Spiral harm

By Phil Plait | November 8, 2007 1:34 pm

Man, the fun stuff you learn writing a book about wiping out all life on Earth.

I was trying to put some sort of statistical likelihood on how often the Sun plows into a dense cloud of dust called a molecular cloud. These clouds orbit the Milky Way like the Sun does, and were we to smash into one, it could, in theory, deposit enough dust in the Earth’s atmosphere to darken our skies somewhat. This would lower the Earth’s albedo (reflectivity), preventing prevent sunlight from reaching the surface, which in turn would cause a global drop in temperature and trigger an Ice Age.

Clouds like this are pretty rare, and tend to hang out in the spiral arms of galaxies. So how close are we to a spiral arm? I did some searching and found out that the Sun is about 6400 light years from the nearest arm, called the Perseus Arm.

I didn’t know that! Interesting.

The Sun orbits the center of the Milky Way at about 200 km/sec. Assuming the arm is just sitting out there like a cosmic traffic jam, we’ll enter it in about 10 million years or so. In reality, the arms move around the center as well, so it’ll actually take us a bit longer. That sounds like a safe enough buffer for me! Also, entering an arm doesn’t guarantee smacking into a cloud, which are actually small as astronomical objects go. We probably don’t do this except once every billion years or so.

So the odds of being killed by entering a molecular cloud are essentially zero for the next few megayears, but then sharply increase (but are still very low) after that. I find this is true a lot in astronomical doomsday scenarios; the odds of dying because the Sun turns into a red giant are zero for the next 6 billion years, but then go to 100% as soon as it expands. However, the Sun is slowly getting hotter with time (as helium builds up in its core due to nuclear fusion), and by my rough calculations the Earth will be too hot for life in a mere 200 million years or so (in 100 million the Earth’s temperature will increase by 10 degrees Fahrenheit, causing a giant global warming, and in one billion years our temperature will be the boiling point of water). So again, we’re safe… for now.

<evil laugh: mwuh hahahahahahaha>


Comments (39)

Links to this Post

  1. Seed's Daily Zeitgeist: 11/9/2007 - General Science | November 12, 2007
  1. Duane

    I always thought we were right at the inner edge of a spiral arm, and that there was little if anything in between any two arms in our galaxy. Are we, actually, sitting between two arms then?

    This means that the most likely places for life to evolve in our galaxy is not in the arms, but in between them (I’m surmising!)

  2. christopher scott

    like galaxy like mother!

  3. Torbjörn Larsson, OM

    the Earth will be too hot for life in a mere 200 million years or so

    Great. Life has been playing it lazy for 3.8 Gy or so, and now you tell us we have only 5 % lifetime left to acquire a second life? Way before the Andromeda crashes into the Milky Way the first time (IIRC T2 Gy ?) and risks ripping us or our sun out to a solitary life, the atmosphere is sputtered away (IIRC 4 Gy ?), or the Sun goes giant (6 Gy).

    So, is Mars going to be helpful or do we need to terraform Titan or some other moon with plentiful of volatiles? (Okay, maybe utopian solutions is a topic for the next book.)

  4. Hank Roberts

    But what about

    Found at: http://www.solstation.com/x-objects/chimney.htm

    “denser gas may already have blown into the Solar System earlier (possibly 33,000 and 60,000 years ago) (Priscilla Chapman Frisch, 1997). Astronomers hypothsize that such gas clouds can suppress the Solar Wind so that interstellar gas and dust enters the Solar System in quantities great enough to affect the Sun and life on Earth.”

  5. Brent

    So, what galaxy is that standing in for the Milky Way in the photo? It’s beautiful.

  6. Mango

    How confident are you in that 200 million year number, Phil?

    If it’s true, that’s really freaky. I don’t expect to be around then, but if you think about it, humans evolved just under the wire. I think it also means that following human extinction there isn’t a very good chance of another intelligent species evolving, digging up the remnants of our civilization, and learning all about us.

    I always imagined that might happen someday.


  7. John Powell

    For a fictional treatment of the Cosmic Cloud scenario you can read “Exit Earth” by Martin Caidin (also author of the novel “Cyborg”, aka “The Six Million Dollar Man”).



  8. Thorin

    This statement is confusing:
    “This would lower the Earth’s albedo (reflectivity), preventing sunlight from reaching the surface, which in turn would cause a global drop in temperature and trigger an Ice Age.”

    Lower reflectivity should facilitate sunlight reaching the surface.

    I completely understand that you’re trying to say, but written that way it reads counter-intuitively.
    Something along the lines of “increased atmospheric dust density…” or something like that would make more sense.

  9. PaulT

    Just a quick comment, since one of the professors in my department does a lot of work on pattern rotation speed (the speed with which the spiral arms rotate):

    What I had heard was that the Sun’s location in the Galaxy is very close to co-rotation, the radius at which the stars and the arms rotate at the same speed. So by that reasoning, the Sun would NEVER enter a spiral arm (which is certainly much safer for us here on Earth!). I understand this is the idea behind the “Galactic Habitable Zone” (a less well-established concept than the CHZ within a solar system) which would consist of planetary systems which orbit near co-rotation in galaxies with prograde arms, like us. Keeps them out of nasty clouds, away from Type II SNe, etc. , that can sterilize them right quick.

  10. dkary

    Two quick comments here:
    1. As Thorin points out, a lower albedo means more light is absorbed, not less. I wonder if what Phil is saying is that more light is absorbed in the upper atmosphere, hence it doesn’t make it to the ground?
    2. I have heard the claim that we are at or near the “co-rotation” point in the Milky Way, where the orbital speed is about the same as the pattern speed of the spiral arms. The net effect of this would be that we don’t go into and out of spiral arms, we just go around the Milky Way at the same speed the arms do. I don’t know if this is true or not, but certainly you do need to look at how fast those arms are moving in order to make a reasonable guess on how long it will before (and if) we go through one.

  11. Hmmm. Here’s a question. If we’re closest to the Perseus Arm, why are we always associated with the Orion Arm (other than S-F writers are notoriously lazy as a group)?

  12. Nuts. I had put in the thing about lowering albedo, and should have taken it out. Basically it just prevents sunlight from hitting the surface, cooling us.

    I don’t think there’s been a good study of the specific effects of cosmic dust in the air. You have to worry about the dust itself warming up. That may be a small effect, though.

  13. Thorin

    “I wonder if what Phil is saying is that more light is absorbed in the upper atmosphere, hence it doesn’t make it to the ground?”

    I think that’s what he was after which I why I didn’t say it was straight out wrong just that it read counter-intuitively.

  14. Jess Tauber

    Hey BA- what happens if ID forbid our system should plow right through the center of a Bok globule? How dense are those things? Enough to block sunlight from Earth? Enough to significantly increase Earth’s mass? What about the sun- if enough ‘metals’, or H/He get dumped on it, would it change the trajectory of the star’s future history, if only slightly?

    I would imagine also that astronomy, as a profession or hobby, would suffer if we did end up in one of those things. I’m in the dark, too, here!

  15. Mark Martin

    According to Fred Hoyle, those black clouds are intelligent beings, but largely oblivious to the consequences of their transits through the varied planetary systems.

  16. I have to say, that I could not agree with you in 100% regarding nnial 2007 – salvatore iaconesi – del.icio.us poetry, but it’s just my opinion, which could be wrong :)

  17. Darth Curt

    So what you’re say Phil, is that we’ve got some time to kill. Figuratively speaking of course. Very cool Galaxy pic… I always thought we were in the Western Arm… perhaps I’m just remembering that from “Guide to the Galaxy” (The novel, not the radio play, or (ech) movie)

  18. Kira

    Perhaps I’m wrong here, but I thought molecular clouds were composed of molecular hydrogen, not dust.

  19. The sun’s temperature gradually increasing doesn’t seem like a huge issue, since it seems like it’s slow enough that life would simply adapt, as it always has. Also, though, the sun is losing mass through the solar wind. So we’re probably very, very slowly moving away from the sun. I wonder how much further out we’ll be in a few billion years…

  20. That Neil Guy


    Have you looked through Isaac Asimov’s A Choice of Catastrophes, which (from what you’ve described) covers similar ground to your book?

    Just curious!

  21. Molecular clouds have H2 and lots of dust. The densities can be quite high, as much as 10^6 particles per cc.

    That Neil Guy: yeah, I have a copy on my desk! I’ll be reading it soon. My book will be submitted to my editor before I can read Asimov’s though.

  22. Wow. That 200 million years number is pretty depressing. I too was hoping to be dug up by future civilizations.

  23. Crux Australis

    Aren’t the arms pressure waves that travel backwards relative to the orbital motion of the stars? So it would actually take us a little less than 10 million years?

  24. John Foudy

    There are plenty of species alive today that are more intelligent than our ancestors (whatever they were) were 40 million years ago- that means plenty of time for us to go and for some random species slip into a sex-selection feedback loop that increases brain size

  25. Acleron

    If you could track back in time as well, do any of the extinction periods on earth correlate with interaction with excess particle density? I’ve read this as a cause but wonder if there is any better data to help or deny the hypothesis.

  26. Phil,

    I have to chime in with the others here: to my knowledge, the Sun is at an orbit where it co-rotates with the spiral arm system (or nearly).

    Further, I have heard that if Earth had ever entered a molecular cloud, we would find evidence of it in the sediments (though it was not mentioned what kind), and that we have not to this day found such evidence, which, along with the co-rotation fact, suggests that the Sun has actually never flown through a molecular cloud, ever.

    As for crossing the spiral arms, there’s another, greater hazard than molecular clouds there: the presence of molecular clouds implies star-forming regions, which implies young stars, which implies the presence of massive stars that haven’t yet, but may be about to, go bang in a supernova.

    So you don’t want to get close to the arms.

    Which makes it all the more salient that purportedly we have hints that the Sun indeed never has.

  27. forrest noble

    200 million years is a very long time for humanity. By then, if our species still exists, I believe we will have planet hopped, star hopped, and also would have the ability to very slowly move our planet’s distance in or out from the sun as needed.


  28. But then, some clouds may be intelligent and come straight for us…


  29. James

    Isn’t the Earth presently passing through passing through a Local Interstellar Cloud (LIC) which is flowing away from the Scorpius-Centaurus Association of young stars?
    See http://antwrp.gsfc.nasa.gov/apod/ap000412.html

  30. Wouldn’t the solar wind just blow the cloud away? After all, that’s what happens to the gas from comets…

  31. !AstralProjectile

    All the carbon in the gas cloud will percolate into the core of the sun, increasing the contribution of the carbon cycle to the overall fusion reaction, thereby causing the sun to go nova!! :-0 (Theory ripped of from Asimov’s “The Currents of Space”)

    Any of you folks read James Tiptree’s novella “Up the walls of the World”? Pretty good read with a parsecs-long, slightly-denser-than-a-vacuum critter that can alter time and cause suns to go nova?

  32. SteveInNC

    I’m fairly sure that G-type stars like the sun have very little matter exchange between the core and the convective zone, so the carbon would stay in the outer layer (http://en.wikipedia.org/wiki/Sun#Structure), though I’m sure it would have some effect on luminosity or color. Small stars (M-class) are believed to be fully convective, though their temperature would be too low for the carbon cycle to occur.

  33. For some reason I’m getting the distinctly amusing mental image of deploying Space Roombas to suck up the nasty bad space dust bunnies…

  34. Loaf Of Bread

    Hey, 200 million years is a fairly nice chunk of time in evolutionary terms. Wasn’t it only about 60 million years ago that the dinosaurs were running the show? We could nuke ourselves tomorrow, and there would still be lots of time for some other intelligent species to evolve and figure out how to get away from the third rock from the sun.

  35. Yawning Angel

    I think we should be able to see a Bok Globule Long, long before it starts to come our way, it ours its.
    These figures are ALL rather rubbery anyway. Carbon and other methods of dating are really litle better than Guesses any way. Or so I’m am told.
    And, WHAT ABOUT the BIG Picture???? You talk about 4-6 billion years hence but thats nuthing on a Cosmic Scale.
    Aren’t We ALL heading towards the Virgo cluster and smash into 100’s of Galaxies in the next 100 Billion years or So????
    Think Big You People!

  36. forrest noble

    Yawning Angel, don’t worry about the Great Attracter. In 100 billion years or sooner the sun will have long since burned out probably– and our species, if it’s still around, will have had plenty of time to star hop, galaxy hop many times as well, or maybe travel beyond the now visible universe– that too is thinking big, without resorting to science fiction I believe.

  37. drazil

    I think global warming will cause the earth to be inhospitable for life long before the 200 million years

  38. forrest noble

    drazel, It’s that global cooling– the dimming of the sun be polution that will get us–didn’t you see the movie.


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