Planet Earth and Planet Mercury have one thing in common: They are the only magnetized planets in the Solar System; The only ones on which you could navigate using a magnetic compass. Earth’s magnetic field is strong, protecting us from bursts of hot plasma spit out by the Sun. But Mercury’s magnetosphere is much weaker. And the rocky little world orbits 2/3 closer to the Sun. The Solar Wind scours Mercury’s poles, blasting sodium and oxygen particles; charging them… … much as oxygen and nitrogen in Earth’s atmosphere fluoresce to form auroras in our polar skies. Sun storms can damage satellites in Earth orbit… …and NASA’s MESSEGER science probe, in Mercury orbit, must weather a constant hurricane of charged particles… … as it continues its quest to understand the small planet closest to our star. Planets like Mercury and Mars have lost their atmospheres to the scalding Sun. Only Earth’s magnetosphere protects us from a similar fate. For SPACE.com, I’m Dave Brody