Magnetized planets, including Earth and Saturn, are surrounded by a magnetosphere, a magnetic bubble that generally repels the solar wind. Magnetic reconnection—the breaking and rejoining of magnetic field lines—occurs at Earth's magnetopause (the boundary of the magnetosphere), allowing solar wind energy to enter the magnetosphere. At Earth, magnetic reconnection is the main driver of magnetosphere dynamics. At Saturn's magnetopause, however, spacecraft have found limited evidence for magnetic reconnection to date. Theory suggests that magnetic reconnection is more likely to happen when the ratio of plasma pressure to magnetic field pressure, known as plasma beta, is lower.