Deep in the interior of Earth, there is a hot, solid inner core, which is surrounded by an outer core of electrically conductive liquid iron. The heat of the solid inner core creates convection currents in the liquid outer core. The rotation of the planet also acts on the liquid material to create whirlpool-like motions in the fluid. These motions of the conductive liquid iron generate an electric current, which in turn produces a magnetic field. The continued motions of the conductive fluid through this magnetic field maintains Earth's global magnetic field.

This self-sustaining mechanism for maintaining a magnetic field is known as the dynamo effect. Other materials, such as ionized gases, can generate a dynamo to create magnetic fields. The Sun, other stars, and other planets, also possess magnetic fields.

Earth's global magnetic field has magnetic poles located near the geographic North and South Poles, and the shape of the field is similar to the magnetic dipole field of a bar magnet. The magnetic field surrounds the planet and extends into space, forming Earth's magnetosphere. The interaction between the magnetic field and charged particles of the solar wind (the stream of charged particles ejected from the Sun) shapes the magnetosphere.

Although Earth's magnetic field deflects most of the solar wind, some solar particles are able to flow into Earth's atmosphere along the magnetic field lines at the North and South Poles. There they can excite the gases in the atmosphere to create colorful light displays known as auroras. Earth's magnetic field also allows compasses to be useful for navigation.

But magnetic fields are not permanent. Earth's magnetic field changes strength, and the poles drift. It even completely reverses direction occasionally so that the poles flip (the last reversal was about 740,000 years ago). The Sun has a complex magnetic field that reverses every 22 years; it also has local magnetic fields that arch over small areas, which are responsible for creating sunspots. Mars also has irregular, local magnetic fields that are likely the remnants of a global magnetic field. Billions of years ago, Mars had a planetary magnetic field that was produced by a dynamo similar to Earth's, but for some reason (possibly giant asteroid impacts) the field was shut down.

Scientists study the magnetic fields of stars and planets to gain a better understanding of the origin and evolution of the solar system. For example, studying planetary magnetic fields can provide information about the internal structure of a planet or how its atmosphere may have changed. Studying magnetic fields can also lead to a better understanding of the development of life.

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