# Can Magnetars destroy planets?

Magnetars are the most magnetic things in our universe. Because of how powerful their magnetic fields are, they could split every single atom apart from each other in our body.

Because of this could they destroy planets (rocky or not)? If so, how close would the planet have to be?

Perhaps one way of answering this is to note that the (approximately) dipole magnetic fields of a magnetar fall off radially as something like $r^{-3}$. So at the magnetar surface the fields could be of order $10^{11}$ Teslas (at a maximum), but these are reduced by a factor of $\sim 8$ at 2 magnetar radii, a factor of $\sim 27$ at 3 magnetar radii etc.

Now given that the radius of a magnetar is going to be $\sim 10$ km, then the fields die away pretty quickly.

So let's say the Earth orbited a magnetar. The field strength at $1.5 \times 10^{11}$ m is going to be $\sim 10^{11} \times (10^4/1.5\times10^{11})^3 = 3 \times 10^{-11}$ Teslas - much smaller than the Earth's magnetic field.

To put this in context. The field at the surface of a small neodymium magnet are of order 1 Tesla and they don't disrupt your atoms when you pick them them up. To get a field this strong from a magnetar you would have to get to within about 50,000 km of it. The synchrotron radiation field would have fried you before then and any planet would have been ripped apart by tidal forces about an order of magnitude further away. The Roche limit given by $$R_{Roche} \sim 1.26 R_1 \left(\frac{\rho_1}{\rho_2}\right)^{1/3},$$ where $\rho_1$ is the density of the neutron star ($\sim 3 \times 10^{17}$ kg/m$^{3}$), $\rho_2$ is the density of the Earth ($\sim 5500$ kg/m$^3$) and $R_1$ is the radius of the neutron star (10 km). This yields $R_{roche} = 480,000$ km.

Completely disregarding magnetic fields for now, an extremely dense object like a magnetar (which is a kind of pulsar, which itself is a kind of neutron star) could destroy planets just with gravity (see Roche Limit). Now, if the planet is far away from the star, things might not be necessarily that ominous. The problem with electromagnetic forces is that their range is not as far-reaching as gravity. In fact, the first exoplanets ever detected were found around a pulsar (PSR B1257+12), and these objects already have pretty damn strong magnetic fields.

Here's the thing: atoms nuclei are held up by outrageously strong nuclear forces, but they are even more short-ranged than electromagnetic forces. I haven't run any numbers on this, but I think that these forces are so strong, that in order to break apart the atoms of the rocky planet just by using the magnetic fields, the planet would have to be so close to the magnetar, that its gravity would already have destroyed the planet.

However, the way gravity and electromagnetism precisely work on such extraordinary environments like the regions around a magnetar are still mysteries to be solved. Magnetism is counter-intuitive and the calculations are hard (been there). Even so, I think gravity would still win.

• Thanks for your answer. If gravity didn't destroy the planet first, would the magnetic field from the Magnetar destroy planets or not? Feb 21 '15 at 16:33
• Well, as far as I can tell, given a sufficiently strong magnetic field, it will distort the electronic cloud around an atom's nucleus, but this is just considering one atom. The way chunks of matter containing various atoms behave under these circumstances can be extremely complex. But destruction is a very probable outcome, at some point. I'm afraid I can't tell you much more than that. Feb 21 '15 at 17:02
• Atoms are held together with electromagnetic forces also. Perhaps you are thinking of outrageously strong nuclear forces? Feb 21 '15 at 20:26
• Yes, indeed. I will edit the answer. Feb 21 '15 at 20:28
• Maybe the best way is to look at the forces that bind molecules, instead of atoms, because those are weaker. Magnetic forces can certainly disrupt them, but again, the range is small compared to gravity. Feb 21 '15 at 20:38