Triton, or Neptune I

Triton is the largest satellite of Neptune. It is the seventh largest satellite in the solar system and is almost certainly a dwarf planet captured from the Kuiper belt. The principal evidence for this claim is that its orbit around Neptune is retrograde and so very unlikely to have been formed in orbit around the growing planet.

The over-exposed disk of Neptune appears in the centre of this image. Triton is nearby and in the 8-o'clock position relative to the planet. Neptune was at magnitude 7.95 at the time of observation whereas Triton was 13.58.