Death from the Spirals! Maybe not so much.

Death from the Spirals! Maybe not so much.

Death from the Spirals! Maybe not so much.

Bad Astronomy
The entire universe in blog form
June 29 2009 7:38 AM

Death from the Spirals! Maybe not so much.

In my book Death from the Skies!, I talk about the many ways our galactic neighborhood can wipe out life on Earth: nearby supernovae, magnetar hissy fits, episodic oscillations of the Sun carrying us into a stream of dangerous intergalactic cosmic rays, and more.

Given that litany of disastrous scenarios, you wish to breathe a momentary sigh of relief that perhaps one of these things can be scratched off the list: the Earth plowing into the galaxy's spiral arms.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


The Milky Way is a disk galaxy, a flattened circle of stars, gas, and dust. The spiral arms are like traffic jams of material; gravitational speed bumps where gas can pile up and form stars. There aren't that many more stars in the arms than between them, it's just where the biggest and brightest tend to be, so they light the place up.

But there are more and denser gas clouds in the arms on average, and every few hundred million years the Sun drifts into and out of these regions. It's been supposed that wandering into a nebula like this can potentially affect the Earth's environment. If the dust is dense enough it can block sunlight, cooling the Earth, causing an ice age.

But a new study shows this may not be the case. Universe Today has the details, but basically there appears to be a 140 million year period to changes in Earth's climate, which was suspiciously close to the period between encounters with our galaxy's arms. The new study -- using actual data of how dense clouds of gas and dust orbit the Milky Way to calculate when our Sun moves into the spiral arms -- finds this period does not line up with any galactic goings-on.

Does that mean we're safe? Well, no. First, all those other impending disasters are still out there, though of course the good news is that they are incredibly rare and basically won't happen for millions of years (in other words, don't fret). Second, the period of climate change is still apparently real, it's just that it's not tied to the Sun's orbit around the center of the Milky Way. If it is real, then something must be causing it. We've taken one cause off the list, but what else are we missing?

To be honest, I won't be rushing to edit that chapter in my book just yet. This study looks good, but I'll wait and see what other scientists say. With another few dozen million years to go, I have plenty of time.