Planets around white dwarfs

The fate of a planetary system like our own, as the parent star expands through the red giant phase and becomes a white dwarf (WD), has been a topic of some discussion. For an Earth-like inner planet, the conducting core may remain intact, even though severe ablation occurs of the outer layers. We argue that a planetary core in orbit around a WD may reveal its presence through its interaction with the magnetosphere of the WD. As the planet moves through the magnetosphere, electrical currents will be generated, which will heat the atmosphere of the WD near its magnetic poles. The results of such a heating may be detected in the optical as H alpha emission. Ohmic dissipation will result in the slow decay of the planetary orbit, and such a planet will merge with the WD in less than a Hubble time, unless the initial orbital 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.