The Drake formula is a statistical method of forecasting the possible number N of technically evolved extraterrestrial and galactic civilizations able to communicate with the human species. It is based on seven different factors that can be grouped into factors of type A, fA (“Astrophysicist”) and type B, fB (“Astrobiological”). The quantitative analysis of these factors at the time of the presentation of the formula was subjective and highly variable for both factors fA and fB. Current scientifical and technological development has made it possible to refine the quantitative estimates of the fA group whose definition is now less uncertain. In group fA the parameter ne is understood as the number of planets capable of sustaining life. By means of ne Drake defines this possibility exclusively from the geometric point of view. In particular, the planet’s orbit must be included in the circumstellar space in which the planetary temperature allows the presence of liquid water. This is not enough because, for liquid (and gaseous) water to be present on the planet’s surface, it is also essential that the planet has a magnetic field of adequate intensity to shield the flow of charged particles coming from its star (solar wind). The solar wind is able to break up and disperse the liquid and gaseous water molecules and any organic molecules in times much shorter than theoretically necessary for the formation of life and above all, except for singularities, than necessary for evolution to arrive at intelligent life. Here the planetary magnetic field parameter nm is introduced into the Drake formula and its statistical probability of existence is discussed.
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