  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.
