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Abstract: "We use the most up to date Milky Way model and solar orbit data in order to test the hypothesis that the Sun's galactic spiral arm crossings cause ma

by DrScump 10y ago

Abstract: "We use the most up to date Milky Way model and solar orbit data in order to test the hypothesis that the Sun's galactic spiral arm crossings cause mass extinction events on Earth. To do this, we created a new model of the Milky Way's spiral arms by combining a large quantity of data from several surveys. We then combined this model with a recently derived solution for the solar orbit to determine the timing of the Sun's historical passages through the Galaxy's spiral arms. Our new model was designed with a symmetrical appearance, with the major alteration being the addition of a spur at the far side of the Galaxy. A correlation was found between the times at which the Sun crosses the spiral arms and six known mass extinction events. Furthermore, we identify five additional historical mass extinction events that might be explained by the motion of the Sun around our Galaxy. These five additional significant drops in marine genera that we find include significant reductions in diversity at 415, 322, 300, 145 and 33 Myr ago. Our simulations indicate that the Sun has spent ~60% of its time passing through our Galaxy's various spiral arms. Also, we briefly discuss and combine previous work on the Galactic Habitable Zone with the new Milky Way model." TL;DR: our Sun's bumpy orbit through the spiral arms of out galaxy means periodic increases of comets and other bodies being jarred loose and later hitting Earth, resulting in mass extinctions. The full paper is a free download from this page.

PhasmaFelis 10y ago
...Huh. I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. (Barring "up/down" oscillation if the orbital plane isn't perfectly aligned with the galaxy's.) There must be some kind of overall structure to maintain distinct spiral arms, right? Does the Solar System actually make a complete circuit of the bulk of the Milky Way? Is that unusual?
- rtkwe 10y ago
  The Spiral arms aren't really cohesive bunches of stars but just bunches of relatively new stars that burn brighter than the older stars outside the spiral arms. http://www.space.com/24642-spiral-galaxies-milky-way-shape-explained.html http://www.space.com/24642-spiral-galaxies-milky-way-shape-e...
  - Keysh 10y ago
    No, in many spiral galaxies the spiral arms are present in older stars as well; that's why they're considered density waves in the stars. The contrast is stronger for younger stars, but it's there in old stars, too.
- gizmo686 10y ago
  >I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. That was the original assumption. However, basic [0] orbital mechanics says that objects with smaller orbits orbit faster. If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more. A more modern hypothesis is that the arms are more like waves of high density. Individual stars move through the the arm. Since this high density region exerts gravity, stars that are moving out of it are slowed down, and stars that are moving into it are sped up. The end result is that orbiting stars tend to spend a little bit more time in the arm region then they would under a pure orbit. In doing so, they contribute to the mass of the arm, causing other stars to also spend more time in the arm then they otherwise would. The end result is a stable wave as all the individual stars move into and out of it throughout their orbits. https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory [0] As in, just applying Newtonian gravity.
  - mirimir 10y ago
    Right, density waves. But as rtkwe and the Wikipedia article note, there's also increased star formation in the higher-density arms. So more stuff hitting Earth also seems plausible.
  - LoSboccacc 10y ago
    a visualization of the current model orbital path http://m.imgur.com/Eup0Yoa http://m.imgur.com/Eup0Yoa
    - PhantomGremlin 10y ago
      It's hard to be sure. But observing both the Wiki animation and this GIF, it appears that stars don't orbit at a constant radius from the galactic center. Rather, they appear to move significantly closer and further. Does that seem right? If so, that's much different than planets orbiting our solar system. To a first approximation our orbits appear circular. Yes I know there's a certain eccentricity but for most planets it's not much. I don't like the idea of our solar system getting too close to the galactic center. It gives me a vague sense of dread, like "bad things can happen there".
- jamestnz 10y ago
  The spiral arms are not fixed masses. If they were, they would quickly wind themselves up tighter and tighter, as the inner areas are orbiting faster than the outer parts. Within a short time the spiral arms would be dispersed. Instead, you can think of the spiral arms as being like the highest-density part of a traffic jam. In a traffic jam, cars are slowing down and bunching onto the back of the queue, while other cars are finally trickling off the front of the queue and speeding back up. In this way the traffic jam itself stays in a stable position, while individual cars constantly move through it. In other words, it isn't the same stars that make up the spiral arms over time. As stars orbit the galactic center, they move in and out of the spiral arms.
  - colechristensen 10y ago
    Put a different way, a galaxy's spiral arms are like waves.
  - anentropic 10y ago
    Mind == blown
  - cema 10y ago
    A good explanation. I think one detail may be corrected: a traffic jam typically goes backwards, with respect to the direction of the cars forming the jam, at a speed much slower than the speed of the traffic. Perhaps a similar behavior is observed with the galactic arms?
  - DrScump 10y ago
    The author explains it pretty well in the Batchelor interview: https://audioboom.com/posts/4827661-mass-extinction-and-the-structure-of-the-milky-way-joint-horner?t=0 https://audioboom.com/posts/4827661-mass-extinction-and-the-...
flukus 10y ago
> TL;DR: our Sun's bumpy orbit through the spiral arms of out galaxy means periodic increases of comets and other bodies being jarred loose and later hitting Earth, resulting in mass extinctions. Isn't only one of the mass extinctions (the dinosaur ending one) somewhat tightly linked to a comet impact? It seems as though it's finding a pattern that does no fit the data.
- metaobject 10y ago
  I believe a few of the mass extinctions are thought to have been caused by gamma ray bursts (GRBs) that were close enough to earth to either affect the atmosphere or the orgamisms on the surface.
- mcv 10y ago
  Are the causes of other mass extinctions known? I can imagine comet impacts can be hard to trace over billions of years. Especially when it's not just a single big one, but a period of increased impacts. Also, I believe the dinosaur extinction was not just a comet impact, but a combination of an impact and a period of increased volcanic activity. Could the volcanic activity be triggered by comets? I think the jury is still out, but it's an interesting field of study across many different fields (astronomy, biology, geology and probably more), and it has obvious implications for humanity's future on Earth, in space, and the chances of finding other life out there.
  - flukus 10y ago
    > Could the volcanic activity be triggered by comets? That's an interesting question. My completely unqualified opinion would be no, an comet couldn't trigger the massive and ongoing volcanic activity that a mass extinction requires. I'd like to hear from someone more informed though.
    - cifvts 10y ago
      Could a big enough comet impact cause the Earth behave like a liquid and therefore a wave destabilize the crust and increase volcanic events?

by DrScump 10y ago

PhasmaFelis 10y ago

...Huh. I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. (Barring "up/down" oscillation if the orbital plane isn't perfectly aligned with the galaxy's.) There must be some kind of overall structure to maintain distinct spiral arms, right? Does the Solar System actually make a complete circuit of the bulk of the Milky Way? Is that unusual?

rtkwe 10y ago
The Spiral arms aren't really cohesive bunches of stars but just bunches of relatively new stars that burn brighter than the older stars outside the spiral arms. http://www.space.com/24642-spiral-galaxies-milky-way-shape-explained.html http://www.space.com/24642-spiral-galaxies-milky-way-shape-e...
- Keysh 10y ago
  No, in many spiral galaxies the spiral arms are present in older stars as well; that's why they're considered density waves in the stars. The contrast is stronger for younger stars, but it's there in old stars, too.
gizmo686 10y ago
>I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. That was the original assumption. However, basic [0] orbital mechanics says that objects with smaller orbits orbit faster. If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more. A more modern hypothesis is that the arms are more like waves of high density. Individual stars move through the the arm. Since this high density region exerts gravity, stars that are moving out of it are slowed down, and stars that are moving into it are sped up. The end result is that orbiting stars tend to spend a little bit more time in the arm region then they would under a pure orbit. In doing so, they contribute to the mass of the arm, causing other stars to also spend more time in the arm then they otherwise would. The end result is a stable wave as all the individual stars move into and out of it throughout their orbits. https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory [0] As in, just applying Newtonian gravity.
- mirimir 10y ago
  Right, density waves. But as rtkwe and the Wikipedia article note, there's also increased star formation in the higher-density arms. So more stuff hitting Earth also seems plausible.
- LoSboccacc 10y ago
  a visualization of the current model orbital path http://m.imgur.com/Eup0Yoa http://m.imgur.com/Eup0Yoa
  - PhantomGremlin 10y ago
    It's hard to be sure. But observing both the Wiki animation and this GIF, it appears that stars don't orbit at a constant radius from the galactic center. Rather, they appear to move significantly closer and further. Does that seem right? If so, that's much different than planets orbiting our solar system. To a first approximation our orbits appear circular. Yes I know there's a certain eccentricity but for most planets it's not much. I don't like the idea of our solar system getting too close to the galactic center. It gives me a vague sense of dread, like "bad things can happen there".
jamestnz 10y ago
The spiral arms are not fixed masses. If they were, they would quickly wind themselves up tighter and tighter, as the inner areas are orbiting faster than the outer parts. Within a short time the spiral arms would be dispersed. Instead, you can think of the spiral arms as being like the highest-density part of a traffic jam. In a traffic jam, cars are slowing down and bunching onto the back of the queue, while other cars are finally trickling off the front of the queue and speeding back up. In this way the traffic jam itself stays in a stable position, while individual cars constantly move through it. In other words, it isn't the same stars that make up the spiral arms over time. As stars orbit the galactic center, they move in and out of the spiral arms.
- colechristensen 10y ago
  Put a different way, a galaxy's spiral arms are like waves.
- anentropic 10y ago
  Mind == blown
- cema 10y ago
  A good explanation. I think one detail may be corrected: a traffic jam typically goes backwards, with respect to the direction of the cars forming the jam, at a speed much slower than the speed of the traffic. Perhaps a similar behavior is observed with the galactic arms?
- DrScump 10y ago
  The author explains it pretty well in the Batchelor interview: https://audioboom.com/posts/4827661-mass-extinction-and-the-structure-of-the-milky-way-joint-horner?t=0 https://audioboom.com/posts/4827661-mass-extinction-and-the-...

flukus 10y ago

> TL;DR: our Sun's bumpy orbit through the spiral arms of out galaxy means periodic increases of comets and other bodies being jarred loose and later hitting Earth, resulting in mass extinctions. Isn't only one of the mass extinctions (the dinosaur ending one) somewhat tightly linked to a comet impact? It seems as though it's finding a pattern that does no fit the data.

metaobject 10y ago
I believe a few of the mass extinctions are thought to have been caused by gamma ray bursts (GRBs) that were close enough to earth to either affect the atmosphere or the orgamisms on the surface.
mcv 10y ago
Are the causes of other mass extinctions known? I can imagine comet impacts can be hard to trace over billions of years. Especially when it's not just a single big one, but a period of increased impacts. Also, I believe the dinosaur extinction was not just a comet impact, but a combination of an impact and a period of increased volcanic activity. Could the volcanic activity be triggered by comets? I think the jury is still out, but it's an interesting field of study across many different fields (astronomy, biology, geology and probably more), and it has obvious implications for humanity's future on Earth, in space, and the chances of finding other life out there.
- flukus 10y ago
  > Could the volcanic activity be triggered by comets? That's an interesting question. My completely unqualified opinion would be no, an comet couldn't trigger the massive and ongoing volcanic activity that a mass extinction requires. I'd like to hear from someone more informed though.
  - cifvts 10y ago
    Could a big enough comet impact cause the Earth behave like a liquid and therefore a wave destabilize the crust and increase volcanic events?