Magnetosphere

A magnetosphere is the bubble of space around a planet where its magnetic field dominates. It deflects most of the solar wind — the stream of charged particles flowing from the Sun — and traps some particles in belts around the planet.

Earth's magnetic field is generated by moving liquid iron in its outer core. Where solar wind particles do get funnelled in near the poles, they make the upper atmosphere glow as aurorae.

Mercury, Earth, Jupiter, Saturn, Uranus and Neptune all have global magnetic fields. Mars lost its field billions of years ago. How much a magnetosphere is needed to keep an atmosphere is still debated: Venus has no global field but keeps a very thick atmosphere.

Read more in Step 4: Adding an atmosphere.