Bobbing for extinctions

By Phil Plait | May 6, 2008 10:20 am

The ways the Universe can deal out death are as numerous as they are terrifying. Asteroid impacts, nearby stars exploding, wandering black holes… I spent a year or so thinking of nearly every method of cosmic catastrophe I could while writing Death from the Skies!*.

I wrote a whole chapter on what dangers lurk in our own Milky Way galaxy, and I was surprised to find out the Sun’s orbit around the center of the galaxy is a potential problem. The galaxy is flat, like a CD (in fact, the proportion is right if you stack about 4 CDs together). The Sun does not orbit the center of the galaxy in a nice, flat plane, like planets do around the Sun. Instead, it bobs up and down like a cork in water, making about four cycles for every one time it orbits the galaxy (which takes about 200 or so million years).

In my research, I came across the idea that when the Sun is at the apex of its bobbing, towards galactic north, it’s about 100 light years above the galactic plane. That’s far enough up that the magnetic fields of the galaxy are weaker, and it’s these fields that protect the Sun (and the planets, meaning us) from intergalactic cosmic rays, subatomic particles that zip around space between galaxies. When the Sun is up high, these cosmic rays can strike us, and we have to endure this particulate rain for millions of years. The radiation can do bad things, like damage the ozone layer or induce genetic mutations.

When researchers plotted the times of the Sun’s most northerly excursions (which happen every 64 or so million years), they lined up in time with many mass extinctions on Earth. Uh oh.

The good news is that this only happens at one part of the Sun’s orbit, so while we’re deep in the plane of the galaxy we’re protected and safe.

Or, actually, things get worse.

A new result has just been announced that says that when the Sun is in the thick of the Milky Way’s plane, tides from the galaxy can induce comets from the outer solar system to plunge down toward the Sun, meaning many will hit the Earth and potentially cause mass extinctions.

Well, nuts.

According to the new study, this happens every 35 – 40 million years, which is not too far off from the calculations in the older study. Since the Sun moves up and down in the plane, it actually plunges through the mid-plane twice each cycle. If it reaches its apex every 64 million years, then it should pass through the mid-plane every 32 million years, which is reasonably close to what the second study says.

So as if it’s not bad enough that we get irradiated at the top of the orbit, we get pummeled by comets twice as often!

Bummer.

So sure, having trillion ton chunks of rock and ice rain down every few 30 million years is bad and all, but that’s not the worst part! Horrifyingly, this news came too late for me to include in the book!

We have to keep our perspective on these things, after all.



*Coming to a bookstore near you on October 20!

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!, Science

Comments (46)

  1. Jeffry White

    Fascinating. And to think that there are still people who think that the universe is hospitable to life.

  2. Pop

    What to do? What to do? The sky is falling, the sky is falling! It seems strange that human-kind is trying to extend life time averages while we find more and more ways for it all to end. Personally, I’d like a nice long lifetime, but not for it to end as we dipsy-doodle through the galactic plane and take a comet or asteroid strike. Rather, I’d prefer being shot by a jealous wife, or irate husband. Unfortunately, the chance of that happeninng is on the order of me living several millions of years. Keep smiling as we cruse along hoping to dodge the “big one.”

  3. Mapnut

    So where are we in the cycle right now? I have to plan ahead.

  4. Paul Parnell

    I’m finding it hard to believe that the radiation level is much higher outside the galaxy. The galactic magnetic field is very very weak isn’t it? Maybe there would be some kind of analog to the Van Allen belts.

    And how does radiation damage the ozone layer? Its actually created by ionizing ultraviolet radiation isn’t it?

  5. funkopolis

    Just save it for volume II, “More Death From The Skies”. Or “Death From More Skies”. Or “More Death, Same Sky”. Or “Why, Sky, Why? What Have I Ever Done To YoAaRAaaaaaiiiiiiggggggh!”

  6. Ginger Yellow

    So, to paraphrase: in the long run, we’re all really dead.

  7. So, why does the Sun “bob up and down” along the way? I would assume that what’s really happening is the Sun is in orbit within some local cluster of stars, and this cluster is what is orbiting the galactic center. Would that be a valid assumption?

    And what sort of view does the Earth have when we’re 100 light years “above” the galactic plane? (How thick is the plane? Are we actually “above” it, or simply further away from the middle?)

  8. Spiv

    “where are we in this?”

    We pass through about every 35-40 million years, last big extinction maybe linked to it is 65m/years ago. So we’re coming up on this in only 5-15 million years! Oh Nose!

  9. From what I’ve read we’re on our way up, but still 20 million years from the apex.

    The Sun bobs due to some sort of perturbation that happened long ago. The galactic plane is massive, so when the Sun dives through it, there is a force that pulls it back toward the plane. Since the plane is mostly empty space, the Sun passes right through, and begins the cycle anew. I have a lengthier and more detailed explanation in the book…

  10. Michelle

    It’s OK. I’m fairly sure I’ll be dead by then.

  11. Kate

    You know, as a woman, I’m used to having to calculate cycles of potential doom for mankind…

    ….but seriously…

    Phil, what is the “force” you refer to when you say that the Sun is being pulled back to the central plane? Gravity? Something else? (I realize gravity is the most obvious answer, but the use of that mysterious, undefined word “force” makes me hope there’s a really cool science lesson in there. :)

    …and how do we know this perturbation is old, and not a new phenomenon? (As in , how do we know that the interaction causing this “up and down” movement wasn’t in the last … say 20 million years and the solar system is actually in the process of being ejected form the galaxy?) Is it simply the correlation between the geologic/fossil records of extinctions that is the main evidence, or are there other clues?

  12. FordPrefect

    Does in start with: “DON’T PANIC” written in big capital letters (very important ), like the hitch-hikers guide to galaxy?

  13. FordPrefect

    I am referring to your book obviously :P

  14. Matthew

    Correct me if I’m wrong, Phil, but wouldn’t the Sun encounter a weaker Galactic magnetic field when it reaches its southern extreme, as well? That would mean that we’d suffer increased intergalactic cosmic rays every 32 Myrs, rather than every 64 Myrs.

  15. Zeugnitz

    Pop et al. – the timescales of events like these are massive enough that there’s no point in trying to apply them either to your own personal life or to current human endeavors.

    The only things where it might be worth considering things on an astronomical scale are defenses against potentially manageable threats (such as asteroids) – which are still far from acute – and future colonization efforts beyond our planet…

    …of course the lifespan of our sun is another issue that needs to be taken into account, but in my humble estimate, by the time it’s a current issue, either the human civilization isn’t going to be around to worry about that or we’ll be well into the colonization phase and probably already have a presence outside our solar system.

    Sure, it’s entirely possible that all life on the planet will be wiped out in an instant at any time due to some massive cosmic event, but that has very little meaning in terms of what we should pursue either as individuals or as a community.

    If you start thinking of your own life in terms of the potential for massive events that are entirely beyond your control, you’ll descend either into insanity or apathy.

    I wouldn’t encourage such thinking. It’s simply counterproductive, and I would hope that most people would want to make the best of their own lives and the future of humanity, even if there is the possibility of total doom – or even an eventual certain doom (such as the heat death of the universe).

    As an individual, you know you aren’t going to be around forever, anyway. The same applies to humanity. As a reality – even if our universe is doomed, there may be other universes, and other realities, and other civilizations.

  16. Sman

    After a quick glance at the Geologic Time Chart, it appears the Periods and Epochs have a random distribution. ???

  17. Jon

    So if the Sun passes through the galactic plane, does it have a corresponding negative apex, where it peaks at ~100 light years below the galactic plane? It’s probably too simplistic of a model, but if the Sun’s motion is roughly like a sinusoid centered at the galactice plane, then the periodicity of the apex events and the pass through the galactic plane should be the same, no?

    Also, do we know if the Sun’s oscillations are decreasing over time? I hope we don’t settle out into the bumpy center of the galactic plane!

  18. molc

    does this bobbing have anything to do with the galactic alignment the mayans “predicted”

  19. Ed

    I was going to comment (incitefully so, I’m sure) that the most useless waste of time and money I have ever seen was the series on TV recently about what the earth would be like once all the humans were gone. Then I suddenly realized……….The Aliens funded It!
    So my philosophy now is: Eat, Drink, and be Merry, for in 20-30 million years or less we all die.

  20. Dave Hall

    That bobbing explains the unsteadiness I feel in the AM. And I was ready to blame it in the Gin and Tonics.
    Maybe you can include the trillion ton rain of rocks in the revised edition–unless something big hits earth before then.

  21. Tim G

    The direction of the net gravitational force on each of the solar system’s planets and asteroids point more or less toward the sun, which contains 99.86% of the solar system’s mass, resulting in simple elliptical orbits.

    The galaxy’s mass is more spread out, which is why the orbits of stars within are a bit more complicated.

    Anyway, the universe seems like a hazardous place. What’s the meaning of life? I don’t know, just survive, reproduce and enjoy the relative tranquility we have today.

  22. Bas Timmerman

    I think I am going to warn Luis Sancho, maybe he can sue the sun to prevent this disaster from happening.

  23. Prathapan

    I am a little bit confused, your second paragraph last sentence in brackets.
    Wow 200 million years or so to travel or orbit the galaxy.
    At what speed the sun is travelling.

  24. Bobryuu

    Maybe you should start a second edition already, Phil…

  25. Is there a similar effect when we’re at the most southerly point of the wobble? If not, why not?

  26. Kate: Gravity is the restoring force that pulls us back to the galactic disk, yes. There’s a classic problem in most astrophysics texts where you computer (roughly) the oscillation timescale in fact. I have fond memories of that problem.

    Also, random thought: at what point are we experiencing hostile conditions so frequently that we can’t really blame these for mass extinctions? If we’re getting abused when we’re in or out of the galactic disk, that almost seems as if it points to a more constant extinction rate, not punctuated massive die-offs. (Bias disclaimer: I’ve always sort of suspected that most attempts to correlate mass extinctions with periodicities are trying to put predictability on what are basically random events, so I’m inclined to be perhaps more skeptical than most.)

  27. Michael Lonergan

    What happens when the sun reaches the bottom of it’s bob?

  28. Helioprogenus

    Although the data seems to line up correctly with the rate and timing of mass extinctions and the sun’s position in the plane of the milky way, I think it’s a little presumptious at this point to assume these oscillations and perturbations can correlate with mass extinctions. What we lack is enough data to reach those conclusions. I admit that it is an intriguing possibility but lets try to restrain ourselves a little from overreaching yet.

    Do we even know if the strength of the galactic magnetic field is uniform? I vaguely recall reading somewhere that it’s stronger North of the Galactic plane then the south. Perhaps there are areas with protective bubbles that we have yet to map.

  29. Horrifyingly, this news came too late for me to include in the book!

    So you want us to hold of buying until the second edition comes out?
    :D

  30. Dziban

    Scaremongering.

  31. Tom Marking

    Actually, there’s quite a bit of evidence that the largest mass extinction event called the Permian Extinction was caused by a massive volcanic episode which produced the Siberian Traps, and not by any astronomical phenomenon at all. So, as if death from the sky wasn’t enough of a bummer, there is also death from below.

    http://palaeo.gly.bris.ac.uk/Essays/wipeout/default.html

    A major catastrophe 251 million years ago left life teetering on the brink of oblivion. Now for the first time we have a clear picture of what caused it, says leading palaeontologist Michael Benton

    251 MILLION years ago, at the end of the Permian period, life on Earth was almost completely wiped out by an environmental catastrophe of a magnitude never seen before or since. All over the world complex ecosystems were destroyed. In the sea, coral reefs, fishes, shellfish, trilobites, plankton, and many other groups disappeared. On land, the sabre-toothed gorgonopsian reptiles and their rhinoceros-sized prey, the dinocephalians and pareiasaurs, were wiped out forever. Only 5 per cent of species survived the catastrophe, and for the next 500,000 years life itself teetered on the brink of oblivion. What terrible event could have wrought such havoc?

    .
    .
    .

  32. Tom Marking

    “it’s about 100 light years above the galactic plane. That’s far enough up that the magnetic fields of the galaxy are weaker, and it’s these fields that protect the Sun”

    I’m pretty skeptical about the relative weakening of the Milky Way’s magnetic field across a vertical distance of only 100 light-years since the thickness of the galactic disk in terms of stars is something like 1,000 light-years. I haven’t been able to find much on the galactic magnetic field, but for what it’s worth:

    http://www.scholarpedia.org/article/Galactic_magnetic_fields

    “Magnetic Field in the Milky Way -
    According to radio synchrotron, optical polarization and Zeeman splitting data, the average strength of the total magnetic field in the Milky Way is about 6 ?G near the Sun and increases to 20-40 ?G in the Galactic center region. Radio filaments near the Galactic center and dense clouds of cold molecular gas host fields of up to several mG strength (Heiles & Crutcher, in Wielebinski & Beck 2005). Outside the central region, the large-scale field is mostly parallel to the plane of the Galactic disk. Faraday rotation measurements from the polarized radio emission of pulsars with known distances allow to investigate the structure of the Milky Way’s magnetic field in three dimensions with much higher resolution than in external galaxies. The overall field structure follows the optical spiral arms, like in other galaxies, but additionally one large-scale field reversal in the disk, inside the solar radius, and several distortions near star-forming regions were discovered (Wielebinski & Beck 2005). More large-scale field reversals in the disk have been proposed, but need confirmation by improved observations.”

    So in the vicinity of the sun the galactic magnetic field strength is 6 millionths of a gauss. In comparison the geomagnetic field strength at the earth’s surface is about 0.5 gauss or 80,000 times stronger. I’m not sure what the galactic magnetic field lines are supposed to do. Sure, charged cosmic ray particles (mostly protons) will spiral around them but if the earth is in the path of one of these spiral trajectories it’s going to get hit anyway. Unless there is some confinement mechanism similar to the Van Allen radiation belts at work I don’t see how the galactic field helps us that much.

  33. Tom Marking

    “it’s about 100 light years above the galactic plane. That’s far enough up that the magnetic fields of the galaxy are weaker, and it’s these fields that protect the Sun”

    I’m pretty skeptical about the relative weakening of the Milky Way’s magnetic field across a vertical distance of only 100 light-years since the thickness of the galactic disk in terms of stars is something like 1,000 light-years. I haven’t been able to find much on the galactic magnetic field, but for what it’s worth:

    http://www.scholarpedia.org/article/Galactic_magnetic_fields

    “Magnetic Field in the Milky Way -
    According to radio synchrotron, optical polarization and Zeeman splitting data, the average strength of the total magnetic field in the Milky Way is about 6 ?G near the Sun and increases to 20-40 ?G in the Galactic center region. Radio filaments near the Galactic center and dense clouds of cold molecular gas host fields of up to several mG strength (Heiles & Crutcher, in Wielebinski & Beck 2005). Outside the central region, the large-scale field is mostly parallel to the plane of the Galactic disk. Faraday rotation measurements from the polarized radio emission of pulsars with known distances allow to investigate the structure of the Milky Way’s magnetic field in three dimensions with much higher resolution than in external galaxies. The overall field structure follows the optical spiral arms, like in other galaxies, but additionally one large-scale field reversal in the disk, inside the solar radius, and several distortions near star-forming regions were discovered (Wielebinski & Beck 2005). More large-scale field reversals in the disk have been proposed, but need confirmation by improved observations.”

    So in the vicinity of the sun the galactic magnetic field strength is 6 millionths of a gauss. In comparison the geomagnetic field strength at the earth’s surface is about 0.5 gauss or 80,000 times stronger. I’m not sure what the galactic magnetic field lines are supposed to do. Sure, charged cosmic ray particles (mostly protons) will spiral around them but if the earth is in the path of one of these spiral trajectories it’s going to get hit anyway. Unless there is some confinement mechanism similar to the Van Allen radiation belts at work I don’t see how the galactic field helps us that much.

  34. Mark Hansen

    Dziban,
    Yes, scaremongering. We’ve only got another 20 million years to party on. No time to get the things done that really matter. No time at all.

  35. Old news Phil! Dr Melott at KU has been telling us about this thing for years now! Someone takes it, dresses it up with a fancy schmancy computer model and suddenly it’s news? Pfft!

  36. csrster

    Talking of extinctions …

    I think I have some Bad Astronomy here …
    http://www.guardian.co.uk/science/2008/may/07/starsgalaxiesandplanets.spaceexploration

    Apparently a 40m diameter asteroid travelling at 28000mph and weighing 1.1m tonnes has the destructive energy of “84 Hiroshimas”.

    My gut tells me right away that this is orders of magnitude out. So sure am I of my gut that I’m going to post this first and then I’m going to go away and work out if I’m right :-)

  37. csrster

    Ok, interestingly it turns out to be only about one order of magnitude out.

    The mass of the asteroid is about right for a 40m sphere a few times the density of water.

    28000 mph translates to 10^4 m/s giving an energy of around .5×10^17 joules.

    1 kiloton is 4×10^12 joules so the asteroid has about 10^4 kilotons of kinetic energy.

    Little Boy yielded about 15 kilotons so the asteroid is about 650 Hiroshimas.

  38. Moving the goalposts already?

    Aggrieved book buyer: I spent $$$ on this book and the world hasn’t been destroyed once already!

    BA: Ah, but that’s just because I didn’t mention the way the world will be destroyed! Buy my next book for the real deal!

    ABB: Oh, no, I’m not throwing good money after bad. You’ll have to destroy the world on your ownsome. I can’t afford it.

    (Of course, if the world is destroyed, I’d be happy to buy any sequels you care to write.)

  39. Torbjörn Larsson, OM

    Bias disclaimer: I’ve always sort of suspected that most attempts to correlate mass extinctions with periodicities are trying to put predictability on what are basically random events, so I’m inclined to be perhaps more skeptical than most.

    Me two. While the famous K/T mass extinction is correlated with and may have been topped up by an impactor, apparently there are indications that continental and sea changes from continental drift softened up the ecological systems beforehand.

    The usual and perhaps more likely suspects of large species and pathogens easily immigrating and wreaking havoc. If so, the underlying supercontinental cycle have a much larger periodicity than the extinction events, so such causes are contingent.

    Add to that fact that mass extinctions are depending on definition, and it should be hard work attributing each extinction to a periodic factor.

    It is an interesting but not especially scaring subject. While on the one side life has weathered a large number of extinctions, on the other we currently live in what could be the fastest extinction event ever. That doesn’t seem to cause people to loose much sleep. But impactors gets us “Armageddon” on the screen…

  40. Tom Marking

    “While the famous K/T mass extinction is correlated with and may have been topped up by an impactor, apparently there are indications that continental and sea changes from continental drift softened up the ecological systems beforehand.”

    It is also associated with a period of severe volcanism which formed the Deccan traps.

    http://en.wikipedia.org/wiki/Deccan_Traps

    “The bulk of the volcanic eruption occurred at the Western Ghats (near Bombay) some 66 million years ago. This series of eruptions may have lasted fewer than 30,000 years in total. The gases released in the process may have played a role in the Cretaceous–Tertiary extinction event, which included the extinction of the dinosaurs.”

  41. alphonso

    Just read this post (thanks to F Cain for the link).
    All I can say is…

    bugger.

  42. Baxter J

    Well, frankly, I don’t see what the commotion – **interrupted by asteroid impact**

  43. Don Schindhelm

    I didn’t see anyone mention that there is growing evidence for the theory that our sun is a binary star, and that the reason for the bobbing above and below the galactic plane is because of the orbit around a center of gravity (most likely a small black hole) between the two stars. See orbit of binary stars http://www.oglethorpe.edu/faculty/~m_rulison/Astronomy/Chap%2017/Images/binary_star_orbit.gif

    The discovery of exoplanet Sedna and its strange orbit is just one recent piece of evidence that supports this theory. The theory is that we cannot see or detect our second sun, which ranges only 1 to 3 light years away locked in an elongated elliptical orbit around the black hole. Our Sun’s sister star will supposedly peek out from behind the black hole, for a brief period of time, during the galactic alignment.

    Following the logic of this theory, the mass extinctions by “Comet Showers” is made possible, by the Sun’s twin star when the two come into closer proximity around the center causing an intense gravitational disturbance of the Ort Belt , home to trillions of comets. During that time period the solar system would be careening with comets. (Time to stop focusing on Homeland Security and start revving up our Home Planet Security)

    A big proponent of the binary star theory is The Binary Research Institute. http://www.binaryresearchinstitute.org

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