28978 Ixion is a plutino (an object that has a 2:3 orbital resonance with Neptune). It is very likely to be a dwarf planet, although the IAU has not officially classified it as such. Light-curve-amplitude analysis shows only small deviations, which suggests that Ixion is a spheroid with small albedo spots and hence a dwarf planet. It has a diameter of approximately 650 km, making it about the fifth largest plutino. It is moderately red in visible light and has a surface made of a mixture of tholin and water ice.
It was discovered on May 22, 2001. Ixion was discovered by the Cerro Tololo Inter-American Observatory (807). It is named after Ixion, a figure from Greek mythology