Re: Great Filter Material
Thu, 24 Jun 1999 17:59:39 -0700

Eliezer S. Yudkowsky, <>, writes:
> What makes the Sun special? [Edited a bit...]
> > Its orbit is significantly less elliptical than that of
> > other stars of its age and type, and hardly inclined at
> > all to the Galactic plane. What's more, the Sun is
> > orbiting very close to the "corotation radius" for the
> > Galaxy-the place at which the angular speed of the
> > spiral pattern matches that of the stars.

I would be curious to know just how elliptical the orbits are of more typical stars. The galactic disk does not get much thicker until you get quite a ways into the center (as far as we know - the gross structure of the galaxy is still not that well known). So unless these stars have egg-shaped orbits, much more elliptical than typical solar system orbits, it should not be an issue.

As for the corotation radius, assuming this is correct (I didn't know we knew how fast the spiral arms are rotating) then unless we have spent the last 200 million years in a gap between spiral arms, I don't see that it helps to move slowly through them. You still spend a fixed percentage of time in high density and low density regions, independent of how fast you move through them.

> > And
> > its small inclination to the Galactic plane prevents
> > abrupt crossings of the plane that would stir up the
> > Sun's Oort Cloud and bombard the Earth with comets.

It's not clear that spending all the time in the galactic disk is better, if being near other stars stirs up the Oort cloud.

> > Because life-bearing stars have to be close to the
> > corotation radius, that rules out more than 95 per
> > cent of stars in the Galaxy in one fell swoop.

The corotation radius seemed to be the weakest argument of all the points he made.

Actually it seems from most of his arguments that it is being near other stars that is a danger. In that case the stars to look at would be those farthest from the galactic center, not stars like the sun.

Here is one other paragraph from the article:

> > "Most astronomers disagree with Gonzalez," adds SETI researcher Dan
> > Werthimer of the University of California at Berkeley. "Our Sun is
> > pretty average. In any case, you don't need a star exactly like our Sun
> > for life."