In general, magnetic fields are produced by electrical currents. There are at least two ways in which to get magnetic fields which may be relevant to planets:

• there can be large-scale electrical currents
• there can be something analogous to a bar magnet in the planets

Both of the above produce magnetic fields with the familiar shape of a bar magnet:

Amazingly enough, if you look at the magnetic field of the Earth and at local regions on the Sun,

the field shapes are very similar to that of a simple bar magnet.

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Magnetic Fields and the Interiors of Terrestrial Planets The interior of the Earth (and most of the other Terrestrial planets) is too hot for a permanent bar magnet to exist. So, in order for the Earth to have a magnetic field, it seems as if the interior of the Earth must be able to support strong electrical currents. This suggests that the core of the Earth should be something like liquid (molten) iron. This important property of the core of the Earth thus follows simply from the fact that the Earth has a reasonably strong magnetic field! (There are other issues involved, however.)