(55637) 2002 UX25

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(55637) 2002 UX25
20131105 2002 UX25 hst.png
2002 UX25 as seen by Hubble Space Telescope
Discovered by Spacewatch (291)
Discovery date October 30, 2002
MPC designation (55637) 2002 UX25
Cubewano (MPC)[2]
Extended (DES)[3]
Orbital characteristics[6]
Epoch July 23, 2010 (JD 2455400.5 )
Aphelion 48.923 AU
(7 318.7 Gm)
Perihelion 36.815 AU
(5 507.4 Gm)
42.869 AU
(6 413.1 Gm)
Eccentricity 0.14121
280.69 yr (102,522 d)
4.54 km/s
Inclination 19.398°
Known satellites One ~190–260 km in diameter[4][5]
Physical characteristics
Dimensions ~650 km[5]
Mass (1.25±0.03)×1020 kg[5]
Mean density
0.82±0.11 g/cm3[5]
0.075 m/s2
0.227 km/s
14.382 h[6]
Albedo 0.115+0.05
Temperature ≈43 K
Spectral type
(pushing red)
B−V=1.12, V−R=0.61[9]
B−V=0.95, V−R=0.56[10][11]
2002 UX25 (apparent magnitude 19.9) as viewed with a 24" telescope

(55637) 2002 UX25 is a Spitzer dwarf-planet candidate that orbits the Sun in the Kuiper belt beyond Neptune. Its orbit takes roughly 280 years, and it has one known moon. This moon makes it much easier to calculate mass, and when the size is known, also a density. The density has been a surprise to astronomers, about 0.82 g/cm3.[13]

It is a trans-Neptunian object with an absolute magnitude of 3.6,[6] making it highly likely to be a dwarf planet. The Spitzer Space Telescope results estimate it to be about 681 km in diameter.[8] It was discovered on October 30, 2002, by the Spacewatch program.[1] It is a mid-sized cubewano similar to (20000) Varuna.

Brightness and color[edit]

A variability of the visual brightness was detected which could be fit to a period of 14.38 or 16.78 h (depending on a single-peaked or double peaked curve).[14]

The Spitzer Space Telescope has estimated it to have a size of 681+116
.[8] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300, in spite of similar brightness and orbit elements.


The discovery of a satellite of 2002 UX25 was reported in IAUC 8812 on 22 February 2007.[4] The satellite was detected using the Hubble Space Telescope in August 2005.[4] The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5.[15] It orbits the primary in 8.3 days, with a semimajor axis of 4770±40 km, yielding a system mass of 1.25×1020 kg.[5] Assuming the same albedo as the primary, the magnitude suggests the satellite has a diameter of 190 km; assuming a 5% albedo (typical of other similar sized, cold, classical KBOs) suggests a diameter of 260 km.[5]


2002 UX25 has a perihelion (closest approach to the Sun) of 36.7 AU,[6] which it will next reach in 2065.[6] As of 2010, 2002 UX25 is 41 AU from the Sun.[12]

The Minor Planet Center (MPC) classifies 2002 UX25 as a cubewano[2] while the Deep Ecliptic Survey (DES) classifies it as scattered-extended.[3] The DES using a 10My integration (last observation: 2009-10-22) shows it with a minimum perihelion (qmin) distance of 36.3 AU.[3]

2002 UX25 has been observed 60 times with precovery images back to 1991.[6]


Density comparison
What Density Notes
Settled snow 0.2–0.3 g/cm3 [16]
Very wet snow and firn 0.7–0.8 g/cm3 [16]
2002 UX25 0.82 g/cm3 [5]
Glacier ice 0.83–0.917 g/cm3 [16]
Tethys 0.984 g/cm3 [17]
Liquid water 1 g/cm3 [16]

2002 UX25 is one of the largest known solid objects in the Solar System that is less dense than water. Why this should be is not well understood: objects of its size in the Kuiper belt are typically rocky and dense, and to have a similar composition to others of its kind, it would have to be exceptionally porous, which is unlikely given the compactability of water ice.[5] Its density of 0.82 g/cm3 shocked astronomers.[13]

Dwarf planet status[edit]

The Spitzer Space Telescope has estimated 2002 UX25 to have a diameter of 681+116
,[8] and most icy objects around 400 km in diameter are believed to be spherical.[18] Mike Brown's website lists it as a highly likely dwarf planet.[19] Light-curve analysis has questioned whether it would truly qualify as a dwarf planet.[20][21]


  1. ^ a b Marsden, Brian G. (2002-11-01). "MPEC 2002-V08 : 2002 UX25". IAU Minor Planet Center. Harvard-Smithsonian Center for Astrophysics. Retrieved 2011-07-05. 
  2. ^ a b "MPEC 2009-C70 :Distant Minor Planets (2009 FEB. 28.0 TT)". Minor Planet Center. 2009-02-10. Retrieved 2011-07-05. 
  3. ^ a b c Marc W. Buie. "Orbit Fit and Astrometric record for 55637" (2009-10-22 using 60 observations). SwRI (Space Science Department). Retrieved 2009-03-12. 
  4. ^ a b c Daniel W. E. Green (2007-02-22). "IAUC 8812: Sats OF 2003 AZ_84, (50000), (55637), (90482)". International Astronomical Union Circular. Archived from the original on 19 July 2011. Retrieved 2011-07-05. 
  5. ^ a b c d e f g h M.E. Brown, 2013, "The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets"
  6. ^ a b c d e f g "JPL Small-Body Database Browser: 55637 (2002 UX25)" (2009-10-22 last obs). Retrieved 2008-12-09. 
  7. ^ S. Fornasier, E. Lellouch, T. Müller, P. Santos-Sanz, P. Panuzzo, C. Kiss, T. Lim, M. Mommert, D. Bockelée-Morvan, E. Vilenius, J. Stansberry, G.P. Tozzi, S. Mottola, A. Delsanti, J. Crovisier, R. Duffard, F. Henry, P. Lacerda, A. Barucci, & A. Gicquel (2013). TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70–500 µm.
  8. ^ a b c d e John Stansberry, Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". arXiv:astro-ph/0702538 [astro-ph]. 
  9. ^ TNO Colors
  10. ^ Tegler, Stephen C. (2007-02-01). "Kuiper Belt Object Magnitudes and Surface Colors". Retrieved 2009-12-30. 
  11. ^ Doressoundiram (2004). "The Meudon Multicolor Survey (2MS) of Centraurs and Trans-Neptunian objects". Retrieved 2006-11-06. 
  12. ^ a b "AstDys (55637) 2002UX25 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2009-11-19. 
  13. ^ a b Ron Cowen - Nature - Astronomers surprised by large space rock less dense than water Kuiper belt object challenges planet-formation theories. (13 November 2013)
  14. ^ Rousselot, P.; Petit, J.-M.; Poulet, F.; Sergeev, A. Photometric study of Centaur (60558) 2000 EC98 and trans-neptunian object (55637) 2002 UX25 at different phase angles, Icarus, 176, (2005) pp. 478–491.Abstract.
  15. ^ Distant EKO The Kuiper Belt Electronic newsletter, March 2007
  16. ^ a b c d Typical densities of snow and ice (kg/m³)
  17. ^ Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2
  18. ^ Mike Brown. "The Dwarf Planets". Archived from the original on 29 January 2008. Retrieved 2008-01-20. 
  19. ^ Michael E. Brown. "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 2011-08-25. 
  20. ^ Gonzalo Tancredi and Sofía Favre (October 13, 2008). "Dwarf Planet & Plutoid Headquarters". Portal Uruguayo de Astronomía. Retrieved 2010-09-22.  (Which are the dwarfs in the Solar System?)
  21. ^ Tancredi, Gonzalo (2009). "Physical and dynamical characteristics of icy "dwarf planets" (plutoids)". Proceedings of the International Astronomical Union Symposium S263 5: 173–185. Bibcode:2010IAUS..263..173T. doi:10.1017/S1743921310001717. 

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