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This article is about 20000 Varuna, a minor planet. For other uses of the word, see Varuna (disambiguation)

20000 Varuna

Artist's conception of Varuna
Discovery
Discovered by	R. McMillan (Spacewatch)
Discovery date	28 November 2000
Designations
MPC designation	20000 Varuna
Pronunciation	/ˈværənə/ VARR-ə-nə [3]
Alternate name	2000 WR106
Minor planet category	TNO (cubewano)[1] Scat-Ext[2]
Orbital characteristics
Epoch 14 July 2004 (JD 2453200.5)
Aphelion	6 781.985 Gm (45.335 AU)
Perihelion	6 120.810 Gm (40.915 AU)
Semi-major axis	6 451.398 Gm (43.129 AU)
Eccentricity	0.051
Orbital period	103 440.6 d (283.20 a)
Average orbital speed	4.53 km/s
Mean anomaly	89.673°
Inclination	17.2°
Longitude of ascending node	97.296°
Argument of perihelion	271.631°
Physical characteristics
Dimensions	800 km (avg of thermals)[4] 500 ± 100 km (Spitzer adopted)[5] (scalene ellipsoid)?[6][7]
Mass	≈3.7 × 1020? kg[6][8]
Mean density	0.992 g/cm³[6]
Equatorial surface gravity	0.15 m/s²
Escape velocity	0.39 km/s
Sidereal rotation period	0.132 16 d (3.17 h)
Albedo	0.037–0.26[4]
Temperature	≈43–41 K
Spectral type	(moderately red) B-V=0.93 V-R=0.64[9]
Apparent magnitude	19.9 (opposition)[10]
Absolute magnitude (H)	3.7[11]

20000 Varuna is a large classical Kuiper Belt object (KBO) and a potential dwarf planet. It previously had the provisional designation 2000 WR₁₀₆ and has been precovered in plates dating back to 1953.

Name

Varuna is named after the Hindu deity, Varuṇa. Varuṇa was one of the most important deities of the ancient Indo-Aryans, and he presided over the waters of the heaven and of the ocean and as the guardian of immortality.^[12] Due to his association with the waters and the ocean, he is often identified with Greek Poseidon and Roman Neptune.

Size

The size of the large KBOs can be determined by simultaneous observations of thermal emission and reflected sunlight. Unfortunately, thermal measures, intrinsically weak for distant objects, are further hampered by the absorption of the Earth atmosphere as only the weak ‘tail’ of the emissions is accessible to Earth-based observations. In addition, the estimates are model-dependent with the unknown parameters (e.g. pole orientation and thermal inertia) to be assumed. Consequently, the estimates of the albedo vary resulting in sometimes substantial differences in the inferred size.

Estimates for the size of Varuna have varied from 500 to 1060 km.^[4] The two most recent estimates from Spitzer are closer to the 500km range and inconsistent with the 2005 estimate of a size of 936 ⁺²³⁸₋₃₂₄ km,^[15] based on earlier results (900 ⁺¹²⁹₋₁₄₅)^[13] and (1060 ⁺¹⁸⁰_−-220),^[14]. This inconsistency of the Spitzer results with the earlier (sub-millimetre) observations was recently addressed by the original authors (Stansberry et al.); given a number of difficulties in Varuna case, the authors are inclined to favor the sub-millimetre results (Jewitt, Lellouch) for this object over those from Spitzer.^{[16 ]}

Varuna was predicted to occult a magnitude 14.7 star in Gemini on December 7, 2008.^[17] This type of event in principle is of the type that might have allowed at least a lower limit to be placed on Varuna's size. If multiple observers at different locations record the event, several chords across Varuna might have been measured, which would have allowed the precise size to be measured.^[18] Predictions suggested the event was visible only from South America and southern Africa.^[19] The collaboration of observers did not report a conclusive observation of the event,^[20] but they have also not yet released their peer-reviewed data summary.

Orbit

Orbits of Varuna (blue) and Pluto (red).

Varuna is classified as a classical trans-Neptunian object and follows a near-circular orbit with a semi-major axis of ≈43 AU, similar to that of Quaoar but more inclined. Its orbital period is similar to Quaoar at 283 Earth years. The graph shows the polar view (top; Varuna’s orbit in blue, Pluto’s in red, Neptune in grey). The spheres illustrate the current (April 2006) positions, relative sizes and colours. The perihelia (q), aphelia (Q) and the dates of passage are also marked. Interestingly, the orbits of Varuna and Pluto have similar inclination and are similarly oriented (the nodes of both orbits are quite close).

At 43 AU and on a near-circular orbit, unlike Pluto which is in 2:3 orbital resonance with Neptune, Varuna is free from any significant perturbation from Neptune. The ecliptic view illustrates the comparison of Varuna's near-circular orbit with that of Pluto (highly eccentric, e=0.25), both similarly inclined.

Physical characteristics

Varuna has a rotational period of approximately 3.17 hours (or 6.34 hours, depending on whether the light curve is single or double-peaked). Given the rapid rotation, rare for objects so large, Varuna is thought to be an elongated spheroid (ratio of axis 2:3), with a mean density around 1g/cm³ (roughly the density of water).^[7] Examination of Varuna's light curve has found that the best-fit model for Varuna is a triaxial ellipsoid with the axes a,b,c in relations: b/a = 0.63 − 0.80, c/a = 0.45 − 0.52 and a bulk density of 0.992 g/cm³.^[6] Since the discovery of Varuna, another, even larger, rapidly rotating (3.9 h) object Haumea has been discovered, also thought to have an elongated shape.^[21]

The surface of Varuna is moderately red (similar to Quaoar) and small amounts of water ice have been detected on its surface.^[22]

References

External links

• "Size and Albedo of Kuiper Belt Object (20000) Varuna". David Jewitt's Home Page. http://www.ifa.hawaii.edu/faculty/jewitt/varuna.html. Retrieved 2005-08-15.   Site updated Reretrived on 2006-07-10
• Orbital simulation from JPL (Java) / Ephemeris