What is a magnetosphere and why is it important?

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I’m reading the Expanse series (they’re fantastic, highly recommended) and they always talk about planets needing a magnetosphere for human life. I’ve read the definition and it doesn’t really tell me what about it makes it so critical for humans. Can any one explain it?
Thanks and sorry if I tagged this wrong.

In: Physics

The magnetosphere is the sphere-like enclosure around a planet created by magnetic fields created in the core.

It has the effect of shielding the planet from harmful cosmic radiation, which can damage organic matter and strip the atmosphere from the planet.

Planets that are geologically active – that is, they have a hot, liquid metal outer core – generate a *huge* magnetic field around them. That magnetic field is the magnetosphere. On Earth, one of the earliest apparent uses was for compasses, which point towards the magnetic poles of this magnetic sphere. Compasses are *useful* but they’re not *necessary*.

What *is* necessary is the role the magnetosphere plays in blocking solar winds. The Sun doesn’t just blast out light, it also blasts out charged particles just like a gigantic constant nuclear explosion (which is what it is). As the name “solar wind” might suggest, this constant blast of charged particles is a lot like wind coming from the Sun, pushing at whatever it runs into. It’s a very very *very* light wind, it won’t move a planet. But it *will* move the very light gasses around the planet. The solar wind can slowly, over the course of eons, strip away a planet’s atmosphere. The magnetosphere blocks those charged particles like a force field, channeling them *around* the planet so they don’t strip away our atmosphere.

Obviously losing our atmosphere would be a bad thing, because we need the atmosphere full of oxygen to breathe, and to insulate the planet so it stays within a livable temperature range. But the atmosphere plays another role, which is to absorb harmful high-energy light like UV, Xray, or even the occasional gamma ray. Most of that comes from the Sun, but some high-energy rays come from outer space. The atmosphere is like a huge, thick blanket that blocks those rays from hitting the surface, mostly. Sure, you can still get a sunburn from the leftover UV rays that make it through. But think about how dangerous those UV rays are *with* the atmosphere protecting us and imagine how much worse it would be without that.

The solar wind is usually the equivalent of a constant light breeze, but occasionally the surface of the Sun explodes with solar flares, which send out unimaginably large blasts of charge particles, which are powerful enough to ionize the atmosphere and burn through it to scour the surface of the Earth clean of living things. Luckily, the magnetosphere helps with those, too. Because the particles coming from the Sun are charged, they follow the lines of the magnetic field and again, mostly go *around* the planet. Some of them get funneled to where the magnetic field lines bunch up and meet, which is at the poles – which is what causes the Northern and Southern Lights. Without the magnetosphere protecting us, every solar flare that hit us would have the potential to kill much of the life on Earth.

The Sun also has a magnetosphere! The Sun’s is so large that it extends beyond the orbit of Neptune! And it protects the whole solar system in much the same way that Earth’s protects just Earth. There are super high energy charge particles coming from outer space, from sources like supernovae, which would overwhelm the Earth’s magnetosphere. But the Sun’s magnetosphere is strong enough to block them.