28978 Ixion

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This article is about the minor planet. For other uses, see Ixion (disambiguation).

28978 Ixion
Synodic rotation period
12.4±0.3 h[10]
15.9±0.5 h[11]
0.108±0.002 geometric[12]
0.037±0.007 Bond[12]
Temperature64+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 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

small Solar System bodies and spherical dwarf planets.[20] Ixion is currently not known to have a natural satellite
, so its mass and density remain unknown.

History

Discovery

Cerro Tololo Observatory

Ixion was discovered on 22 May 2001 by a team of American astronomers at the

Lawrence Wasserman identified Ixion in digital images of the southern sky taken with the 4-meter Víctor M. Blanco Telescope at Cerro Tololo.[23][21] Ixion was first noted by Elliot while compiling two images taken approximately two hours apart,[24][21] which revealed Ixion's slow motion relative to the background stars.[a] At the time of discovery, Ixion was located in the constellation of Scorpius.[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.[21][26] The discovery supported suggestions that there were undiscovered large trans-Neptunian objects comparable in size to Pluto.[21][27] Since Ixion's discovery, numerous large trans-Neptunian objects, notably the dwarf planets Haumea, Eris, and Makemake, have been discovered; in particular, Eris is almost the same size as Pluto.[19]

The discovery of Ixion was formally announced by 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.[c]

At the time of discovery, Ixion was thought to be among the largest trans-Neptunian objects in the

minor planet number 28978 on 2 September 2001.[32]

Name

Ixion imaged by the MPG/ESO telescope's Wide Field Imager at the La Silla Observatory in 2001[27]

This minor planet is named after the

plutinos (objects in a 3:2 orbital resonance with Neptune) to be named after mythological figures associated with the underworld.[33] 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.[34] 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.[34] Rather than being grateful, Ixion became lustful towards 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.[34] 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.[35]

The name for Ixion was suggested by E. K. Elliot, who was also involved in the naming of Kuiper belt object 38083 Rhadamanthus.[2][35] The naming citation was published by the Minor Planet Center on 28 March 2002.[36]

The usage of

planetary 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 (), 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 (). These symbols are occasionally mentioned on astrological websites, but are not used broadly.[37]

Orbit and rotation

aphelion
(Q) dates are shown for both Pluto and Ixion.

Ixion is classified as a

semi-major axes around 39 AU.[39]

Like Pluto, Ixion's orbit is elongated and inclined to the

aphelion (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, 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 small light curve amplitude of less than 0.15 magnitudes.[10][11][40] 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.[41][40] 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.[42] 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.[11] 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.[10]

Physical characteristics

Size and brightness

Size estimates for Ixion
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]
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 a measured diameter of 710 km (440 mi), with an optical

2003 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 astronomer Michael Brown and the Minor Planet Center.[19][54]

Ixion was the largest and brightest Kuiper belt object found when it was discovered.[53][21][29] 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 planet Ceres and comparable in size to Charon.[21] 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.[27]

In 2002, astronomers of the

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).[44]

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.[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 was 650+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 the Herschel Space Observatory along with Spitzer in 2013.[51]

On 13 October 2020, Ixion

microbars for any possible atmosphere of Ixion.[9]

Possible dwarf planet

Astronomer

arcseconds from Ixion,[1][57] 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]

Spectra and surface

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

The surface of Ixion is very dark and unevolved, resembling those of smaller, primitive Kuiper belt objects such as

solar radiation and cosmic rays.[38]

The red color of Ixion's surface originates from the irradiation of water- and organic-containing

heterogeneities across its surface, which may also explain the conflicting detections of water ice in various studies.[60]

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 materials

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 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 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.[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 a

coma around Ixion, placing an upper limit of 5.2 kilograms per second for Ixion's dust production rate.[61]

Exploration

The

New Horizons spacecraft, which successfully flew by Pluto in 2015, observed Ixion from afar using its long 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 high phase angle of 64 degrees, enabling the determination of the light scattering properties and photometric phase 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 an

orbital insertion around either.[62] 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.[63] 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.[63]

Notes

  1. arcseconds from its original position first observed by Cerro Tololo.[23]
  2. ^ The given equatorial coordinates of Ixion during 22 May 2001 is 16h 16m 06.12s and −19° 13′ 45.6″,[23][5] which is close to the Scorpius constellation's coordinates around 17h and −40°.[25]
  3. ^ In the convention for minor planet provisional designations, the first letter represents the half-month of the year of discovery while the second letter and numbers indicate the order of discovery within that half-month. In the case for 2001 KX76, the first letter 'K' corresponds to the second half-month of May 2001 while the succeeding letter 'X' indicates that it is the 23rd object discovered on the 77th cycle of discoveries (with 76 cycles completed). Each cycle consists of 25 letters representing discoveries, hence 23 + (76 cycles × 25 letters) = 1,923.[28]
  4. ^ The plutino classification is named after the dwarf planet Pluto, largest member of this group.
  5. ^ The current estimates of Pluto and Charon's diameters are 2376 km and 1212 km, respectively.[52] One-third of Pluto's diameter is 792 km and three-fifths of Charon's diameter is 727 km—compare to Ixion's diameter of 710 km.

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External links
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