A search for X-ray emission from Saturn, Uranus and Neptune

We present an analysis of X-ray observations of the trans-Jovian planets Sa turn, Uranus and Neptune with the ROSAT PSPC in comparison with X-ray obser vations 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 obta ined. These upper limits show that Jupiter-like X-ray luminosities can be e xcluded 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 an d total luminosity observed from Saturn are consistent with thick-target br emsstrahlung caused by electron precipitation as occurring in auroral emiss ion from the Earth.