Linking Mass Extinctions To the Sun's Journey In the Milky Way

schwit1 writes "In a paper published today on the Los Alamos astro-ph preprint service, astronomers propose that as many as eleven past extinction events can be linked to the Sun's passage through the spiral arms of the Milky Way. (You can download the paper here [pdf].) From the paper: '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.'"

Comments

Oort cloud?

[Anonymous Coward]

I only read the abstract, but while it proposes a correlation it did not speculate on the exact cause of the extinction. I wonder if passing 'nearer' (I use the term loosely) to higher concentrations of stars might disturb the Oort cloud, sending more comets than normal careening in towards the inner solar system ... or if we might catch stragglers from other stars' own Oort Clouds.

Re:Oort cloud?

[icebike]

That is how I read it, or simply wandering comets, asteroids, broken free of what ever they were orbiting. Even interstellar dust concentrations perturbing our own asteroids might be enough.

But I was more surprised to learn the Sun was not traveling in rough unison with a (relatively) fixed spiral arm. Is this normal for all stars?
If all stars are wandering why do spiral arms exist at all? Why wouldn't the Milky Way simply be a disk?

Re:Oort cloud?

[icebike]

“We find they are forming spiral arms,” explains D’Onghia. “Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there.”

No mention of Shock waves, or even a hint of what might cause such shock, or how such shock could be transmitted in the vacuum of space.

Density waves, (shock waves) another term for Stochastic Star Formation theory, is no longer the leading theory of the existence of spiral arms. Its not the 1960s any more.

This shock wave theory suggest that stars are relatively uniformly distributed, even in the inter-arm gaps, but because of density waves inducing star birth at their leading edge and star death at their trailing edge, the arms simply appear brighter.
Hubble pretty much put that theory to bed. see http://en.wikipedia.org/wiki/NGC_1300 The inter-arm gaps are real.

Further the so called shock wave theory (Stochastic Star Formation) postulates that stars on average do not actually leave their "arm", and the visual effect of the arm at any give place pretty much spans the life of a star. (born on the leading edge, dead by the trailing edge). Yet this story suggests the Sun has wandered through the arm(s) several times.

Further, even when perturbations from a passing galaxy might have triggered them via gravity, the arms persist. and in some galaxies even after
the perturbations disappear. So what is driving these? What would cause "shock waves"?

The 60's are calling, and they want their theory back.

Re:Oort cloud?

[dkf]

No mention of Shock waves, or even a hint of what might cause such shock, or how such shock could be transmitted in the vacuum of space.

Via the interstellar medium, of course. It's pretty tenuous, but most certainly is capable of sustaining phenomena like shock waves. Which isn't to say that that's necessarily the particular process that is dominant in the galactic arms; it could also be something relating to magnetism, as the physics of a flowing magnetically-coupled medium is viciously difficult to work with (i.e., highly non-linear). And I've got no idea what happens at the phase change boundaries between the parts of the ISM which are plasmas and the parts which are conventional (tenuous) gasses; phase changes can do "interesting" things.

As for what's powering it all, you've got some exceptionally powerful energy sources out there. Black holes in particular can pump vast amounts of energy into the surrounding volume of space. The stellar wind from very high mass stars would be another interesting source.

Re:Rubish

[Dutchmaan]

**might** be explained

Isn't that pretty much what "correlation" means?

Re:Rubish

[Anonymous Coward]

It means every time a mass extinction occurs on Earth, a galactic spiral arm is contructed.

Re:Rubish

[Dutchmaan]

I know that's supposed to be tongue in cheek, but correlation is just another way of saying, we need to look into this with more detail. So if that's your hypothesis and there's correlation between the events, then who am I to judge if you decided to study it with greater detail.

Re:Rubish

[wonkey_monkey]

So why are you calling it "rubbish?" They've found a correlation. That's interesting. No-one's claiming to have discovered the mechanism. Correlation is not causation. You seem to have inferred that because someone's found a correlation, they must also be claiming causation.

Nemesis: Debunked theory

[Misagon]

There has previously been a theory that these mass reoccurring extinctions would have been created by the near passing of a hypothetical star that we would have been unable to detect because it would be on the other side of the Oort cloud.
I suppose that this new finding will debunk that theory for good.

The hypothetical star had been named Nemesis. I know of it only because I ready about it in a novel by Asimov recently.

Re:suuuure

[icebike]

in other news...many people die in hospitals, therefore hospitals may cause death.

And indeed they do.

http://www.health-care-reform.net/causedeath.htm
http://en.wikipedia.org/wiki/Hospital-acquired_infection

So don't be so quick to dismiss the possibility of causation, simply because it was discovered by correlation.
Falsely assuming no-causation is every bit as much as a statistics induced error as falsely assuming causation.

With correlation you have a reason to look for causation. Without correlation, looking for causation is just shooting in the dark.

Re:Intergalactic space

[symbolset]

It is possible for the Sun to be flung out of the galaxy by passing too close to a much larger star. Stars are flung out of galaxies quite frequently in much the same way that asteroids and comets are frequently flung out of the solar system. The Earth would be unlikely to survive such an event. But if it did, no, there is nothing out there between galaxies that is more harmful than whirling through this relatively dense dust and grit.

Explanation is elsewhere

[monatomic]

http://news.nationalgeographic.com/news/2007/04/070420-extinctions.html Other researchers found that time periods Earth is exposed to large amounts of cosmic rays is correlated with mass extinction events. This is a possible explanation.

Re:Rubish

[Teancum]

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.

That is how the authors of this paper reported their findings in the actual article's abstract. As for how some random Slashdot poster reported this idea, does it really matter? If you are complaining about Slashdot itself and lame editorship on the part of those who review these stories on the Slashdot staff, that is something else entirely and not something to complain about to the paper's authors.

Besides, they claim it is a causation, or that events which somehow happen during those crossings in turn trigger these extinction events. Unfortunately we have a data sample of one solar system to compare against right now to see if there might be any substance to the mechanism.

Re:Rubish

[hairyfeet]

Well I'm not a scientist but with a little common sense being applied and knowing what we know it makes sense. After all what do we know so far? Well we know there is this HUGE band of rock at the outer edge of our system called the Oort Cloud, there is also another huge band of rocks in the inner system which may have been a failed planet. If going through the arm caused changes to the gravitational fields way out there in the oort cloud it probably wouldn't take much to get one of those chunks moving, after all that is where it is believed all our long term comets came from so there had to be something in the past that got the comets going. So if you look at how big a period we are looking at and how much debris we have in the inner belt the idea that going through the arm may occasionally cause one of those big inner rocks to get flung at us,possibly by being hit by something fling out of the oort? Really doesn't sound too implausible to me.

To me the thing I find really interesting is how many times our friend Mr Jupiter has saved us by being a big giant garbage collector and pulling all this crap into it that could have easily headed our way if it wasn't there. i honestly wouldn't be surprised if it turned out for every extinction event there was probably thousands that didn't happen because Jupiter sucked it up or slung it off in another direction,given how many times we have seen impacts in just the little bit of time we've been able to see that far out.

So I wish them all the luck, maybe if we can find out exactly what events cause these maybe we can avoid it happening to us, as while I doubt we'd see 100% fatalities of the human race one really nasty impact could easily send us back to the dark ages.

Re:spiral arms?

[richard.cs]

Do the spiral arms move w/respect to all the stars like some sorta density wave?

That's exactly what the spiral arms are, they can't be the same stars orbiting together in that shape as that would imply a rigid body rotation. The situation where everything moves around together as if it were nailed to a rigid cosmic disc doesn't work because the orbit time of the stars at the centre of the galaxy is less than that of the stars at the edge. This is a consequence of the orbital physics, it's essentially the only way the forces can balance.

So, the stars in the centre whiz around quickly (in cosmological time anyway) whilst the ones at the edge take forever. The spirals are simply areas of higher star density but they are not the same stars all the time. This region does rotate but more slowly than the stars contained within it. So, why are there areas of increased star density? No-one's entirely sure but it seems likely that these are actually regions with higher rates of star formation, with many young, short-lived blue stars.

Not exactly a new concept

[Omega Hacker]

I've got a novel by John Brunner written in 1982 called The Crucible of Time (), which documents a (very non-human) species through its scientific awakening. Throughout the book they're discovering that their planet is getting closer to a cloud of debris dense enough to massively devastate the surface, possibly shatter the planet. In the end they manage to build enough arks to save the species. The foreward reads:

"It is becoming more and more widely accepted that the Ice Ages coincide with the passage of the Solar System through the spiral arms of our galaxy. ..."

Re:Rubish

[meerling]

It's been brought up before, and rather quickly debunked, not because a number of extinctions do occur at the periods of crossing a galactic arm, but rather the numerous other times when extinctions occur outside of the galactic arms, and the times it passes through the arms that the extinction events don't occur.

If you want to see what I'm talking about, just search the science sites about it.

Yes, it is an intriguing idea, but No, it doesn't hold up to scrutiny.

Re:What we need to know...

[MatthiasF]

We are actually passing through a minor one right now, called the Orion Spur.

http://en.wikipedia.org/wiki/File:Artist's_impression_of_the_Milky_Way_(updated_-_annotated).jpg

Re:Rubish

[stenvar]

Correlation is not causation

That doesn't mean what you seem to think it means. "Correlation is not causation" is a statement reminding people that "B causes A" and "X causes A and B" are alternative explanations to "A causes B" when one observes a correlation.

In this case, the only reasonable choice is "galactic orbit causes extinctions" or "the correlation is accidental"; none of the other alternatives are reasonable.

Re:Rubish

[dryeo]

Areas of star formation are more radioactive due to massive blue stars and resulting supernovas when the massive star dies. More star formation happens in the arms.

We ARE a fragile society

[ulatekh]

A technological regress requires a fragile society not just a sudden jolt.

But we are a fragile society. Without even having to bring up the Idiocracy, the fact remains that we're mostly a society of specialists, dependent on the other cogs in the machine for our survival, stupidly mocking the "preppers" who are really just trying to be generalists. A comet strike could easily disrupt this machine and cause it to grind to a halt.