Abell 31 (also known as Sh2-290) is a planetary nebula of nearly ten light-years wide, located about 2,000 light-years away from Earth in the constellation of Cancer. Although it is one of the largest planetary nebulae of the sky, Abell 31 is not very bright.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose in the 18th century because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through small optical telescopes.