A magnetosphere's a highly magnetized region around and possessed by an astronomical object. Earth's surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede's magnetized, but too weak to trap solar wind plasma. Mars has patchy surface magnetization. The term magnetosphere has also been used to describe regions dominated by the magnetic fields of celestial objects, for example pulsar magnetospheres. History of magnetosph… (More on Magnetosphere)