28978 Ixion

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A large planetoid in orbit of Sol, 28978 Ixion lies beyond the orbit of Pluto with a semimajor axis of 39.65 AU, and has an estimated size of about 650 kilometers.

The object is called a plutino (an object that has a 2:3 orbital resonance with planet Neptune). It is considered very likely to be a spheroid dwarf planet with small albedo spots. It is moderately red in visible light and has a surface made of a mixture of tholin and water ice.

Physical characteristics

Ixion has a high albedo and is smaller than the asteroid Ceres. It is moderately red (slightly redder than 50000 Quaoar) in the visible light. It also has a higher albedo (>0.15) than the mid-sized red cubewanos. There may be an absorption feature at the wavelength of 0.8 μm in its spectrum, which is commonly attributed to the alteration of surface materials by water. In the near-infrared the spectrum of Ixion is flat and featureless. Water ice absorption bands at 1.5 and 2 μm are absent. This is in contrast to Varuna, which has a red spectral slope in the near-infrared as well as prominent water absorption bands.

Both visible and infrared spectroscopic results indicate that Ixion's surface is a mixture of water ice, dark carbon and tholin, which is a heteropolymer formed by irradiation of clathrates of water and organic compounds. It is possible that Ixion could develop a coma or temporary atmosphere when it is closer to perihelion.

Orbit and rotation

Ixion and Pluto follow similar but differently oriented orbits: Ixion’s perihelion is below the ecliptic whereas Pluto's is above it.

Uncharacteristically for bodies locked in resonance with Neptune (such as Orcus), Ixion approaches Pluto with less than 20 degrees of angular separation. Ixion's orbital period is almost 250 standard years, about 0.5% larger than Pluto's. Ixion does demonstrate some regular changes in brightness, which are thought to be caused by its rotation.