(55637) 2002 UX25
2002 UX25 and satellite, as seen by Hubble
Discovered bySpacewatch (291)
Discovery siteKitt Peak National Obs.
Discovery date30 October 2002
(55637) 2002 UX25
Cubewano (MPC)[2]
Extended (DES)[3]
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 2
Observation arc25.26 yr (9,228 days)
Earliest precovery date12 October 1991
Aphelion48.491 AU
Perihelion36.485 AU
42.488 AU
276.95 yr (101,157 days)
4.54 km/s
0° 0m 12.96s / day
≈ 5 September 2066[4]
±3 days
Known satellites1
(ø: 190–260 km)[5][6]
Physical characteristics
Mean diameter
665±29 km[7]
659±38 km[8]
Mass(1.25±0.03)×1020 kg[6]
Mean density
0.82±0.11 g/cm3
(assuming equal densities
for primary and satellite)[6]
0.80±0.13 g/cm3[8]
Equatorial surface gravity
0.075 m/s2
Equatorial escape velocity
0.227 km/s
14.382±0.001 h[9]
Temperature≈ 43 K
19.8 [11]
3.87±0.02,[9] 4.0[1]

(55637) 2002 UX25 (provisional designation 2002 UX25) is a trans-Neptunian object that orbits the Sun in the Kuiper belt beyond Neptune. It briefly garnered scientific attention when it was found to have an unexpectedly low density of about 0.82 g/cm3.[12]

2002 UX25 has an absolute magnitude of about 4.0,[1] and Spitzer Space Telescope results estimate it to be about 681 km in diameter.[13] The low density of this and many other mid sized TNOs implies that they have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets.[14]

It was discovered on 30 October 2002, by the Spacewatch program.[15]

Numbering and naming

This minor planet was numbered (55637) by the Minor Planet Center on 16 February 2003 (M.P.C. 47763).[16] As of 2023, it has not been named.[17]


2002 UX25 (vmag 19.9) as viewed with a 24" telescope

2002 UX25 has a perihelion of 36.7 AU,[1] which it will next reach in 2065.[1] As of 2020, 2002 UX25 is 40 AU from the Sun.[11]

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

It has been observed 212 times with precovery images dating back to 1991.[1]

Physical characteristics

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).[18] The light-curve amplitude is ΔM = 0.21±0.06.[9]

The analysis of combined thermal radiometry of 2002 UX25 from measurements by the Spitzer Space Telescope and Herschel Space Telescope indicates an effective diameter of 692 ± 23 km and albedo of 0.107+0.005
. Assuming equal albedos for the primary and secondary it leads to the size estimates of ~664 km and ~190 km, respectively. If the albedo of the secondary is half of that of the primary the estimates become ~640 and ~260 km, respectively.[6] Using an improved thermophysical model slightly different sizes were obtained for UX25 and its satellite: 659 km and 230 km, respectively.[8]

2002 UX25 has red featureless spectrum in the visible and near-infrared but has a negative slope in the K-band, which may indicate the presence of the methanol compounds on the surface.[7] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300, in spite of similar brightness and orbital elements.


With a density of 0.82 g/cm3, assuming that the primary and satellite have the same density, 2002 UX25 is one of the largest known solid objects in the Solar System that is less dense than water.[12] Why this should be is not well understood, because objects of its size in the Kuiper belt often contain a fair amount of rock and are hence pretty dense. To have a similar composition to others large KBOs, it would have to be exceptionally porous, which was believed to be unlikely given the compactability of water ice;[6] this low density thus astonished astronomers.[12] Studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is brittle and can support significant porosity in objects significantly larger than 2002 UX25, particularly if rock is present; the low density could thus be a consequence of this object failing to warm sufficiently during its formation to significantly deform the ice and fill these pore spaces.[19]

Density comparison
What Density
Settled snow 0.2–0.3 [20]
Slush/firn 0.7–0.8 [20]
2002 UX25 0.82 [6]
Glacier ice 0.83–0.92 [20]
Tethys 0.984 [21]
Liquid water 1 [20]


A simulated circular orbit of 210-km-diameter moon at a distance of 4770 km

S/2005 (55637) 1
Discovered byBrown et al.
Discovery date2007
Orbital characteristics
4750±40 km[22]
8.3095 d (prograde)[22]
Satellite of2002 UX25
Physical characteristics
Mean radius
105±15 km[23]

The discovery of a minor-planet moon was reported in IAUC 8812 on 22 February 2007.[5] The satellite was detected using the Hubble Space Telescope in August 2005.[5] The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5.[24] It orbits the primary in 8.309±0.0002 days,[9] at a distance of 4770±40 km, yielding a system mass of (1.25±0.03)×1020 kg.[6][9] The eccentricity of the orbit is 0.17±0.03.[9]

This moon is estimated to be 210±30 km in diameter.[7] Assuming the same albedo as the primary, it would have a diameter of 190 km, assuming an albedo of 0.05 (typical of other cold, classical KBOs of similar size) a diameter of 260 km.[6]


  1. ^ a b c d e f g "JPL Small-Body Database Browser: 55637 (2002 UX25)" (2017-01-16 last obs.). Jet Propulsion Laboratory. Retrieved 24 February 2018.
  2. ^ a b "MPEC 2009-C70 :Distant Minor Planets (2009 FEB. 28.0 TT)". Minor Planet Center. 10 February 2009. Retrieved 5 July 2011.
  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 12 March 2009.
  4. ^ JPL Horizons Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is 3-sigma.)
  5. ^ a b c Daniel W. E. Green (22 February 2007). "IAUC 8812: Sats OF 2003 AZ_84, (50000), (55637), (90482)". International Astronomical Union Circular. Archived from the original on 19 July 2011. Retrieved 5 July 2011.
  6. ^ 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". The Astrophysical Journal Letters. 778 (2): L34. arXiv:1311.0553. Bibcode:2013ApJ...778L..34B. doi:10.1088/2041-8205/778/2/L34. S2CID 17839077.
  7. ^ a b c d Fornasier, S.; Lellouch, E.; Müller, P., T.; et al. (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". Astronomy & Astrophysics. 555: A92. arXiv:1305.0449v2. Bibcode:2013A&A...555A..15F. doi:10.1051/0004-6361/201321329. S2CID 119261700.
  8. ^ a b c d Brown, Michael E.; Butler, Bryan J. (20 June 2017). "The Density of Mid-sized Kuiper Belt Objects from ALMA Thermal Observations". The Astronomical Journal. 154 (1): 19. arXiv:1702.07414. Bibcode:2017AJ....154...19B. doi:10.3847/1538-3881/aa6346.
  9. ^ a b c d e f "(55637) 2002 UX25". www.johnstonsarchive.net. Archived from the original on 28 June 2012. Retrieved 21 May 2020.
  10. ^ a b c Hainaut, O. R.; Boehnhardt, H.; Protopapa, S. (October 2012). "Colours of minor bodies in the outer solar system. II. A statistical analysis revisited" (PDF). Astronomy & Astrophysics. 546: 20. arXiv:1209.1896. Bibcode:2012A&A...546A.115H. doi:10.1051/0004-6361/201219566. S2CID 54776793.
  11. ^ a b "AstDys (55637) 2002UX25 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 12 December 2020.
  12. ^ a b c Cowen, Ron (2013). "Astronomers surprised by large space rock less dense than water". Nature News. doi:10.1038/nature.2013.14135. S2CID 123788849.
  13. ^ John Stansberry; Will Grundy; Mike Brown; Dale Cruikshank; John Spencer; David Trilling; et al. (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope" (PDF). In M. Antonietta Barucci; Hermann Boehnhardt; Dale P. Cruikshank (eds.). The Solar System Beyond Neptune. University of Arizona press. pp. 161–179. arXiv:astro-ph/0702538. Bibcode:2008ssbn.book..161S. ISBN 978-0-8165-2755-7.
  14. ^ W.M. Grundy, K.S. Noll, M.W. Buie, S.D. Benecchi, D. Ragozzine & H.G. Roe, 'The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)', Icarus (forthcoming, available online 30 March 2019) Archived 7 April 2019 at the Wayback Machine DOI: 10.1016/j.icarus.2018.12.037,
  15. ^ Marsden, Brian G. (1 November 2002). "MPEC 2002-V08 : 2002 UX25". IAU Minor Planet Center. Harvard-Smithsonian Center for Astrophysics. Retrieved 5 July 2011.
  16. ^ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 24 February 2018.
  17. ^ "55637 (2002 UX25)". Minor Planet Center. Retrieved 12 December 2020.
  18. ^ 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.
  19. ^ "The Mutual Orbit, Mass, and Density of Transneptunian Binary" (PDF). 7 April 2019. Archived from the original (PDF) on 7 April 2019. Retrieved 21 May 2020.
  20. ^ a b c d "Typical densities of snow and ice (kg/m3)". Archived from the original on 1 January 2014. Retrieved 21 May 2020.
  21. ^ Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2
  22. ^ a b c d "2002 UX25 System parameters". Will Grundy. Retrieved 29 August 2023.
  23. ^ "(55637) 2002 UX25". Retrieved 28 July 2023.
  24. ^ Distant EKO The Kuiper Belt Electronic newsletter, March 2007