Comment by eloff | Hacker News Reader

Comment by eloff | Hacker News Reader

I saw two independently published papers the other day showing a 27.5M year cycle. One was this paper talking about cycles in volcanic activity, another was talking about mass extinctions. Article here: https://phys.org/news/2020-12-mass-extinctions-land-dwelling... . I'm going to hazard a guess that the former is very likely a direct cause of the latter. We suspect this to be true of most of the worst mass extinctions, like the Permian extinction. With the dinosaurs it might have been the Chicxulub asteroid impact, or that might have merely been the finishing blow to a system under severe stress from the Decan Traps flood basalt eruptions in India that had been underway for millions of years by that point. There are plenty of indications that species were declining well before the impact.

The solar system takes 230 million years to make one complete orbit around the Milky Way, so I don't think this is likely caused by our passage through the galaxy. It's more likely an internal rhythm in activity deep within the Earth.

I believe the solar system actually cycles from below the plane of the galaxy to above the plane of the galaxy. I've heard numbers between 25M years and 40M years. Debris on that plane could shake comets loose from the oort cloud. Other theories on dark matter support the idea that crossing through dark matter could increase the temperature in the cores of planets, leading to increased seismic activity.

Those are good points. Maybe it does have to do with our galactic orbit.

In addition to the galactic orbit the disk itself has it's own local gravitational pull that keeps stars in the plane by pulling up/down when a stars trajectory takes it below/above the local disk. When we are out of the plane we loose the protective cover all the shielding the disk provides from the galactic core (electrons go zing). When in the middle of the disk there is a higher density of mater to interact with (rocks go boom). Our typical exposure to "orbits" earth & moon tends to be that they are roundish flat & smooth and we might be tempted to think the suns orbit around the galactic core is smooth, but it may be better to think of us as also bobbing up and down the highth of the disk as we go.

That's an interesting way to visualize it. As usual, every phenomena is more complex the more you dig into it.

> Other theories on dark matter support the idea that crossing through dark matter could increase the temperature in the cores of planets, leading to increased seismic activity.

Source? That’s fascinating, almost beyond belief, and I’d love to dig into related research.

Seen various versions of this around: https://astronomy.com/magazine/2019/07/dark-matters--shadowy...

I wouldn't be too quick in dismissing the Galaxy. The galaxy isn't uniform so something could easily be related to one of the harmonics. And 27.5M years happens to be quite close 1/8th of a galactic year, so something to do with the earth passing in/out galactic arms or similar is not out of the question.

To be slightly pedantic: our sun is a star in the galaxy, so as the entire galaxy rotates we would not pass in and out of a galactic arm. We would rotate with it.

Maybe not. https://en.wikipedia.org/wiki/Density_wave_theory

The galaxy isn't a rigid spinning object though, it's composed of many stars acting under gravitational influences. Like our solar system, objects closer in will orbit faster than objects further out. Rather than fixed structures, the arms are more like a standing wave of greater density.

Some related links

https://en.wikipedia.org/wiki/Density_wave_theory

https://en.wikipedia.org/wiki/Galaxy_rotation_curve

https://www.nature.com/articles/s41550-018-0627-5

As I understand it, galaxies don't rotate like rigid bodies: stars further from the centre take longer to go round than ones closer in, much like planets further from the sun take longer to complete an orbit.

If stars that start off inside an arm remain so, but the outer part of the arm goes round more slowly, the arm would get wound 'tighter' as time goes on, and this doesn't seem to happen.

I think what's really going on isn't yet well understood, but it's likely to be that the arms are something more like a standing pattern, and stars do indeed move in and out of the arms as time goes on (something like the way a traffic jam can move more slowly than the individual cars that it's made of).

Fair point. So maybe we should say that our sun would occasionally be a part of the galactic arm as the pattern morphs and re-forms. A fun concept.

It turns out that the stars further from the center don't take as long as one might think. Observations of this phenomenon led to the development of the theory of dark matter.

https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Not pedantic at all, I for one have never speculated about galactic dynamics

So in the solar system the inner planets have faster orbits than the outer planets, I assume it would be the same on a galactic level - if we’re a slow poke out on the edge, could there be some region of the galaxy that orbits the core 8x faster than us such that we’re periodically effected by it?

Edit LOL my sibling comments have filled in some info for me on the mean time

Most stars would not pass in and out of each spiral arm of a galaxy.

Perhaps it’s just our Sun that does it.

230 million years? perhaps our Solar system is traversing each spiral arms of our Milky Way.

Just reaching.

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