# How would the strong magnetic field of a white dwarf affect humans inhabiting a planet that orbits such a star?

Magnetic white dwarfs (MWD) comprise almost 2 % of all white dwarfs and they are characterized by having a strong magnetic field, whose strength varies between 1 T and 100 kT. Compared to the Earth's magnetic field, whose strength varies between 25 µT and 65 µT, the magnetic field of a MWD is between 40 000 and 10⁹ times stronger than that of our planet.

On the other hand, the existence of exoplanets orbiting white dwarfs has already been proposed and they are likely to have a semi-major axis less than 0.02 AU. This made me wonder: is this distance really safe for the human's health or for electronic devices? I mean, suppose there is a planet orbiting a MWD with a strong magnetic field at a distance less than 0.02 AU. This planet is terraformed and inhabited by humans, forming a society like ours, very technologically advanced. So how would the magnetic field of the host star affect the inhabitants of this planet?, would they be able to tolerate it or would they die?, what would happen to commonly used electronic devices?, would they be damaged?

Please note that I am setting aside all the inconvenience of terraforming an exoplanet orbiting a white dwarf for practical reasons.

• The field strength you mention is on surface of white dwarf. In the distance of habitable zone it is much lower. – Juraj Dec 8 '19 at 13:37
• All their compasses would point up and down instead of north and south. – A. I. Breveleri Dec 9 '19 at 5:21

# It wouldn't.

The field of a magnetic dipole has a strong radial dependence; it falls off proportional to $$r^{-3}$$, where $$r$$ is the distance to the dipole. The values you list are the strengths of the white dwarfs' magnetic fields at their surfaces. The field strength at the orbit of a planet would be substantially weaker. As an example, take GRW +70 8247, the first known magnetic white dwarf. It has a radius of 2 km (on the low end) and a surface field of $$B_{\text{surf}}\sim30\text { kT}$$ (Koester & Chanmugan 1990). Therefore, the magnetic field at 0.02 AU should be $$B(0.02\text{ AU})=B_{\text{surf}}\left(\frac{0.02\text{ AU}}{2\text{ km}}\right)^{-3}\approx10^{-14}\text{ Tesla}$$ and we see that this is tiny in comparison to the surface magnetic field of Earth.

Most white dwarfs should fall in the same range, with some reaching field strengths of $$\sim10^{-13}\text{ T}$$ at 0.02 AU. After looking at these numbers, I was curious, of course, about magnetars, which are neutron stars with extremely powerful magnetic fields. I wanted to see if even they could produce any drastic effects. A magnetar might have a radius of about 10 km and a surface field of $$B_{\text{surf}}\sim10^{11}\text{ T}$$, at the upper end, producing $$B(0.02\text{ AU})\approx4\times10^{-6}\text{ T}$$ - only an order of magnitude lower than Earth's magnetic field at Earth's surface. I think that even this wouldn't be as severe as what you're looking for.

As a side note, I wanted to mention that the "0.02 AU" distance you cite is likely the outermost limit of the orbits of habitable exoplanets around a white dwarf. Planets can certainly exist outside that zone, and in fact many white dwarfs will retain any planets of their progenitors that survived the star's red giant phase. As red giants (including AGB stars) may reach radii of 1-2 AU, surviving planets will likely orbit beyond this, unless migration due to tidal forces takes place.

1. The negative factors for life appearance are lack of any energy or strongly chaotic energies. In the described situation nothing of the kind is present.
2. If life appeared in some conditions, they are good for it. Any strong regular energy will be USED. As the planet with all on it is moving in the strong magnetic field, they all are electric batteries.
3. Absolutely no problem to use electro technologies there. For they know electricity from birth. And the changing external magnetic influence can be blocked by a simple Faraday cage.
4. As for our, earthly devices, the visitors should use devices made for army for the use in the nuclear war. Again - Faraday cage and grounding solve the problem.