28978 Ixion (; provisional designation ) is a large trans-Neptunian object. It is located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Ixion is classified as a plutino, a dynamical class of objects in a 2:3 orbital resonance with Neptune. It was discovered in May 2001 by astronomers of the Deep Ecliptic Survey at the Cerro Tololo Inter-American Observatory, and was announced in July 2001. The object is named after the Greek mythological figure Ixion, who was a king of the Lapiths.
In visible light, Ixion appears dark and moderately red in color due to organic compounds covering its surface. Water ice has been suspected to be present on Ixion's surface, but may exist in trace amounts hidden underneath a thick layer of organic compounds. Ixion has a measured diameter of , making it the fourth-largest known plutino. It appears to be a transitional object between irregularly-shaped small Solar System bodies and spherical dwarf planets. Ixion is currently not known to have a natural satellite, so its mass and density are unknown.
History Discovery Ixion was discovered on 22 May 2001 by a team of astronomers at the Cerro Tololo Inter-American Observatory in Chile. Led by Robert L. Millis, the discovery took place through the Deep Ecliptic Survey, an astronomical survey for Kuiper belt objects located near the ecliptic plane, using telescopes of the National Optical Astronomy Observatory. On the night of 22 May 2001, American astronomers James Elliot and Lawrence Wasserman identified Ixion in digital images of the southern sky taken with the 4-meter Víctor M. Blanco Telescope at Cerro Tololo. Ixion was first noted by Elliot while compiling two images taken approximately two hours apart, which revealed Ixion's slow motion relative to the background stars. At the time of discovery, Ixion was located in the constellation of Scorpius.
The discoverers of Ixion noted that it appeared relatively bright for a distant object, implying that it might be rather large for a trans-Neptunian object. The discovery supported suggestions that there were undiscovered large trans-Neptunian objects comparable in size to Pluto. Since Ixion's discovery, numerous large trans-Neptunian objects, notably the dwarf planets Haumea, , and Makemake, have been discovered; in particular, Eris is almost the same size as Pluto.
The discovery of Ixion was formally announced by the Minor Planet Center in a Minor Planet Electronic Circular on 1 July 2001. It was given the provisional designation , indicating that it was discovered in the second half of May 2001. Ixion was the 1,923rd object discovered in the latter half of May, as indicated by the last letter and numbers in its provisional designation.
At the time of discovery, Ixion was thought to be among the largest trans-Neptunian objects in the Solar System, as implied by its high intrinsic brightness. These characteristics of Ixion prompted follow-up observations in order to ascertain its orbit, which would in turn improve the certainty of later size estimates of Ixion. In August 2001, a team of astronomers used the European Southern Observatory's Astrovirtel virtual observatory to automatically scan through archival precovery photographs obtained from various observatories. The team obtained nine precovery images of Ixion, with the earliest taken by the Siding Spring Observatory on 17 July 1982. These precovery images along with subsequent follow-up observations with the La Silla Observatory's 2.2-meter MPG/ESO telescope in 2001 extended Ixion's observation arc by over 18 years, sufficient for its orbit to be accurately determined and eligible for numbering by the Minor Planet Center. Ixion was given the permanent minor planet number 28978 on 2 September 2001.
Name and symbol
This minor planet is named after the Greek mythological figure Ixion, in accordance with the International Astronomical Union's (IAU's) naming convention of naming plutinos (objects in a 3:2 orbital resonance with Neptune) after mythological figures related to the underworld. In Greek mythology, Ixion was the king of the Lapiths who was sentenced to an eternity of being bound to a burning solar wheel in the underworld. The name of Ixion was suggested by Elaine K. Elliot, the wife of one of the Deep Ecliptic Survey members. The naming citation was published by the Minor Planet Center on 28 March 2002.
Various symbols for Ixion have been proposed for astrology purposes. Sandy Turnbull proposed a symbol for Ixion (), which includes the initials I and X as well as depicting the solar wheel that Ixion was bound to in Tartarus. Denis Moskowitz, a software engineer in Massachusetts who designed the symbols for most of the dwarf planets, substitutes the Greek letter iota (Ι) and xi (Ξ) for I and X, creating a variant (). These symbols are occasionally mentioned on astrological websites, but are not used broadly. Another symbol is the wheel of Ixion, .
Orbit
Ixion orbits the Sun's barycenter with a semi-major axis of 39.5 astronomical units (AU), putting it in the Kuiper belt beyond Neptune's orbit. Ixion takes 248 years to complete one orbit, which places it in a 2:3 mean-motion resonance with Neptune. For every two revolutions it makes around the Sun, Neptune makes exactly three. It is therefore a member of the plutinos, a large population of trans-Neptunian objects named after its largest member Pluto.
Ixion's orbit closely resembles that of Pluto's, as both are plutinos. Ixion has an orbital eccentricity of 0.24 and an inclination of with to the ecliptic, slightly greater than Pluto's inclination of . Over the course of its current orbit, Ixion's distance from the Sun ranges from 29.9 AU at perihelion (closest distance) to 49.1 AU at aphelion (farthest distance). Ixion will come to perihelion on 24 September 2070. Due to gravitational perturbations by the giant planets, the orbital elements of Ixion vary over time. Simulations of Ixion's orbit show that in 10 million years, its semi-major axis can fluctuate between , eccentricity between 0.20 and 0.30, and inclination between 15.0 and 19.5°. This means Ixion's perihelion can become as low as and its aphelion as high as .
Rotation The rotation period of Ixion is uncertain; various photometric measurements suggest that it displays very little variation in brightness, with a small light curve amplitude of less than 0.15 magnitudes. Initial attempts to determine Ixion's rotation period were conducted by astronomer Ortiz and colleagues in 2001 but yielded inconclusive results. Although their short-term photometric data was insufficient for Ixion's rotation period to be determined based on its brightness variations, they were able to constrain Ixion's light curve amplitude below 0.15 magnitudes. Astronomers Sheppard and Jewitt obtained similarly inconclusive results in 2003 and provided an amplitude constraint less than 0.05 magnitudes, considerably less than Ortiz's amplitude constraint. In 2010, astronomers Rousselot and Petit observed Ixion with the European Southern Observatory's New Technology Telescope and determined Ixion's rotation period to be hours, with a light curve amplitude around 0.06 magnitudes. Galiazzo and colleagues obtained a shorter rotation period of hours in 2016, though they calculated that there is a 1.2% probability that their result may be inaccurate.
Physical characteristics Size, shape, and brightness + History of size estimates for IxionYearDiameter (km)References200220032005200520052007 20072013201320212026
Observations of stellar occultations from 2020 to 2023 have shown that Ixion is a nearly spheroidal body with an area equivalent diameter of . The shape of Ixion is slightly flattened, with its longest diameter being and its shortest diameter being . Ixion's shape shows little change between different dates of occultation observations, indicating that it is mostly axisymmetric about its rotation axis (having a slightly triaxial shape). Compared to Pluto and its moon Charon, Ixion is less than one-third the diameter of Pluto and three-fifths the diameter of Charon. Ixion is the fourth-largest plutino with a measured diameter, after , , and Pluto.
Ixion falls in the diameter range where trans-Neptunian objects are typically observed with densities lower than that of water ice (). However, Ixion's mass and density are unknown because it is not known to have any natural satellites or moons. The Hubble Space Telescope observed Ixion in 2002 and 2006, but no moons brighter than 0.5% of Ixion's brightness were detected beyond an angular separation of 0.5 arcseconds. Stellar occultation observations from 2020 to 2023 showed no evidence of rings thicker than a kilometre in width and 0.1 in optical depth.
Optical observations of Ixion show that it has an absolute magnitude of 3.8 and a geometric albedo (reflectivity) of 0.11. In terms of absolute magnitude, Ixion was the brightest object discovered by the Deep Ecliptic Survey and is among the twenty brightest trans-Neptunian objects known according to astronomer Michael Brown and the Minor Planet Center.
When Ixion was discovered, it was thought to be the largest and brightest Kuiper belt object. Under the assumption of a low albedo, it was presumed to have a diameter around , which would have made it larger than the dwarf planet and almost the size of Charon. Since then, more accurate measurements from infrared space telescopes and stellar occultations have revised Ixion's diameter downward, with the most accurate measurement being .
Possible dwarf planet Astronomer Gonzalo Tancredi considered Ixion a likely dwarf planet candidate because its diameter is large enough to become theoretically round under hydrostatic equilibrium, and its rotational brightness variation is low enough to suggest a nearly spherical shape. American astronomer Michael Brown considers Ixion to highly likely be a dwarf planet, placing it at the lower end of the "highly likely" range.
Spectra and surface
The surface of Ixion is very dark and unevolved, resembling those of smaller, primitive Kuiper belt objects such as Arrokoth. In the visible spectrum, Ixion appears moderately red in color, similar to the large Kuiper belt object . Ixion's reflectance spectrum displays a red spectral slope that extends from wavelengths of 0.4 to 0.95 μm, in which it reflects more light at these wavelengths. Longward of 0.85 μm, Ixion's spectrum becomes flat and featureless, especially at near-infrared wavelengths. In the near-infrared, Ixion's reflectance spectrum appears neutral in color and lacks apparent absorption signatures of water ice at wavelengths of 1.5 and 2 μm. Although water ice appears to be absent in Ixion's near-infrared spectrum, Barkume and colleagues have reported a detection of weak absorption signatures of water ice in Ixion's near-infrared spectrum in 2007. Ixion's featureless near-infrared spectrum indicates that its surface is covered with a thick layer of dark organic compounds irradiated by solar radiation and cosmic rays.