Hi all, I'm new here to the BB, but a fan of the site. I thought I'd heard recently that there was a theory floating around that the Earth periodically travels through a 'dirty' layer of the galaxy and that this may be what is responible for the reoccurance of mass extinctions on the planet. If I remember correctly, the idea goes something like; the solar system in it's orbit around the Milky Way, also moves vertically in the galactic plane, like a horse on a merry-go-round. At some point on its journey it goes through a field of debris every X million years.

Has anybody here heard this theory? Is the any creedance to it? What is the evidence for the cycle?

Thanks!

Heard it? I made it up (well, me and ten thousand other people). Check this out: on the other side of the Milky Way, there is a small galaxy in collision with the Milky Way. Since they are in collision, they probably don't rotate together, right? They've been colliding for more than a couple cycles of the Milky Way rotation. There ya go.

The only problem is that galaxy collisions are a lot like you and me throwing balls of air at each other from twenty feet apart and watching them collide in the middle.

Well, then would the added gravitational influence be enough to 'shake loose' some objects in the Oort cloud or Kuiper's Belt?

And what's the rotational time of the MW? 250 million years?

Thanks!

Well, different parts of the Milky Way surely have different periods, but I think that is approx. the length of time it takes us.

There's another aspect of the theories that has the Solar System alternately above and then below the main plane of the Milky Way, and somehow regularly being stressed. I can't imagine how it'd have much effect on the Oort cloud, but who knows? Maybe all those stresses added together produce a detectable result.

While we're on the topic of these mass extinctions, what other phenomena can do this sort of thing?

Obviously there's the collisions with space rocks, which would have made up most of the extinctions in early Earth history, when they hadn't yet been thrown out by Jupiter. Another one might be supernovae. Get one of those going off too close and you're history.

Frankly, those are the only likely astronomical things I can think of that would catastrophically effect Earth life.

Maybe once in a while another star comes within 20000 AU of the Sun and messes around with Oort orbits, which has the slight chance of sending one at Earth. This seems to be not a serious threat, though.

This is going to sound really weird, and I don't have the reference with me to give the complete explanation, but I heard a proposal that the Earth ran into some form of space cloud that slowly depleted the Earth's atmosphere of a some of its oxygen, that is enough was depleted that the larger animals (the dinosaurs) could not intake enough oxygen and suffocated, but the smaller animals (i.e. mammals) were able to survive. Unfortunately, I don't know where the author got this idea from, either. Pretty weird, huh?

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"As I lay beneath the Southern Cross, the stars tell more than I could" . . . David Meece

I found this story covering a 26 million year mass extinction cycle (apparently, we are in the middle of the current cycle! [img]/phpBB/images/smiles/icon_smile.gif[/img] ):

http://www.bbc.co.uk/education/darwi.../massintro.htm

http://park.org/Canada/Museum/extinction/patterns.html

Apparently 3 were caused by massive glaciation, the others mostly impact oriented. But I couldn't find anyone offering an explanation for the reoccurances.


<font size=-1>[ This Message was edited by: RafaelAustin on 2002-10-17 14:17 ]</font>

Well what's even weirder...(is that a word?) is that I heard once that some guy had a theory that the dinosaurs farted too much therefore producing too much methane and then died. A little strange i would say

Methinks the evil flying dragon doth protest too much. [img]/phpBB/images/smiles/icon_smile.gif[/img]

Welcome to the board.

Leave it to Grapes. [img]/phpBB/images/smiles/icon_rolleyes.gif[/img]

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"As I lay beneath the Southern Cross, the stars tell more than I could" . . . David Meece