A Search for X-ray emission from Saturn, Uranus and Neptune
  We present an analysis of X-ray observations of the trans-Jovian planets Saturn, Uranus and Neptune with the ROSAT PSPC in comparison with X-ray observations of Jupiter. For the first time a marginal X-ray detection of Saturn was found and 95% confidence upper limits for Uranus and Neptune were obtained. These upper limits show that Jupiter-like X-ray luminosities can be excluded for all three planets, while they are consistent assuming intrinsic Saturn-like X-ray luminosities. Similar X-ray production mechanisms on all trans-Jovian planets can therefore not be ruled out, and spectral shape and total luminosity observed from Saturn are consistent with thick-target bremsstrahlung caused by electron precipitation as occurring in auroral emission from the Earth. 
Jan-Uwe Ness and Juergen H.M.M. Schmitt