Is the solar system endangered by passing through a spiral arm of the Milky Way?

On Thu, 27 May 1999, David wrote:

> In reference to cyclic extincions on Earth several explanations exist. =
> One being that our solar system in between the arm of the Milky Way. In =
> 10,000 years or more the solar system will pass into an arm of the Milky =
> way. The transition from isolationism to communal life is devastating on =
> life.=20
>
> Is this a active theory or has this been debunked by an other theory. Or =
> am I dreaming this?

There are some scientists who think that mass extinctions may happen when
the sun passes through the plane of our galaxy. Our galaxy is pretty
flat (like a big record), and the sun bobs up and down like a cork as it
moves around the galactic center. Each time we pass through the disk of
the galaxy, some gravitational effects MAY be big enough to upset the
Oort cloud of comets, which could cause some comets to rain down on the
Earth. This happens every 25 to 30 million years or so.

This theory is very highly controversial. It was developed because some
scientists think that there is a regular period between mass extinctions
on the Earth. This supposed periodicity is also very controversial.
Personally, I don't believe it, but there are many good scientists who do.

The sun does pass through spiral arms from time to time, but we are
currently in between arms, and in will be millions of years before we go
through one again. These also may cause extinctions for the same reasons
as above, but, again, this is very controversial.

So, the chances that there will be a mass extinction due to
astronomical causes in the next 10,000 years are very small (although
there IS a real chance). I think we have more to worry from our own
handling of the planet than the Universe's treatment of the Earth.

Thanks for writing!

Sincerely,
Kurtis Williams

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