  We analyze ISO archive data of the nearby bright emission-line star 51 Oph,
previously classified as a proto-planetary system similar to beta Pic. The
infrared spectrum reveals the presence of gas-phase emission bands of hot
(approx 850 K) CO, CO_2, H_2O and NO. In addition to this, partially
crystalline silicate dust is present. The solid-state bands and the energy
distribution are indicative of dust that has formed recently, rather than of
debris dust. The presence of hot molecular gas and the composition of the
circumstellar dust are highly unusual for a young star and are reminiscent of
what is found around evolved (AGB) stars, although we exclude the possibility
of 51 Oph belonging to this group. We suggest several explanations for the
nature of 51 Oph, including a recent episode of mass loss from a Be star, and
the recent destruction of a planet-sized body around a young star.
