The ways the Universe can deal out death are as numerous as they are terrifying. Asteroid impacts, nearby stars exploding, wandering black holes... I spent a year or so thinking of nearly every method of cosmic catastrophe I could while writing Death from the Skies!*.
I wrote a whole chapter on what dangers lurk in our own Milky Way galaxy, and I was surprised to find out the Sun's orbit around the center of the galaxy is a potential problem. The galaxy is flat, like a CD (in fact, the proportion is right if you stack about 4 CDs together). The Sun does not orbit the center of the galaxy in a nice, flat plane, like planets do around the Sun. Instead, it bobs up and down like a cork in water, making about four cycles for every one time it orbits the galaxy (which takes about 200 or so million years).
In my research, I came across the idea that when the Sun is at the apex of its bobbing, towards galactic north, it's about 100 light years above the galactic plane. That's far enough up that the magnetic fields of the galaxy are weaker, and it's these fields that protect the Sun (and the planets, meaning us) from intergalactic cosmic rays, subatomic particles that zip around space between galaxies. When the Sun is up high, these cosmic rays can strike us, and we have to endure this particulate rain for millions of years. The radiation can do bad things, like damage the ozone layer or induce genetic mutations.
When researchers plotted the times of the Sun's most northerly excursions (which happen every 64 or so million years), they lined up in time with many mass extinctions on Earth. Uh oh.
The good news is that this only happens at one part of the Sun's orbit, so while we're deep in the plane of the galaxy we're protected and safe.
Or, actually, things get worse.
A new result has just been announced that says that when the Sun is in the thick of the Milky Way's plane, tides from the galaxy can induce comets from the outer solar system to plunge down toward the Sun, meaning many will hit the Earth and potentially cause mass extinctions.
According to the new study, this happens every 35 - 40 million years, which is not too far off from the calculations in the older study. Since the Sun moves up and down in the plane, it actually plunges through the mid-plane twice each cycle. If it reaches its apex every 64 million years, then it should pass through the mid-plane every 32 million years, which is reasonably close to what the second study says.
So as if it's not bad enough that we get irradiated at the top of the orbit, we get pummeled by comets twice as often!
So sure, having trillion ton chunks of rock and ice rain down every few 30 million years is bad and all, but that's not the worst part! Horrifyingly, this news came too late for me to include in the book!
We have to keep our perspective on these things, after all.