 Hubble Space Telescope image of Ixion taken in 2006[1] | |
| Discovery[2] | |
|---|---|
| Discovered by | Deep Ecliptic Survey |
| Discovery site | Cerro Tololo Obs. |
| Discovery date | 22 May 2001 |
| Designations | |
| (28978) Ixion | |
| Pronunciation | /ɪkˈsaɪ.ən/[3] |
Named after | ΙξίωνIxīōn |
| 2001 KX76 | |
| TNO · plutino[4] · distant[5] | |
| Adjectives | Ixionian/ɪksiˈoʊniən/[6] |
| Symbol | .svg) or.svg) (astrological) |
| Orbital characteristics[2] | |
| Epoch 17 December 2020 (JD 2459200.5) | |
| Uncertainty parameter 3 | |
| Observation arc | 35.93 yr (13,122 days) |
| Earliestprecovery date | 17 July 1982 |
| Aphelion | 49.584AU |
| Perihelion | 30.019 AU |
| 39.802 AU | |
| Eccentricity | 0.24579 |
| 251.11yr (91,717d) | |
| 289.587° | |
| 0° 0m 14.13s / day | |
| Inclination | 19.600° |
| 71.011° | |
| ≈ 24 September 2070[7] ±1 day | |
| 298.314° | |
| Physical characteristics | |
| Dimensions | 756.9 km × 684.9 km(projected, occultation)[8][9] |
| 709.6±0.2 km[9] | |
| 12.4±0.3h[10] 15.9±0.5 h[11] | |
| 0.108±0.002geometric[12] 0.037±0.007Bond[12] | |
| Temperature | 64+0.7 −1.1 K[9] |
| IR (moderately red)[13][14] B–V=1.009±0.051[15] V–R=0.61±0.03[15] V–I=1.146±0.086[15] | |
| 19.8[16] | |
| 3.774±0.021[17][12] 3.6 (assumed)[2][5] | |
28978 Ixion (/ɪkˈsaɪ.ən/, provisional designation2001 KX76) is a largetrans-Neptunian object. It is located in theKuiper belt, a region of icy objects orbiting beyondNeptune in the outerSolar System. Ixion is classified as aplutino, a dynamical class of objects in a 2:3orbital resonance withNeptune. It was discovered in May 2001 by astronomers of theDeep Ecliptic Survey at theCerro Tololo Inter-American Observatory, and was announced in July 2001. The object is named after theGreek mythological figureIxion, who was a king of theLapiths.
Invisible light, Ixion appears dark andmoderately red in color due toorganic compounds covering its surface. Waterice 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 710 km (440 mi), making it the fourth-largest known plutino. It appears to be a transitional object between irregularly-shapedsmall Solar System bodies and sphericaldwarf planets.[18] Ixion is currently not known to have anatural satellite, so its mass and density are unknown.
Ixion was discovered on 22 May 2001 by a team of American astronomers at theCerro Tololo Inter-American Observatory inChile.[5][19] The discovery formed part of theDeep Ecliptic Survey, a survey conducted by American astronomerRobert Millis to search forKuiper belt objects located near theecliptic plane using telescopes at the facilities of theNational Optical Astronomy Observatory.[20][19] On the night of 22 May 2001, American astronomersJames Elliot andLawrence Wasserman identified Ixion in digital images of thesouthern sky taken with the 4-meterVíctor M. Blanco Telescope at Cerro Tololo.[21][19] Ixion was first noted by Elliot while compiling two images taken approximately two hours apart,[22][19] which revealed Ixion's slow motion relative to the background stars.[a] At the time of discovery, Ixion was located in theconstellation ofScorpius.[b]
The discoverers of Ixion noted that it appeared relatively bright for a distant object, implying that it might be rather large for a TNO.[19][24] The discovery supported suggestions that there were undiscovered large trans-Neptunian objects comparable in size to Pluto.[19][25] Since Ixion's discovery, numerous large trans-Neptunian objects, notably thedwarf planetsHaumea,Eris, andMakemake, have been discovered; in particular, Eris is almost the same size as Pluto.[26]
The discovery of Ixion was formally announced by theMinor Planet Center in aMinor Planet Electronic Circular on 1 July 2001.[21] It was given theprovisional designation2001 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.[c]
At the time of discovery, Ixion was thought to be among the largest trans-Neptunian objects in theSolar System, as implied by its highintrinsic brightness.[19][24] 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.[28][25] In August 2001, a team of astronomers used theEuropean Southern Observatory'sAstrovirtelvirtual observatory to automatically scan through archivalprecovery photographs obtained from various observatories.[25] The team obtained nine precovery images of Ixion, with the earliest taken by theSiding Spring Observatory on 17 July 1982.[29][30] These precovery images along with subsequent follow-up observations with theLa Silla Observatory's 2.2-meterMPG/ESO telescope in 2001 extended Ixion'sobservation arc by over 18 years, sufficient for its orbit to be accurately determined and eligible for numbering by the Minor Planet Center.[25][30] Ixion was given the permanentminor planet number 28978 on 2 September 2001.[31]
This minor planet is named after theGreek mythological figureIxion, in accordance with theInternational Astronomical Union's (IAU's) naming convention which requiresplutinos (objects in a 3:2orbital resonance withNeptune) to be named after mythological figures associated with theunderworld.[32] In Greek mythology, Ixion was the king of the legendaryLapiths ofThessaly and had marriedDia, a daughter ofDeioneus (or Eioneus), whom Ixion promised to give valuable bridal gifts.[33] 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.[33] Rather than being grateful, Ixion became lustful towards Zeus's wife,Hera. Zeus found out about his intentions and created the cloudNephele in the shape of Hera, and tricked Ixion into coupling with it, fathering the race ofCentaurs.[33] For his crimes, Ixion was expelled from Olympus, blasted with athunderbolt, and bound to aburning solar wheel in the underworld for all eternity.[34]
The name for Ixion was suggested by E. K. Elliot, who was also involved in the naming of Kuiper belt object38083 Rhadamanthus.[2][34] The naming citation was published by the Minor Planet Center on 28 March 2002.[35]
The usage ofplanetary symbols is discouraged in astronomy, so Ixion never received a symbol in the astronomical literature. There is no standard symbol for Ixion used by astrologers either. Sandy Turnbull proposed a symbol for Ixion (.svg)
), which includes the initials I and X as well as depicts 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 (.svg)
). These symbols are occasionally mentioned on astrological websites, but are not used broadly.[36] Another symbol is the wheel of Ixion,.svg)
.[37]
Ixion is classified as aplutino, a large population ofresonant trans-Neptunian objects in a 2:3mean-motionorbital resonance with Neptune.[d] Thus, Ixion completes two orbits around theSun for every three orbits that Neptune takes.[4][38] 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.[19][24] Ixion orbits the Sun at an average distance of 39.8 AU (5.95 billion km; 3.70 billion mi), taking 251 years to complete a full orbit.[2] This is characteristic of all plutinos, which have orbital periods around 250 years andsemi-major axes around 39 AU.[39]
Like Pluto, Ixion's orbit is elongated and inclined to theecliptic.[39] Ixion has anorbital eccentricity of 0.24 and anorbital inclination of 19.6degrees, slightly greater than Pluto's inclination of 17 degrees.[2][39] Over the course of its orbit, Ixion's distance from the Sun varies from 30 AU atperihelion (closest distance) to 49.6 AU ataphelion (farthest distance).[2] 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[update], Ixion is approximately 39 AU from the Sun and is currently moving closer, approaching perihelion by 2070.[2] 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.[4]
The rotation period of Ixion is uncertain; various photometric measurements suggest that it displays very little variation in brightness, with a smalllight curveamplitude of less than 0.15magnitudes.[10][11][40] Initial attempts to determine Ixion's rotation period were conducted by astronomerOrtiz 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.[41][40] AstronomersSheppard andJewitt obtained similarly inconclusive results in 2003 and provided an amplitude constraint less than 0.05 magnitudes, considerably less than Ortiz's amplitude constraint.[42] In 2010, astronomers Rousselot and Petit observed Ixion with the European Southern Observatory'sNew Technology Telescope and determined Ixion's rotation period to be15.9±0.5 hours, with a light curve amplitude around 0.06 magnitudes.[11] Galiazzo and colleagues obtained a shorter rotation period of12.4±0.3 hours in 2016, though they calculated that there is a 1.2% probability that their result may be erroneous.[10]
| Year | Diameter (km) | Refs |
|---|---|---|
| 2002 | 1055±165 | [43] |
| 2003 | <804 | [44] |
| 2005 | <822 | [45] |
| 2005 | 475±75 | [46] |
| 2005 | 480+152 −136 | [47] |
| 2007 | ~446.3 (Spitzer 1-Band) | [48] |
| 2007 | 573.1+141.9 −139.7 (Spitzer 2-Band) | [48] |
| 2007 | 650+260 −220(adopted) | [48] |
| 2007 | 590±190 | [49] |
| 2013 | ~549 | [50] |
| 2013 | 617+19 −20 | [51] |
| 2021 | 709.6±0.2 | [9] |
Ixion has a measured diameter of 710 km (440 mi), with an opticalabsolute magnitude of 3.77 and ageometric albedo (reflectivity) of 0.11.[9][12] Compared toPluto and its moonCharon, Ixion is less than one-third the diameter of Pluto and three-fifths the diameter of Charon.[e] Ixion is the fourth-largest knownplutino that has a well-constrained diameter, preceding2003 AZ84,Orcus, and Pluto.[39] It was the intrinsically brightest object discovered by the Deep Ecliptic Survey[53] and is among the twenty brightest trans-Neptunian objects known according to astronomerMichael Brown and the Minor Planet Center.[26][54]
Ixion was the largest and brightest Kuiper belt object found when it was discovered.[53][19][28] Under the assumption of a low albedo, it was presumed to have a diameter around 1,200 km (750 mi), which would have made it larger than the dwarf planetCeres and comparable in size to Charon.[19] 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.[25]
In 2002, astronomers of theMax Planck Institute for Radio Astronomy measured Ixion'sthermal emission atmillimeter wavelengths with theIRAM 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.[43] They later reevaluated their results in 2003 and realized that their detection of Ixion'sthermal emission was spurious; follow-up observations with the IRAM telescope did not detect any thermal emission within the millimeter range atfrequencies 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).[44]
With space-based telescopes such as theSpitzer Space Telescope, astronomers were able to more accurately measure Ixion's thermal emissions, allowing for more accurate estimates of its albedo and size.[55][48] 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).[46] 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 was650+260
−220 km (404+162
−137 mi), just beyond Spitzer's 2005 diameter constraint albeit having a large margin of error.[48] Ixion's diameter was later revised to 617 km (383 mi), based on multi-band thermal observations by theHerschel Space Observatory along with Spitzer in 2013.[51]
On 13 October 2020, Ixionocculted a 10th magnitudered giant star (star Gaia DR2 4056440205544338944), blocking out its light for a duration of approximately 45 seconds.[9] The stellar occultation was observed by astronomers from seven different sites across thewestern United States.[9] Of the ten participating observers, eight of them reported positive detections of the occultation.[8] Observers from theLowell Observatory provided highly precise measurements of the occultationchord timing, allowing for tight constraints to Ixion's diameter and possibleatmosphere. An elliptical fit for Ixion's occultation profile gives projected dimensions of approximately 757 km × 685 km (470 mi × 426 mi), corresponding to a projected spherical diameter of 709.6 ± 0.2 km (440.92 ± 0.12 mi). The precise Lowell Observatory chords place an upper limitsurface pressure of <2microbars for any possible atmosphere of Ixion.[9]
AstronomerGonzalo Tancredi considers Ixion as alikely candidate as it has a diameter greater than 450 km (280 mi), the estimated minimum size for an object to achievehydrostatic equilibrium, under the assumption of a predominantly icy composition.[56] Ixion also displays alight curveamplitude less than 0.15magnitudes, indicative of a likelyspheroidal shape, hence why Tancredi considered Ixion as a likely dwarf planet.[57] American astronomer Michael Brown considers Ixion tohighly likely be a dwarf planet, placing it at the lower end of the "highly likely" range.[26] 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 partiallydifferentiated, with aporous 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.[18] However, this notion for Ixion cannot currently be tested: the object is not currently known to have anynatural satellites, and thus Ixion's mass and density cannot currently be measured. Only two attempts with theHubble Space Telescope have been made to find a satellite within anangular distance of 0.5arcseconds from Ixion,[1][58] 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.[49]
The surface of Ixion is very dark and unevolved, resembling those of smaller, primitive Kuiper belt objects such asArrokoth.[12] In thevisible spectrum, Ixion appearsmoderately red in color, similar to the large Kuiper belt objectQuaoar.[59] Ixion'sreflectancespectrum displays a redspectral slope that extends fromwavelengths 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 atnear-infrared wavelengths.[59] In the near-infrared, Ixion's reflectance spectrum appears neutral in color and lacks apparentabsorption signatures of waterice atwavelengths of 1.5 and 2 μm.[38] 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.[60] Ixion's featureless near-infrared spectrum indicates that its surface is covered with a thick layer of darkorganic compounds irradiated bysolar radiation andcosmic rays.[38]
The red color of Ixion's surface originates from the irradiation of water- and organic-containingclathrates by solar radiation and cosmic rays, which produces dark, reddishheteropolymers calledtholins that cover its surface.[41] The production of tholins on Ixion's surface is responsible for Ixion's red, featureless spectrum as well as its low surface albedo. Ixion's neutral near-infrared color and apparent lack of water ice indicates that it has a thick layer of tholins covering its surface, suggesting that Ixion has undergone long-term irradiation and has not experienced resurfacing byimpact events that may otherwise expose water ice underneath.[13][38] While Ixion is generally known to have a red color, visible and near-infrared observations by theVery Large Telescope (VLT) in 2006 and 2007 paradoxically found a bluer color.[61] This discrepancy was concluded to be an indication ofheterogeneities across its surface, which may also explain the conflicting detections of water ice in various studies.[61]
In 2003, VLT observations tentatively resolved a weak absorption feature at 0.8 μm in Ixion's spectrum, which could possibly be attributed to surface materialsaqueously altered by water. However, it was not confirmed in a follow-up study by Boehnhardt and colleagues in 2004, concluding that the discrepancy between the 2003 and 2004 spectroscopic results may be the result of Ixion's heterogenous surface.[41] In that same study, their results from photometric andpolarimetric 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 a geometric albedo of 0.08.[41] Based on combined visible and infrared spectroscopic results, they suggested that Ixion's surface consists of a mixture largely ofamorphous 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 andmethane-richTitan tholins, and 2% water ice.[41]
In 2005, astronomers Lorin and Rousselot observed Ixion with the VLT in attempt to search for evidence of cometary activity. They did not detect acoma around Ixion, placing an upper limit of5.2 kilograms per second for Ixion's dust production rate.[62]
TheNew Horizons spacecraft, which successfully flew by Pluto in 2015, observed Ixion from afar using itslong range imager on 13 and 14 July 2016.[12] The spacecraft detected Ixion at magnitude 20.2 from a range of 15 AU (2.2 billion km; 1.4 billion mi), and was able to observe it from a highphase angle of 64 degrees, enabling the determination of the light scattering properties and photometricphase curve behavior of its surface.[12]
In a study published by Ashley Gleaves and colleagues in 2012, Ixion was considered as a potential target for anorbiter mission concept, which would be launched on anAtlas V 551 orDelta IV HLV rocket. For an orbiter mission to Ixion, the spacecraft have a launch date in November 2039 and use agravity assist from Jupiter, taking 20 to 25 years to arrive.[63] Gleaves concluded that Ixion andHuya were the most feasible targets for the orbiter, as the trajectories required the fewest maneuvers fororbital insertion around either.[63] For aflyby 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.[64] 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.[64]
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| Consensus | _(cropped).jpg)  | ||||||||||||||
| Candidate (for TNOs, D+1σ ≥ 700 km or H ≤ 4.2) |
| ||||||||||||||
