20000 Varuna is a large classical Kuiper Belt object (KBO). It previously had the provisional designation 2000 WR106 and has been precovered in plates dating back to 1953. Under the International Astronomical Union's 2006 draft proposal for the definition of a planet, Varuna would be labelled a dwarf planet if it were proven to be spherical.

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.

A recent thermal model estimates the size at 936 +238/-324 km. This estimate is based on earlier results (900 +129/-145) and (1060 +180/-220).

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 ice). 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.[2] Since the discovery of Varuna, another, even larger, rapidly rotating (3.9 h) object (136108) 2003 EL61 has been discovered, also thought to have an elongated shape.

The surface of Varuna is moderately red (similar to Quaoar) but exceptionally dark (albedo<0.04) compared with other large classical Kuiper Belt objects, suggesting that the surface is largely devoid of ice, although small amounts of water ice have been detected on its surface [9].

SOURCE: Wikipedia - 20000 Varuna