THREE/3 Magnetic Field

STEP ONE: Generate the magnetic field factor on 3.3.1.

STEP TWO: Roll on Table 3.3.2 to determine the strength of the field, compared to Earth.

Table 3.3.1 Magnetic Field Factor

 MagFactor = 10 x   1   x D2 x M0.5 / A 
(P / 24)0.5

... where: P is the rotation period (in hours)
D is the density (in earths)
M is the mass of the planet (in earths)
A is system age (in Gy)

Icy Worlds: For worlds made up mostly of ice multiple Magnetic Field Factor by 0.5.

Note that for tidally locked worlds or for worlds with a rotation period longer than a local year use the local year as the rotation period.

Table 3.3.2 Magnetic Field Strength

1d10  Magnetic Field Factor
≤0.50  0.51 - 1.00  1.01 - 2.00  2.01 - 4.00  ≥4.01
1-3  None  None  1d10 x 0.001  1d10 x 0.050  1d10 x 0.100
4-5  None  None  1d10 x 0.002  1d10 x 0.100  1d10 x 0.200
6-7  None  1d10 x 0.001  1d10 x 0.010  1d10 x 0.200  1d10 x 0.300
8-9  1d10 x 0.001  1d10 x 0.002  1d10 x 0.050  1d10 x 0.300  1d10 x 0.500
10  1d10 x 0.010  1d10 x 0.010  1d10 x 0.100  1d10 x 0.500  1d10 x 1.000


To have a magnetic field of a decent size, a world must have at least part liquid metallic composition, typically in the core. It does not have to be that much, Mercury has a weak magnetic field despite most of its core is believed to be solid. The strength of the magnetic field is also dependant upon how fast a world rotates. It also is likely to be affected by specifics of the core (liquid FeS abundance, in particular), and thus two rather similar worlds like Earth and Venus can have very different magnetic fields (Venus has 1/1000th the magnetic field of Earth). Magnetic fields can vary in strength over time quite much.


Though all worlds probably have some magnetic field, it may be so small as to be uninteresting. Venus and Mars both have very small magnetic fields.


Earth has a magnetic field of about 0.305 Gauss. Thus, to determine the Gauss strength of a world's magnetic field just multiply by 0.305.


Magnetic fields tend to be inclined to the rotational axis of a world. If you wish to simulate this, you may decide the inclination (compared to the axis) by rolling on the Axial Tilt table (2.2.3).


The stronger a magnetic field is, the further away from a world it extends. A world with a weak (0.01-0.001) magnetic field has a magnetospause about 1 world radii away from the surface. For an Earth-sized field, 10 world radii are more typical. Worlds with magnetic fields 10 times the size of Earth's have magnetospauses perhaps 50-100 radii away. Actually, the magnetic field extends much farther in the "tail" direction from the primary, and for gas giants the magnetic field may be significantly offset compared to the center of the planet too.


The magnetic field of a world helps to shield it from the solar wind and cosmic radiation. However, a dense atmosphere may cover for the lack of a average magnetic field. It also generates the phenomena we know as auroras.


Magnetic fields change inclination (on Earth, we use this fact when we see that the deviations of compasses change from decade to decade), but they also reverse (changing North and South poles) from time to time.