The galactic plane
Posted by Mr. Buracas on 2008-05-07
Our galaxy, the Via Lactae, is round and flat. Like a frisbee, only with a bulge in the core. It’s called, oddly enough, the galactic core, and there’s very likely a black hole in that core. (But that’s another story.)
So yeh, we live on the rim of a flattened galaxy. When we view the Milky way from Earth, we’re looking through the edge of it, sort of like when we look at the edge of the rings of Saturn. Here’s a photograph from Japan’s Akari satellite taken in the infrared wavelength (ummm, infrared? huh?).
Remember, we’re taking this picture from within the Milky Way… we’re in the outer reaches!
Notice that the galactic disc is thin. Ish. The Milky Way is about 1 000 light years thick, but over 100 000 light years in diameter.
Now, to the point of this posting. Our solar system is on the edge of the galaxy. But guess what? we’re not standing still. No, we’re slowly rotating around the galactic core (like a satellite around a planet, or a planet around a star). We make one orbit in about 150 million years or so, and this is known as the galactic year.
But our “orbit” also wobbles a bit, up and down with respect to the galactic plane. You can imagine that when we cross through the middle of it, where it’s thicker, that there’s a lot of dust a debris and stuff.
Well, there is. And every 35 to 40 million years we cross the dusty area, and our chances of colliding with a comet increase by ten times. It’s what scientists think wiped out the dinosaurs 65 million years ago. Here’s the article from Cardiff University (it’s in Wales, in the UK):
Scientists at the Cardiff Centre for Astrobiology built a computer model of our solar system’s movement and found that it “bounces” up and down through the plane of the galaxy. As we pass through the densest part of the plane, gravitational forces from the surrounding giant gas and dust clouds dislodge comets from their paths. The comets plunge into the solar system, some of them colliding with the earth.
The Cardiff team found that we pass through the galactic plane every 35 to 40 million years, increasing the chances of a comet collision tenfold. Evidence from craters on Earth also suggests we suffer more collisions approximately 36 million years. Professor William Napier, of the Cardiff Centre for Astrobiology, said: “It’s a beautiful match between what we see on the ground and what is expected from the galactic record.”
