In this step, we will estimate whether the world under development has a significant magnetic field.

The possible cases for a world’s magnetic field will be sorted into four categories: None, Weak, Moderate, and Strong, defined as follows.

• None: The world has no detectable magnetic field and is completely unprotected from the stellar wind. Examples: Venus, Earth’s moon, Mars, most of the gas giant planets’ major satellites.
• Weak: The world has a detectable magnetic field (about 1% as strong as Earth’s), but it offers no significant protection from the stellar wind. Examples: Mercury or Ganymede.
• Moderate: The world’s magnetic field is strong enough to offer limited protection against the stellar wind (about 10% as strong as Earth’s). Examples: None in our planetary system.
• Strong: The world’s magnetic field is at least comparable to that of Earth, sufficient to provide adequate protection against the stellar wind. Examples: Earth, the gas giant planets.

A world’s magnetic field seems to depend on several items:

• The world needs to have a hot, liquid outer core of significant mass, composed largely of iron
• There must be convection taking place in that iron outer core, causing rising and falling currents
• The world must rotate on its axis

If all three of these conditions hold, the iron outer core forms a dynamo which creates a significant magnetic field. This, in turn, helps protect the world’s atmosphere from being stripped away by stellar wind, and also protects the surface of the world from some harmful radiation. In our own planetary system, only Earth and the gas giant planets have strong magnetic fields.

Note that the third condition – that the world must rotate on its axis – is almost universal. Even a tide-locked world still rotates on its axis, and physical modeling seems to indicate that even slow rotation is enough to support a working dynamo. The existence of strong convective heat transfer through a world’s outer core seems to be the critical factor.

Procedure

To determine the strength of a world’s magnetic field at random, roll 3d6 modified as follows:

• +4 if the world has a Soft Lithosphere
• +8 if the world has Early Plate Lithosphere or Ancient Plate Lithosphere and also has Mobile Plate Tectonics
• +12 if the world has Mature Plate Lithosphere and also has Mobile Plate Tectonics

Refer to the Magnetic Field Table entry for the modified roll.

Magnetic Field Table
Modified Roll (3d6)	Magnetic Field
14 or less	None
15-17	Weak
18-19	Moderate
20 or greater	Strong

Examples

Arcadia IV has a Mature Plate Lithosphere and Mobile Plate Tectonics, so Alice rolls 3d6+12 and gets a result of 25. Arcadia IV has a Strong Magnetic Field.

Arcadia V has a Mature Plate Lithosphere but has Fixed Plate Tectonics. Alice rolls an unmodified 3d6 and gets a result of 13. Arcadia V has no significant magnetic field.