GD 356 is unique among magnetic white dwarfs because it shows Zeeman-split Balmer lines in pure emission .
The lines originate from a region of nearly uniform field strength ( \delta B / B \approx 0.1 ) that covers 10 per cent of the stellar surface in which there is a temperature inversion .
The energy source that heats the photosphere remains a mystery but it is likely to be associated with the presence of a companion .
Based on current models we use archival Spitzer IRAC observations to place a new and stringent upper limit of 12 M _ { J } for the mass of such a companion .
In the light of this result and the recent discovery of a 115 min photometric period for GD 356 , we exclude previous models that invoke accretion and revisit the unipolar inductor model that has been proposed for this system .
In this model a highly conducting planet with a metallic core orbits the magnetic white dwarf and , as it cuts through field lines , a current is set flowing between the two bodies .
This current dissipates in the photosphere of the white dwarf and causes a temperature inversion .
Such a planet is unlikely to have survived the RGB/AGB phases of evolution so we argue that it may have formed from the circumstellar disc of a disrupted He or CO core during a rare merger of two white dwarfs .
GD 356 would then be a white dwarf counterpart of the millisecond binary pulsar PSR 1257+12 which is known to host a planetary system .