The magnetosphere is the region of space in which a planet's magnetic field dominates that of the solar wind. It is distorted into a teardrop shape by the solar wind pushing on the dayside and drawing out a long magnetotail on the nightside. Earth's magnetosphere normally extends about 10 Earth radii on the dayside, while its tail stretches out several hundred Earth radii in the anti-sunward direction. It is, however, a highly dynamic structure that responds dramatically to changes in the dynamic pressure of the solar wind and the orientation of the interplanetary magnetic field. Its ultimate source of energy is the interaction with the solar wind. Some of the energy extracted from this interaction goes directly into driving various magnetospheric processes, while some is stored in the magnetotail, to be released later in substorms.

Significant magnetospheres also exist around Mercury and the four gas giants of the Solar System – Jupiter, Saturn, Uranus, and Neptune. The Sun's own, enormous magnetosphere is called the heliosphere.