Magnetosphere might not be the first thing you think of when you think of “breathable air”, but maybe you should be.
I was reading a scifi novel recently, when I came across a description of an inhabited planet and I thought ‘yeah, that couldn’t support life in the real universe’. Because of the magnetosphere.
…Okay, maybe we should back up a bit. Let’s start with the basics.
A magnetosphere is the area around an astronomical object affected by the magnetic field of that object. For example, it’s an area around Earth where Earth’s magnetic field affects stuff. In other words, the magnetic field that powers compasses and the poles.
It’s kind of like a bubble of magnetism around a planet, or other object. Well, actually, not so much a bubble, as a weird bulb that fans out into space at the side facing away from the sun. As you can see in the picture at the top of this article.
Of course, you might now be wondering where the magnetosphere comes from, and what it has to do with breathing.
The engine that runs the magnetosphere is the Earth’s core. Or rather, the convection flows of molten metal in that core. That’s right, the core of the Earth is made of magma, which sloshes around — slowly. That magma consists in part of liquid iron. Because of the Earth’s rotation, the magma rotates, and that causes a magnetic field: the magnetosphere.
That’s also the reason the magnetic field has its pole at the north pole: the iron naturally spins roughly around the Earth’s axis, since the Earth does so as well. At some point in the future, the liquid iron might move, and the field could flip. The north pole would then move to the other side of the planet, to where the south pole is now. It has happened in the past, and could happen again.
You might also be wondering what the magnetosphere does. The answer is simple: it does what all magnetic fields do, repel charged particles. The Earth’s magnetosphere pushes away charged particles, which means they don’t come down to earth.
There are — in fact — lots of charged particles around in space. Say a star goes supernova somewhere in our galactic neighborhood. The star sheds its outer layers and sends out a wave of very lethal charged particles. That wave expands and eventually reaches other stars, and their planets, like Earth. Black holes also cause charged particles, as do many other events in the wider galaxy. Particles could even travel from other galaxies and after a gazillion years end up knocking at our door.
Another source of charged particles is our sun. It sends out a constant wave of the buggers. It’s what we call the solar wind — or at least part of it is.
And if you’re wondering what the big deal about ‘charged particles’ really is… Go read this old post of mine. TL;DR: charged particles are a form of radiation. Prolonged direct exposure to charged particles is not good for you.
Of course, Earth’s atmosphere also protects us from charged particles. The thing is, without the magnetosphere, the solar wind would slowly bleed off Earth’s atmosphere, one particle at a time. It’s what happened to Mars, which has no molten core, and so no magnetic field to speak of.
A breath of life
As you can see, the magnetosphere is pretty important. Without it, Earth would become uninhabitable.
And that’s one of the things to keep in mind about exoplanets. They can exist in the so-called goldilocks zone of the solar system, meaning they could have liquid water, be roughly the size of Earth, and still be incapable of supporting life. Like Mars is.
Back to my original observation. As I wrote at the top of this post, I was reading a scifi novel about an exoplanet. That planet was supposedly four times the size of Earth, but inhabited, and with roughly the same gravity as Earth. That was possible because it was far less dense, holding no metals.
That set of alarm bells in my head. A planet without metal would not have a magnetosphere, meaning it would have no atmosphere. Well, more or less. You could maybe terraform it far enough to support life in domes, or underground caves. Or it could have alien life that wasn’t bothered by radiation. But no, this was supposedly a planet of humans, that walked outside in the open air.
And yeah, it’s nitpicking, and it didn’t matter to the story at all. But, if you ever run into this kind of thing yourself when writing, you can think of this post and fix it. Or, if you’re a reader, you can think of this post and smile.