Comment by DrScump | Hacker News Reader

Comment by DrScump | Hacker News Reader

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.

...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?

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-e...

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.

>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

[0] As in, just applying Newtonian gravity.

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.

a visualization of the current model orbital path http://m.imgur.com/Eup0Yoa

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".

The orbit in the model are supposedly eccentric, but the period for a complete prbit is quite long, allegedly around 220milliom years, and yes the paper is all about bad things happening periodically as we change our local neighboors. But not something we'll have to worry about in quite a while

I think that's just a side effect of the compressed scales used by the animation. If they didn't do that then you would not be able to see the wave effect as clearly.

That's an exaggerated, simple model of spiral density waves; it's not a model for the Sun's orbit, which probably only varies in radius by about 10-15%.

> 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.

I thought that problem was the same as the galaxy rotation curve problem? If we solve (erm) that issue by adding in some dark matter - can't we then also accept a spiral structure to rotate with the galaxy without "winding up" the spiral?

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.

Put a different way, a galaxy's spiral arms are like waves.

Mind == blown

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?

The author explains it pretty well in the Batchelor interview:

https://audioboom.com/posts/4827661-mass-extinction-and-the-...

> 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.

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.

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.

> 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.

Could a big enough comet impact cause the Earth behave like a liquid and therefore a wave destabilize the crust and increase volcanic events?

I live not too far from the Danish location Stevn Klint, a chalk cliff formation where the Alvarez'es found one very neat exhibit for their case. Utterly fascinating: You can climb to the foot of the cliffs, and there, unmistakable and obvious, is the dark, horizontal line in the massive wall of white - you can see it, you can touch it, and you can ponder the very different nature of fossil below and above the line.

There is a local museum rigth next to the cliffs. A few years ago, I spoke to the leader there, a woman with a ph.d. in history, who more or less supplied the final death blow to any lingering respect I might have held for any kind of humanities: Her only take on Walter and Louis Alvarez and their world famous work in her own backyard was how of course they were so arrogant, they had raised hostility among scientists and everybody else the world over.

Arrogant about what?

I took it to be a humanities view of being out of whack with concensus.

what's that to do with anything in the thread

Reply on news.ycombinator.com