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].)

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.

The figure on the right, from their paper, shows the Sun’s orbit in red over the last half billion years. The Sun’s present position is indicated by the yellow spot, and the eleven extinctions are indicated by the circles.

There are obviously a great deal of uncertainties in this conclusion. Most significantly, the shape and history of the Milky Way remains very much in doubt, especially since we reside within it and cannot really get a good look at it. Though in recent years astronomers have assembled a reasonable image of the galaxy’s shape — a barred spiral with two major arms and several minor ones — this picture includes many assumptions that could very easily be wrong.

Nonetheless, the paper’s conclusions are interesting.

The theory is that when the Sun passes through the more crowded spiral arms, there is a greater chance that the Oort Cloud will be disturbed, producing a shower of comets descending into the inner solar system and thereby producing the extinction events.

The authors admit the speculative nature of their results. They also note, however, that if true, this data can be useful for helping to constrain the location of the smaller spurs and arms of the Milky Way.

If we were to assume that the extinctions are solely caused by impacts induced by our passage through spiral arms, then this would clearly allow us to constrain their locations independent of astronomical observations – an interesting test for the current model of galactic structure.

Finally, the authors note that these extinction events might actually helpful for the development of complex life, and therefore a habitable planet’s location within the galaxy might be crucial.

[R]econsidering the possibility that spiral arms do cause extinctions on Earth, we can consider the ramifications of this to possible complex life elsewhere in the Galaxy. If we also consider that extinctions accelerate rather than impede the evolution of complex life, we may better constrain ideas on the Galactic Habitable Zone. It may be the case that habitable planets orbiting stars further from the Galactic centre do not retain a high enough organic turn over rate due to more infrequent spiral arm crossings, or that closer in the extinction rate exceeds the ability for life to recover. Consideration could also be given to the possibility that habitable planet-hosting stars may have highly eccentric orbits around the Galaxy. [emphasis mine]

Essentially, a star’s orbit within a galaxy might play an important role in whether intelligent life evolves. Too far out and not enough extinction events will occur to help the evolution of complex life forms. Too close in and too many extinctions will cut off the development of life in general.

It appears from this work that our Sun might very well be in just the right place.