20000 Varuna (provisional designation ) is a large trans-Neptunian object in the Kuiper belt. It was discovered in November 2000 by American astronomer Robert McMillan during a Spacewatch survey at the Kitt Peak National Observatory. It is named after the Hindu deity Varuna, one of the oldest deities mentioned in the Vedic texts.
Varuna's light curve is compatible with the body being a Jacobi ellipsoid, suggesting that it has an elongated shape due to its rapid rotation. Varuna's surface is moderately red in color due to the presence of complex organic compounds on its surface. Water ice is also present on its surface, and is thought to have been exposed by past collisions which may have also caused Varuna's rapid rotation. Although no natural satellites have been found or directly imaged around Varuna, analysis of variations in its light curve in 2019 suggests the presence of a possible satellite orbiting closely around Varuna. Assumptions that the body is in hydrostatic equilibrium (and thus a dwarf planet) result in a calculated density so low that it is likely not a dwarf planet anyway.
History Discovery
Varuna was discovered by American astronomer Robert McMillan using the Spacewatch 0.9-meter telescope during a routine survey on 28 November 2000. The Spacewatch survey was conducted by McMillan at the Kitt Peak National Observatory near Tucson, Arizona. At the time of discovery, Varuna was located at a moderately dense star field close to the northern galactic equator. Although Varuna was not detected by McMillan's real-time computer software, he was able to identify Varuna moving slowly among the background stars by manually comparing multiple scans of the same region using the blinking technique. After McMillan's observing shift, follow-up observations of Varuna were conducted by astronomer Jeffrey Larsen in order to confirm the object. By the end of Larsen's observing shift, both McMillan and Larsen had made a total of 12 observations that spanned three nights.
The discovery of Varuna was formally announced in a Minor Planet Electronic Circular on 1 December 2000. It was given the provisional designation , indicating that it was discovered during the second half of November 2000. Varuna was the 2667th object observed in the latter half of November, as indicated by the last letter and numbers in its provisional designation. At the time, Varuna was thought to be one of the largest and brightest minor planets in the Solar System due to its relatively high apparent magnitude of 20 for a distant object, which implied that it might be around one-fourth the size of Pluto and comparable in size to the dwarf planet .
Subsequently, after the announcement of Varuna's discovery, precovery images of Varuna were found by German astronomers Andre Knofel and Reiner Stoss at the Palomar Observatory. One particular precovery image, which was taken with the Palomar Observatory's Big Schmidt telescope in 1955, showed that Varuna was located three degrees away from its extrapolated location based on the approximate circular orbit determined in December 2000. The oldest known precovery image of Varuna was taken on 24 November 1954. These precovery images along with additional observations from Japan, Hawaii, and Arizona helped astronomers refine its orbit and determine Varuna's proper classification.
In January 2001, Varuna was assigned the minor planet number 20000 by the Minor Planet Center as its orbit was well determined from precovery images and subsequent observations. The minor planet number 20000 was particularly chosen to commemorate Varuna's large size, being the largest classical Kuiper belt object known at that time and was believed to be as large as Ceres. The number 20000 was also chosen to commemorate the coincidental 200th anniversary of the discovery of Ceres, which occurred in the same month as the numbering of Varuna.
Name Varuna is named after the eponymous Hindu deity Varuna, following the International Astronomical Union naming convention for non-resonant Kuiper belt objects after creator deities. The name was proposed by Indian choreographer Mrinalini Sarabhai, and was approved by the IAU in March 2001. Varuna is one of the oldest Vedic deities of Hindu literature, being mentioned in the earliest hymns of the Rigveda. In Hindu literature, Varuna created and presided over the waters of the heaven and of the ocean. Varuna is the king of gods and men and the universe, and has unlimited knowledge.
Planetary symbols are no longer much used in astronomy, so Varuna never received a symbol in the astronomical literature. There is no standard symbol for Varuna used by astrologers either. Denis Moskowitz, a software engineer in Massachusetts who designed the symbols for most of the dwarf planets, proposed a symbol for Varuna (): it derives from the Devanagari letter va व and Varuna's snake-lasso. This symbol is occasionally mentioned on astrology websites, but is not broadly used. Another sometimes seen is a variant of Neptune ( with a globe and outward-facing tines), as Varuna is the Hindu equivalent of Neptune.
Orbit and classification
Varuna orbits the Sun at an average distance of , taking 279 years to complete a full orbit. Its orbit is nearly circular, with a low orbital eccentricity of 0.056. Due to its low orbital eccentricity, its distance from the Sun varies only slightly over the course of its orbit. Varuna's minimum distance possible (MOID) from Neptune is 12.04 AU. Over the course of its orbit, Varuna's distance from the Sun ranges from 40.3 AU at perihelion (closest distance) to 45.1 AU at aphelion (farthest distance). Varuna's orbit is inclined to the ecliptic by 17 degrees, similar to Pluto's orbital inclination. Varuna had passed its perihelion in 1928 and is currently moving away from the Sun, approaching aphelion by 2071.
With a nearly circular orbit at around 40 to 50 AU, Varuna is classified as a classical Kuiper belt object (KBO), or sometimes a "cubewano". Varuna's semi-major axis of 42.8 AU is similar to that of other large classical KBOs such as Quaoar (a=43.7 AU) and Makemake (a=45.6 AU), although other orbital characteristics such as inclination widely differ. Varuna is a member of the "dynamically hot" class of classical KBOs, meaning that it has an orbital inclination greater than 4 degrees, the imposed maximum inclination for dynamically cold members of its population. As a classical KBO, Varuna is not in orbital resonance with Neptune and is also free from any significant perturbation by Neptune.
Rotation
Varuna has a rapid rotation period of approximately 6.34 hours, derived from a double-peaked solution for Varuna's rotational light curve. Varuna's rotation was first measured January 2001 by astronomer Tony Farnham using the McDonald Observatory's 2.1-meter telescope, as part of a study on the rotation and colors of distant objects. CCD photometry of Varuna's light curve in 2001 revealed that it displays large brightness variations with an amplitude of about 0.5 magnitudes. The measured rotational light curve of Varuna provided two ambiguous rotation periods of 3.17 and 6.34 hours, for a single-peaked and a double-peaked solution, respectively. Additional possible rotation periods of 2.79 and 3.66 hours were also obtained by Farnham, although these values could not be ruled out at the time.
A single-peaked interpretation of Varuna's rotational light curve (3.17 h) would assume a spherical shape for Varuna, with albedo features on its surface that would account for its brightness variations. However, in order for this interpretation to be valid, Varuna must have a density much greater than (roughly the density of water), otherwise it would deform and break apart as the given rotation period exceeds the critical rotation rate of ~3.3 hours for a body with a density of . A double-peaked interpretation of Varuna's rotational light curve (6.34 h) would assume that Varuna's shape is an elongated ellipsoid, with an estimated a/b aspect ratio of 1.5–1.6. The rotational light curve of Varuna was later investigated by astronomers David C. Jewitt and Scott Sheppard during February and April 2001, and concluded that the double-peaked interpretation for Varuna's light curve is the most plausible solution due to the absence of rotational variation in Varuna's color in the visible spectrum.
Examination of past photometric observations of Varuna's light curve has shown that its light curve amplitude had increased by roughly 0.13 magnitudes from 2001 to 2019. This increase in amplitude is due to the combined effects of Varuna's ellipsoidal shape, rotation, and varying phase angle. Geometric models for Varuna's changing amplitude have provided several possible solutions for the orientation of Varuna's rotational poles in ecliptic coordinates, with the best-fit solution adopting a spin axis right ascension and declination of 54° and −65°, respectively. The best-fit pole orientation of Varuna implies that it is being viewed at a near-edge on configuration, in which Varuna's equator nearly faces directly toward Earth.
Varuna's rapid rotation is believed to have resulted from disruptive collisions that have sped up its rotation during the formation of the Solar System. The present collision rate in the trans-Neptunian region is minimal, though collisions were more frequent during the formation of the Solar System. However, Jewitt and Sheppard calculated that the rate of disruptive collisions among large trans-Neptunian objects (TNOs) during the Solar System's formation is extremely uncommon, contradictory to the current abundance of binary and rapidly rotating TNOs that are believed to have originated from such collisions. To explain the abundance of binary and rapidly rotating TNOs, the rate of collisions among TNOs had likely increased as a result of Neptune's outward migration perturbing the orbits of TNOs, thus increasing the frequency of collisions which may have led to Varuna's rapid rotation.
Physical characteristics Size and shape + Size estimates for VarunaYearDiameter (km)MethodRefs2000thermal2002thermal2002best fit albedo2005thermal2005thermal2005thermal2007or 412.3~718.2or ≤744.1thermal(Spitzer 1-Band)2007>thermal(Spitzer 2-Band)2007thermal(adopted)2008thermal2010(long-axis minimum only)occultation2013thermal2013best fit albedo2013occultation2014 (minimum)occultation2019thermal
As a result of its rapid rotation, the shape of Varuna is deformed into a triaxial ellipsoid. Given the rapid rotation, rare for objects so large, Varuna's shape is described as a Jacobi ellipsoid, with an a/b aspect ratio of around 1.5–1.6 (in which Varuna's longest semi-axis a is 1.5–1.6 times longer than its b semi-axis). Examination of Varuna's light curve has found that the best-fit model for Varuna's shape is a triaxial ellipsoid with the semi-axes a, b, and c in ratios in the range of b/a = 0.63–0.80, and c/a = 0.45–0.52.
Due to Varuna's ellipsoidal shape, multiple observations have provided different estimates for its diameter, ranging from . Most diameter estimates for Varuna were determined by measuring its thermal emission, although size estimates have been constrained to smaller values as a result of higher albedos determined by space-based thermal measurements. Observations of stellar occultations by Varuna have also provided varying size estimates. An occultation by Varuna in February 2010 yielded a chord length of , inferred to be across its longest axis. Later occultations in 2013 and 2014 yielded mean diameters of and respectively.
Since the discovery of Varuna, , another larger rapidly rotating (3.9 h) object over twice the size of Varuna, has been discovered and is also thought to have an elongated shape, albeit slightly less pronounced (estimated ratios of b/a = 0.76~0.88, and c/a = 0.50~0.55, possibly due to a higher estimated density approximately ).
Status as a dwarf planet uncertain