Why does the earth have a magnetic field with two poles?

Answer

A magnetic field always has 2 poles: a north pole and a south pole. This is analogous to an electric current, which (can be seen clearly from any battery) has a positive terminal and a negative terminal. Electric current and magnetism are closely related – everyday applications are the electric motor and electromagnets.

For detailed information about the how and why of magnetic fields in planets, I refer you to the website of Urania, the public observatory of Hove. At http://www.urania.be/ Sterrenkunde/zonnesysteem/aarde.php you will find information about the earth, including its magnetic field.

Not all planets have a magnetic field: it depends on whether there are metals in the core. The origin of magnetic fields in other celestial bodies is explained on http://www.urania.be/ Sterrenkunde/zonnesysteem/terresrische.php , where more information can also be found about magnetic fields.