ago sucked in those objects whose orbits might bring them within grasp.)

In terms of modern galactic astronomy, this is small potatoes. But there may be others in the Milky Way, somewhere in between the sizes we’ve mentioned above. And one of these may be our culprit, now hidden behind the dust lanes of the galactic lens.

Source: Popular Science

Benford thinks the best candidate might be a condensed source projecting a beam of positrons and electrons, precessing and sweeping out a disk-like portion of the galactic lens.

An energy source like that would, indeed, be a deadly thing. An interstellar jet, even one barely grazing by the solar system, could explain a lot, such as the anomalous isotopes in those clay layers – if the particle fluxes were high enough to cause elemental transmutation. And it might be no problem for such a beam to overwhelm the ozone layer, causing collapse of the Earthly ecosystem.

Even if the beam passed nearby for only a brief time, it would probably be enough to do great harm. 

(2015 note: Rampino now sees the pattern as being roughly 30 million years.  This would be consistent with an object even closer to galactic center than my earlier hypothesized beast at 2.4 kiloparsecs.  Still, the basic idea here is not disproved.  It belongs on our shelf of possibilities.)

There you have it, still another explanation for a set of mysteries exhumed from under the dust of our ancestors. All the witnesses are long dead, of course. But that doesn’t keep us from sifting through the clues, looking for culprits.

Over the years we’ve heard conjectures of nearby supernovae, wobbling planets, and even colonization from the stars, in order to explain the demise of the dinosaurs and other mass-extinction victims. Though noe of them have rhythmic periodicity.

If the giant-meteorite proponents are right, we might be wise to take some precautions, to keep track of those bits of rock tumbling about the solar system. The other “periodic” solutions, too, each seem to offer their own bogey men to watch out for as well.

Now there’s this new “thing” to worry about, possibly orbiting out there roughly 2.4 kilo-parsecs from the center of the galaxy … just waiting, it would seem, to reach out one more time and get us.

It’s a little unnerving.

Still, one shouldn’t lose too much sleep over it. Dangerous and nasty as the little bugger may be, we probably have another 130 million years to get ready for it. If any money is going to go to chicken little preparations, at this point I think I’d rather spend it on asteroids.

Author’s final (1984) note: Remember where we mentioned TEN recorded ecological holocausts? This paper only dealt with the four greater die-offs, whose apparent regular intervals lead to an interesting conjecture.

But there are six much smaller events in the record as well. Of these, two are “intermediate” in magnitude – one about 80 Myr after the Ordovician disaster, and the other approximately 30 Myr after the Permian.

You can’t do much with two data points, of course. Certainly there’s no way we can imply that each major even is followed by a secondary die-off an average of 55 Myr later, is there?

It is now 65 Myr since the major holocaust of the Cretaceous…

No. The author steadfastly refuses to state that we seem overdue for one of those littler extinctions. That would be stretching things too far.

He hopes.

== ... back to 2015! ==

And there you have it.  A clever -- if somewhat unlikely -- rumination from my younger self.  The article was discussed on the Weird Astronomy page of the Atomic Rockets website: "...just because the assumptions are questionable does not mean that they are wrong."  This "lapping" mechanism has some appeal, whether applied to the thirty or 190 million year cycles.  Still, if wagering, I'd give stronger odds to some version of Mike Rampino's orbital "dipping" process... with or without the recent Dark Matter gloss.

And yet, aren't these marvelous times, as we sift for evidence and plumb the past for mysteries?  Our ancestors, if told of this quest, would be puzzled!

But the best of them -- I think -- would also be proud of us.

You should be too!  Try to get your fellow citizens to realize it, as well.