28978 Ixion

28978 Ixion /ɪkˈsaɪ.ən/, provisional designation 2001 KX76, is a large trans-Neptunian object and a possible dwarf planet. 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 the visible spectrum, Ixion appears moderately red in color while it appears neutral in the near-infrared, likely as a result of the presence of dark organic compounds on its surface. Water ice has been also suspected to be present on Ixion's surface, albeit in trace amounts as most of the water ice is expected to be hidden underneath a thick layer of organic compounds on Ixion's surface. Ixion's diameter is estimated at 617 km (383 mi), making it the fourth-largest known plutino. Several astronomers have considered Ixion to be a possible dwarf planet under the expectation that it is large enough to have assumed a round shape under hydrostatic equilibrium, although studies in 2019 suggest that objects around the size of Ixion may retain significant internal porosity and thus represent a transitional zone between small Solar System bodies and dwarf planets. Ixion is currently not known to have a natural satellite, thus its mass and density remain unknown.

Discovery
xion was discovered on 22 May 2001 by a team of American astronomers at the Cerro Tololo Inter-American Observatory in Chile. The discovery formed part of the Deep Ecliptic Survey, a survey conducted by American astronomer Robert Millis to search for Kuiper belt objects located near the ecliptic plane using telescopes at the facilities 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 TNO. 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, Eris, and Makemake, have been discovered.

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 2001 KX76, 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 had 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. For this reason, the Minor Planet Center assigned Ixion its permanent minor planet number 28978 on 2 September 2001.

Name
Ixion is named after the eponymous Greek mythological figure Ixion, in accordance with the International Astronomical Union's (IAU's) naming convention which requires plutinos (objects in a 3:2 orbital resonance with Neptune) to be named after mythological figures associated with the underworld. In Greek mythology, Ixion was the king of the legendary Lapiths of Thessaly and had married Dia, a daughter of Deioneus (or Eioneus), whom Ixion promised to give valuable bridal gifts. Ixion invited Deioneus to a banquet but instead pushed him into a pitfall of burning coals and wood, killing Deioneus. Although the lesser gods despised his actions, Zeus pitied Ixion and invited him to a banquet with other gods. Rather than being grateful, Ixion became lustful toward's Zeus's wife, Hera. Zeus found out about his intentions and created the cloud Nephele in the shape of Hera, and tricked Ixion into coupling with it, fathering the race of Centaurs. For his crimes, Ixion was expelled from Olympus, blasted with a thunderbolt, and bound to a burning solar wheel in the underworld for all eternity.

The name for Ixion was suggested by James Elliot, who was involved in its discovery by the Deep Ecliptic Survey team. The naming citation was published by the Minor Planet Center on 28 March 2002.

Size and brightness
Comparison of the relative colors and sizes of the four largest plutinos and their moons

Different diameters for Ixion depending on its albedo

Ixion has an optical absolute magnitude of 3.83 and is estimated to have a geometric albedo (reflectivity) of 0.14, corresponding to a diameter of 617 km (383 mi). Compared to Pluto and its moon Charon, Ixion is approximately one-fourth the diameter of Pluto and half the diameter of Charon. Ixion is also the fourth-largest known plutino that has a well constrained diameter, preceding 2003 AZ84, Orcus, and Pluto. It is the intrinsically 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.

At the time of Ixion's discovery, it was the brightest known Kuiper belt object found. Hence, it was thought to be one of the largest Kuiper belt objects discovered due to its high intrinsic brightness. Under the assumption of a low albedo, it was presumed to have a diameter around 1,200 km (750 mi), which would make it larger than the dwarf planet Ceres and comparable in size to Charon. Subsequent observations of Ixion with the La Silla Observatory's MPG/ESO telescope along with the European Southern Observatory's Astrovirtel in August 2001 concluded a similar size around 1,200–1,400 km (750–870 mi), though under the former assumption of a low albedo.

In 2002, astronomers of the Max Planck Institute for Radio Astronomy measured Ixion's thermal emission at millimeter wavelengths with the IRAM 30m telescope and obtained an albedo of 0.09, corresponding to a diameter of 1,055 km (656 mi), consistent with previous assumptions of Ixion's size and albedo. They later reevaluated their results in 2003 and realized that their detection of Ixion's thermal emission was spurious; follow-up observations with the IRAM telescope did not detect any thermal emission within the millimeter range at frequencies of 250 GHz, implying a high albedo and consequently a smaller size for Ixion. The lower limit for Ixion's albedo was constrained at 0.15, suggesting that Ixion's diameter did not exceed 804 km (500 mi).

With space-based telescopes such as the Spitzer Space Telescope, astronomers were able to more accurately measure Ixion's thermal emissions, allowing for more accurate estimates of its albedo and size. Preliminary thermal measurements with Spitzer in 2005 yielded a much higher albedo constraint of 0.25–0.50, corresponding to a diameter range of 400–550 km (250–340 mi). Further Spitzer thermal measurements at multiple wavelength ranges (bands) in 2007 yielded mean diameter estimates around 446 km (277 mi) and 573 km (356 mi) for a single-band and two-band solution for the data, respectively. From these results, the adopted mean diameter was 650+260

−220 km (404+162

−137 mi), just beyond Spitzer's 2005 diameter constraint albeit having a large margin of error. Ixion's diameter was later revised to 617 km (383 mi), based on multi-band thermal observations by the Herschel Space Observatory along with Spitzer in 2013.

On 13 October 2020, Ixion occulted a 10th magnitude star, blocking out its light for a predicted maximal duration of approximately 34 seconds. The stellar occultation was observed by astronomers at Arizona, United States. Of the ten participating observers, eight of them reported positive detections of the occultation. A preliminary elliptical fit for Ixion's occultation profile suggests projected dimensions of approximately 757 km × 685 km (470 mi × 426 mi).

Possible dwarf planet status
The International Astronomical Union has not classified Ixion as a dwarf planet and has not yet officially accepted additional dwarf planets since Makemake and Haumea in 2008. Astronomer Gonzalo Tancredi considers Ixion as a likely candidate as it has a diameter greater than 450 km (280 mi), the estimated minimum size for an object to achieve hydrostatic equilibrium, under the assumption of a predominantly icy composition. Ixion also displays a light curve amplitude less than 0.15 magnitudes, indicative of a likely spheroidal shape, hence why Tancredi considered Ixion as a likely dwarf planet. American astronomer Michael Brown considers Ixion to highly likely be a dwarf planet, placing it at the lower end of the "highly likely" range. However, in 2019, astronomer William Grundy and colleagues proposed that trans-Neptunian objects similar in size to Ixion, around 400–1,000 km (250–620 mi) in diameter, have not collapsed into solid bodies and are thus transitional between smaller, porous (and thus low-density) bodies and larger, denser, brighter and geologically differentiated planetary bodies such as dwarf planets. Ixion is situated within this size range, suggesting that it is at most only partially differentiated, with a porous internal structure. While Ixion's interior may have collapsed gravitationally, its surface remained uncompressed, implying that Ixion might not be in hydrostatic equilibrium and thus not a dwarf planet. However, this notion for Ixion cannot currently be tested: the object is not currently known to have any natural satellites, and thus Ixion's mass and density cannot currently be measured. Only two attempts with the Hubble Space Telescope have been made to find a satellite within an angular distance of 0.5 arcseconds from Ixion, and it has been suggested that there is a chance as high as 0.5% that a satellite may have been missed in these searches.

Spectra and surface[edit]
Comparison of sizes, albedo, and colors of various large trans-Neptunian objects. The gray arcs represent uncertainties of the object's size.

In the visible spectrum, Ixion appears moderately red in color, similar to the large Kuiper belt object Quaoar. 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.

The red color of Ixion's surface results from the irradiation of clathrates of water and organic compounds by solar radiation and cosmic rays, which produces dark, reddish heteropolymers called tholins that cover its surface. The production of tholins on Ixion's surface is responsible for Ixion's red, featureless spectrum as well as its relatively low surface albedo. The neutral color and absence of apparent signs of water ice in Ixion's near-infrared spectrum indicates that the layer of tholins covering its surface must be very thick, suggesting that Ixion has undergone long-term irradiation and has not experienced resurfacing by impact events that may otherwise expose water ice underneath, in contrast to the relatively fresh water ice-rich surface of the similarly-colored Kuiper belt object Varuna. While Ixion is generally known to have a red color (spectral index IR), photometric measurements of Ixion's visible and near-infrared colors with the Very Large Telescope (VLT) in 2006 and 2007 paradoxically obtained a more blue spectral index of BB. This discrepancy was concluded to be an indication of heterogeneities across its surface, which may also explain the conflicting detections of water ice on Ixion's surface in various studies.

Spectroscopic observations of Ixion's visible spectrum with the VLT in 2003 have tentatively identified a weak absorption feature at 0.8 μm, which could possibly be attributed to surface materials aqueously altered by water. However, evidence for this suspected absorption feature remains inconclusive as it was detected near wavelengths where the signal-to-noise ratio in Ixion's spectrum is high and has not been confirmed by subsequent spectroscopic observations. A study of Ixion's spectrum conducted by Boehnhardt and colleagues in 2004 was unable identify any absorption feature at 0.8 μm, concluding that the discrepancy between the 2003 and 2004 spectroscopic results may be the result of Ixion's heterogenous surface. In that same study, their results from photometric and polarimetric observations suggest that Ixion's surface consists of a mixture of mostly dark material and a smaller proportion of brighter, icy material. Boehnhardt and colleagues suggested a mixing ratio of 6:1 for dark and bright material as a best-fit model for Ixion's geometric albedo of 0.08, although more recent measurements made by space-based telescopes after Boehnhardt's study have shown that Ixion has a higher geometric albedo of at least 0.14, thus corresponding to a greater proportion of bright material in Ixion's surface based on Boehnhardt's model. Based on combined visible and infrared spectroscopic results, they suggested that Ixion's surface consists of a mixture largely of amorphous carbon and tholins, with the following best-fit model of Ixion's surface composition: 65% amorphous carbon, 20% cometary ice tholins (ice tholin II), 13% nitrogen and methane-rich Titan tholins, and 2% water ice.

In 2005, astronomers Lorin and Rousselot observed Ixion with the VLT in attempt to search for evidence of cometary activity. They did not detect a coma around Ixion, placing an upper limit of 5.2 kilograms per second for Ixion's dust production rate.

Orbit and rotation
Ixion is classified as a plutino, or an object that has a 2:3 mean-motion orbital resonance with Neptune. That is, it completes two orbits around the Sun for every three orbits that Neptune takes. At the time of Ixion's discovery, it was initially thought to be in a 3:4 orbital resonance with Neptune, which would have made Ixion closer to the Sun. Ixion orbits the Sun at an average distance of 39.8 AU (5.95×109 km), taking 251 years to complete a full orbit. This is characteristic of all plutinos, which have orbital periods around 250 years and semi-major axes around 39 AU.

Like Pluto, Ixion's orbit is elongated and inclined to the ecliptic. Ixion has an orbital eccentricity of 0.24 and an orbital inclination of 19.6 degrees, slightly greater than Pluto's inclination of 17 degrees. Over the course of its orbit, Ixion's distance from the Sun varies from 30.1 AU at perihelion (closest distance) to 39.8 AU at aphelion (farthest distance). Although Ixion's orbit is similar to that of Pluto, their orbits are oriented differently: Ixion's perihelion is below the ecliptic whereas Pluto's is above it (see right image). As of 2019, Ixion is approximately 39 AU from the Sun and is currently moving closer, approaching aphelion by 2070. Simulations by the Deep Ecliptic Survey show that Ixion can acquire a perihelion distance (qmin) as small as 27.5 AU over the next 10 million years.

The rotation period of Ixion is uncertain; various photometric measurements suggest that it displays very little variations in brightness, with a small light curve amplitude 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 15.9±0.5 hours, with a light curve amplitude around 0.06 magnitudes. Galiazzo and colleagues obtained a shorter rotation period of 12.4±0.3 hours in 2016, though they calculated that there is a 1.2% probability that their result may be erroneous.

Exploration
In a study published by Ashley Gleaves and colleagues in 2012, Ixion was considered as a potential target for an orbiter mission that would be launched on an Atlas V 551 or Delta IV HLV rocket. For an orbiter mission to Ixion, the spacecraft have a launch date in November 2039 and use a gravity assist from Jupiter, taking 20 to 25 years to arrive. Gleaves concluded that Ixion and Huya were the most feasible targets for the orbiter, as the trajectories required the fewest maneuvers for orbital insertion around either. For a flyby mission to Ixion, planetary scientist Amanda Zangari calculated that a spacecraft could take just over 10 years to arrive at Ixion using a Jupiter gravity assist, based on a launch date of 2027 or 2032. Ixion would be approximately 31 to 35 AU from the Sun when the spacecraft arrives. Alternatively, a flyby mission with a later launch date of 2040 would also take just over 10 years, using a Jupiter gravity assist. By the time the spacecraft arrives in 2050, Ixion would be approximately 31 to 32 AU from the Sun. Other trajectories using gravity assists from Jupiter or Saturn have been also considered. A trajectory using gravity assists from Jupiter and Saturn could take under 22 years, based a launch date of 2035 or 2040, whereas a trajectory using one gravity assist from Saturn could take at least 19 years, based on a launch date of 2038 or 2040. Using these alternative trajectories for the spacecraft, Ixion would be approximately 30 AU from the Sun when the spacecraft arrives.