20000 Varuna

Lore & Background

Varuna was discovered by Robert McMillan using the Spacewatch 0.9-meter telescope on 28 November 2000. McMillan identified the object by manually comparing multiple scans using the blinking technique, as his real-time software did not detect it. Follow-up observations by Jeffrey Larsen confirmed the object, and the discovery was announced on 1 December 2000. Precovery images from 1954 and 1955 helped refine its orbit. The minor planet number 20000 was assigned in January 2001 to commemorate its large size and the 200th anniversary of Ceres's discovery. The name Varuna, proposed by Indian choreographer Mrinalini Sarabhai and approved by the IAU in March 2001, follows the convention of naming non-resonant Kuiper belt objects after creator deities. Varuna's surface is moderately red due to complex organic compounds, and water ice is present, likely exposed by past collisions that may have also caused its rapid rotation. Its light curve is compatible with a Jacobi ellipsoid shape, and analysis of light curve variations in 2019 suggests a possible closely orbiting satellite. Assumptions of hydrostatic equilibrium yield a density so low that it is likely not a dwarf planet.

Reader's Guide

Varuna's significance lies in its status as one of the largest classical Kuiper belt objects known at its discovery, comparable in size to Ceres. Its rapid rotation period of 6.34 hours and elongated shape (aspect ratio 1.5–1.6) provide insights into the internal structure and collisional history of trans-Neptunian objects. The presence of water ice and organic compounds on its surface, along with evidence of past collisions, informs models of Kuiper belt composition and evolution. The possible satellite suggested by 2019 light curve analysis, though not directly imaged, adds to the understanding of binary systems in the outer Solar System. Varuna's nearly circular orbit at 42.7 AU and its classification as a dynamically hot classical KBO help distinguish between different populations within the Kuiper belt. The object's naming after the Hindu deity Varuna reflects the IAU convention for such bodies, and its numbering as 20000 was a deliberate commemoration of its size and the anniversary of Ceres's discovery.

Did You Know?

• Varuna's rotation period of 6.34 hours is derived from a double-peaked solution for its light curve.
• The oldest known precovery image of Varuna was taken on 24 November 1954.
• Analysis of light curve variations in 2019 suggests the presence of a possible satellite orbiting closely around Varuna.

Discovery and Confirmation

Robert McMillan spotted Varuna on November 28, 2000, while conducting a Spacewatch survey with the 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Arizona. What made the find notable was that his real-time detection software missed the object; instead, McMillan caught it by manually comparing multiple scans of a moderately dense star field near the northern galactic equator using the blinking technique, watching for slow motion against background stars. After his shift ended, astronomer Jeffrey Larsen took over follow-up observations to confirm the moving body. Together, the two observers accumulated twelve observations across three nights before the discovery was formally announced in a Minor Planet Electronic Circular on December 1, 2000, under the provisional designation 2000 WR106. The object's apparent magnitude of 20 at such a distance initially led astronomers to speculate it might rival Ceres in size. German astronomers Andre Knofel and Reiner Stoss later found precovery images at Palomar, the oldest dating to November 24, 1954, which helped refine the orbit. In January 2001, the Minor Planet Center assigned it the number 20000, a choice that both honored its status as the largest classical Kuiper belt object known then and marked the 200th anniversary of Ceres's discovery.

Shape, Surface, and Possible Satellite

Varuna's light curve data points to a body that is not spherical but elongated, consistent with a Jacobi ellipsoid shape produced by rapid rotation. Its surface carries a moderately red hue, which astronomers attribute to complex organic compounds accumulated over time. Water ice is also detectable on the surface, and the prevailing interpretation is that past collisions both exposed this ice and imparted the fast spin that stretches the body. In 2019, careful analysis of subtle variations in the light curve raised the possibility of a small, closely orbiting satellite, though no moon has ever been directly imaged or confirmed. The question of whether Varuna qualifies as a dwarf planet has been raised, but assuming hydrostatic equilibrium leads to a calculated density so low that it likely fails that criterion. In other words, Varuna appears to be a large, irregularly shaped, rapidly spinning Kuiper belt body with a chemically interesting surface and perhaps a small companion, rather than a rounded dwarf planet.

Orbit and Dynamical Classification

Varuna circles the Sun at a mean distance of about 42.7 astronomical units, completing one full orbit in roughly 279 years. Its orbital eccentricity is a low 0.056, meaning the distance from the Sun shifts only modestly, from 40.3 AU at perihelion to 45.1 AU at aphelion. The orbit is tilted 17 degrees relative to the ecliptic, a value comparable to Pluto's inclination. Because of this inclination, which exceeds the four-degree threshold, Varuna is classified as a dynamically hot classical Kuiper belt object, sometimes called a cubewano. Its semi-major axis places it in the same broad neighborhood as other large classical KBOs such as Quaoar and Makemake, though their inclinations differ considerably. The minimum distance Varuna can reach from Neptune is about 12 AU, keeping it well clear of the ice giant. Varuna passed perihelion in 1928 and is now drifting outward, expected to reach aphelion around 2071.

Naming and Cultural Resonance

Following the International Astronomical Union's convention of naming non-resonant Kuiper belt objects after creator deities, Varuna was named for the Hindu god of the same name. The name was proposed by Indian choreographer Mrinalini Sarabhai and received IAU approval in March 2001. In Vedic literature, Varuna is among the oldest deities, appearing in the earliest hymns of the Rigveda. He is described as the king of gods and men, the creator and overseer of the waters of heaven and the ocean, and a figure of unlimited knowledge. In modern astronomical practice, planetary symbols have largely fallen out of use, so Varuna never received an official symbol. However, Massachusetts-based software engineer Denis Moskowitz, who designed symbols for several dwarf planets, proposed one derived from the Devanagari letter va and Varuna's snake-lasso. A variant of the Neptune symbol also appears occasionally on astrology websites, reflecting the traditional pairing of Varuna with Neptune as their respective cultural counterparts.