Plasmas and their embedded magnetic fields affect the formation and evolution of planets and planetary systems. The heliosphere shields the Solar System from galactic cosmic radiation. Earth is shielded by its magnetic field, protecting it from solar and cosmic particle radiation and from erosion of the atmosphere by the solar wind. Planets without a shielding magnetic field, such as Mars and Venus, are exposed to those processes and evolve differently. On Earth, the magnetic field changes strength and configuration during its occasional polarity reversals, altering the shielding of the planet from external radiation sources.