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20000 Varuna

20000 Varuna
Artist's impression of Varuna
Discovered by R. McMillan (Spacewatch)
Discovery date 28 November 2000
MPC designation 20000 Varuna
Pronunciation [1]
Named after
2000 WR106
TNO (cubewano)[2]
Adjectives Varunian
Orbital characteristics[4]
Epoch 23 July 2010 (JD 2455400.5)
Aphelion 45.313 AU (6778.797 Gm)
Perihelion 40.494 AU (6057.848 Gm)
42.904 AU (6418.322 Gm)
Eccentricity 0.056
281.03 a (102646.1 d)
Average orbital speed
4.53 km/s
Inclination 17.2°
Physical characteristics
Dimensions 668+154
Mass ≈ 3.7×1020 kg[6][7]
Mean density
0.992 g/cm3[6]
0.22 m/s2
0.38 km/s
Sidereal rotation period
6.3418 h[5]
Albedo 0.127+0.040
Temperature ≈ 43–41 K
Spectral type
(moderately red)
19.9 (opposition)[9]

20000 Varuna is a large classical Kuiper belt object. It is probably a dwarf planet. It rotates rapidly and hence its shape is probably very elongated.


  • History 1
    • Discovery 1.1
    • Name 1.2
  • Size 2
    • Occultation 2.1
  • Orbit 3
  • Physical characteristics 4
  • Dwarf planet 5
  • References 6
  • External links 7



Varuna was discovered on 28 November 2000 by Rovert McMillan of Spacewatch. It was given the provisional designation 2000 WR106 and has been precovered in plates dating back to 1954.[4]


Varuna is named after a Hindu deity. Varuna was one of the most important deities of the ancient Indians, and he presided over the waters of the heaven and of the ocean and was the guardian of immortality.[10] Due to his association with the waters and the ocean, he is often identified with Greek Poseidon and Roman Neptune. Varuna received the minor planet number 20000 because it was the largest cubewano found so far and was believed to be as large as Ceres.[11]


Size estimates for Varuna
Year Diameter (km) Notes Refs
2001 900 Jewitt [12]
2002 1,060 Lellouch [13]
2005 936 Grundy [14]
2005 >621 Spitzer 2-Band [15][16]
2007 502 Spitzer 1-Band [15]
2010 1003 Chord [17]
2013 668+154
Herschel [5]

The size of the large Kuiper belt objects 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 Earth's atmosphere, because 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 diameter of Varuna have varied from 500 to 1,060 km.[18] Multi-band thermal measurements from the Herschel Space Observatory in 2013 yielded a diameter of 668+154


A 28-second occultation of an 11.1 magnitude star by Varuna was observed from Camalaú, Paraíba, Brazil, on the night of 19 February 2010.[19] Results of the 2010 occultation as seen from São Luís with a duration of 52.5 seconds corresponds with a chord of 1003 km.[17] But Quixadá 255 km away had a negative result suggesting a significantly elongated shape is required for Varuna.[17] Because the occultation occurred near Varuna's maximum brightness, the occultation was observing the maximum apparent surface area for an ellipsoidal shape.[17]


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 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 6.34 hours.[4] It has a double-peaked light curve. 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 1 g/cm3 (roughly the density of water).[20] 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 ratios in the range of b/a = 0.63–0.80, and c/a = 0.45–0.52 and a bulk density of 0.992+0.086
g/cm3.[6] Since the discovery of Varuna, Haumea, another, even larger, rapidly rotating (3.9 h) object, has been discovered and is 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] A recent study of the surface composition of (20000) Varuna.[23] After studying the spectra corresponding to different rotational phases, they do not find any indication of surface variability. They also find that the most probable composition for the surface of Varuna is a mixture of amorphous silicates (25%), complex organics (35%), amorphous carbon (15%) and water ice (25%). However, they also discuss another possible surface composition containing up to a 10% of methane ice. For an object with the characteristics of Varuna, this volatile could not be primordial, so an event, such as an energetic impact, would be needed to explain its presence on the surface.

Dwarf planet

The International Astronomical Union has not classified it as a dwarf planet. However, Brown places it on the high end of "highly likely",[24][25] and Tancredi (2010) classifies it as "accepted" but has not made a direct recommendation for its inclusion.[26]


  1. ^ Merriam Webster's Collegiate Dictionary. From the Sanskrit वरुण
  2. ^ "MPEC 2009-P26 :Distant Minor Planets (2009 AUG. 17.0 TT)". IAU Minor Planet Center. 7 August 2009. Retrieved 16 September 2009. 
  3. ^ M. W. Buie (12 January 2007). "Orbit Fit and Astrometric record for 20000". SwRI (Space Science Department). Retrieved 19 September 2008. 
  4. ^ a b c "JPL Small-Body Database Browser: 20000 Varuna (2000 WR106)". 9 February 2010. Retrieved 2 January 2011. Last observation as of 9 February 2010 
  5. ^ a b c d e Lellouch, E.; Santos-Sanz, P.; Lacerda, P.; Mommert, M.; Duffard, R.; Ortiz, J. L.; Müller, T. G.; Fornasier, S.; Stansberry, J.; Kiss, Cs.; Vilenius, E.; Mueller, M.; Peixinho, N.; Moreno, R.; Groussin, O.; Delsanti, A.; Harris, A. W. (September 2013). TNOs are Cool": A survey of the trans-Neptunian region. IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations""" (PDF). Astronomy & Astrophysics 557: A60.  
  6. ^ a b c P. Lacerda, D. Jewitt (2006). "Densities Of Solar System Objects From Their Rotational Lightcurve".  
  7. ^ Calculated using Lacerda and Jewitt (2007) diameter of 900 km and density of 0.992 g/cm3.
  8. ^ "TNO and Centaur Colors". Archived from the original on 8 September 2006. Retrieved 8 November 2006. 
  9. ^ "HORIZONS Web-Interface". JPL Solar System Dynamics. Retrieved 2 July 2008. 
  10. ^ MW Sanskrit–English dictionary
  11. ^ "M.P.C. 41805" (PDF). The Minor Planet Circulars/Minor Planets and Comets. 9 January 2001. Retrieved 4 July 2010. 
  12. ^ D. Jewitt, H. Aussel, A. Evans (2001). "The size and albedo of the Kuiper-belt object (20000) Varuna" (PDF).  
  13. ^ E. Lellouch; et al. (2002). "Coordinated thermal and optical observations of Trans-Neptunian object (20000) Varuna from Sierra Nevada".  
  14. ^ W. M. Grundy, K. S. Noll, D. C. Stephens (2005). "Diverse albedos of small trans-neptunian objects".  
  15. ^ a b J. Stansberry (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope".  
  16. ^ J. Stansberry; et al. (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from the Spitzer Space Telescope". The Solar System Beyond Neptune.  
  17. ^ a b c d Bruno Sicardy. "The 2010, February 19 stellar occultation by Varuna". 42nd DPS Meeting. Retrieved 12 November 2010. 
  18. ^ W. R. Johnston (2008). "TNO/Centaur diameters and albedos". Retrieved 8 November 2006. 
  19. ^ "RELATÓRIO FINAL OCULTAÇÃO DA ESTRELA UCAC2 41014042 PELO ASTEROIDE VARUNA" (PDF) (in Português). Retrieved 18 September 2010. 
  20. ^ D. Jewitt, S. Sheppard (2002). "Physical Properties Of Trans-Neptunian Object (20000) Varuna".  
  21. ^ D. L. Rabinowitz; et al. (2006). "Photometric Observations Constraining the Size, Shape, and Albedo of 2003 EL61, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt".  
  22. ^ J. Licandro, E. Oliva, M. di Martino (2001). "NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106".  
  23. ^ V. Lorenzi, N. Pinilla-Alonso, J, Licandro, C. Dalle-Ore, J. P. Emery (2014). "Rotationally- resolved spectroscopy of (20000) Varuna in the near-Infrared".  
  24. ^ Michael E. Brown (23 September 2011). "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 23 September 2011. 
  25. ^ Tancredi, G.; Favre, S. (2008). "Which are the dwarfs in the solar system?" (PDF). Asteroids, Comets, Meteors. Retrieved 23 September 2011. 
  26. ^ Tancredi, G. (2010). "Physical and dynamical characteristics of icy "dwarf planets" (plutoids)". Icy Bodies of the Solar System: Proceedings IAU Symposium No. 263, 2009. Retrieved 17 January 2012. 

External links

  • "Size and Albedo of Kuiper Belt Object (20000) Varuna". David Jewitt's Home Page. Retrieved 23 January 2010. 
  • Orbital simulation from JPL (Java) / Ephemeris
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