How is there a difference of potential between two planets? Theoretically, it's not unreasonable, when one considers the environment of space. While it is currently agreed that space is a relative, as opposed to an absolute, vacuum, this means that there is some material/matter in the void between bodies; this makes sense, because there must be SOMETHING, of some kind, for radio waves to propagate..a propagation medium, or something for the waves charge/act upon. Depending on the density/composition of this matter/material, a body passing through it will pick up or lose a charge due to friction, as well as imparted from the ionization of the matter, and radiation in the area. Also, space is FULL of ELECTRO-MAGNETIC radiation (radio, microwave, xrays, etc), all of which are striking planets, asteroids, nebulae, etc. constantly.
Now, after the dryness of the above paragraph, have some examples; in the dry winter air, or fresh out of the dryer, clothes tend to build up a static charge, sometimes quite a high one. The static cling of you clothes is built up due to friction...same thing with aircraft...static build up due to the friction with the air, and even moreso when flying through highly charged storm areas; you've seen the lightning storms, with an electrical arc several thousand feet long, and all due to movement/friction of air, and airborne particles. Similarly, in space, there are ionized (charged) regions of space, and all that elecro-magnetic radiation with its ionizing effects, all acting on any planetary body present in that environment.
I think it probably depends on the planet's own magnetic field strength, and whether it is strong enough to protect the planet from being turned into a terminal for a discharge. Depending also on the planet's structure, it may be able to slowly release a static charge back into it's local environment, much like aircraft do...if you've noticed, on the trailing edge of an aircraft's wing and tail/rudder, you can see little 'whiskers' pointing backwards...these are static discharge wicks, to prevent the build-up of harmful potentials in the aircraft's skin/airframe.