  The planets magnetic field has been explained based on the dynamo theory,
which presents as many difficulties in mathematical terms as well as in
predictions. It proves to be extremely difficult to calculate the dipolar
magnetic moment of the extrasolar planets using the dynamo theory. The aim is
to find an empirical relationship (justifying using first principles) between
the planetary magnetic moment, the mass of the planet, its rotation period and
the electrical conductivity of its most conductive layer. Then this is applied
to Hot Jupiters. Using all the magnetic planetary bodies of the solar system
and tracing a graph of the dipolar magnetic moment versus body mass parameter,
the rotation period and electrical conductivity of the internal conductive
layer is obtained. An empirical, functional relation was constructed, which was
adjusted to a power law curve in order to fit the data. Once this empirical
relation has been defined, it is theoretically justified and applied to the
calculation of the dipolar magnetic moment of the extra solar planets known as
Hot Jupiters. Almost all data calculated is interpolated, bestowing confidence
in terms of their validity. The value for the dipolar magnetic moment, obtained
for the exoplanet Osiris (HD209458b), helps understand the way in which the
atmosphere of a planet with an intense magnetic field can be eroded by stellar
wind. The relationship observed also helps understand why Venus and Mars do not
present any magnetic field.
