Planets around white dwarfs

The fate of a planetary system like our own, as the parent star expands thr ough the red giant phase and becomes a white dwarf (WD), has been a topic o f some discussion. For an Earth-like inner planet, the conducting core may remain intact, even though severe ablation occurs of the outer layers. We a rgue that a planetary core in orbit around a WD may reveal its presence thr ough its interaction with the magnetosphere of the WD. As the planet moves through the magnetosphere, electrical currents will be generated, which wil l heat the atmosphere of the WD near its magnetic poles. The results of suc h a heating may be detected in the optical as H alpha emission. Ohmic dissi pation will result in the slow decay of the planetary orbit, and such a pla net will merge with the WD in less than a Hubble time, unless the initial o rbital separation is greater than about 10 solar radii. We propose that the peculiar emission-line WD GD 356 may be a system in the process of such a merger.