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:
Both of the above produce magnetic fields which the familiar shape which can be represented as:
Amazingly enough, if you look at the magnetic field of the Earth,
it appears to be very similar to the simple bar magnet (everyone played with as a kid).
The interior of the Earth (and most of the other Terrestrial planets) is clearly too warm 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.
An interesting such as the properties of the core of the Earth may thus be deduced simply from the fact that the Earth has a reasonably strong magnetic field!