55637 Uni

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(55637) Uni
20131105 2002 UX25 hst.png
Uni and Tinia as seen by the Hubble Space Telescope
Discovery [1]
Discovered by Spacewatch (291)
Discovery site Kitt Peak National Obs.
Discovery date30 October 2002
Designations
Designation
(55637) Uni
Named after
 Uni
2002 UX25
Cubewano (MPC) [2]
Extended (DES) [3]
Orbital characteristics [1]
Epoch 5 May 2025 (JD 2460800.5)
Uncertainty parameter 0
Observation arc 33.35 yr (12,182 days)
Earliest precovery date12 October 1991
Aphelion 49.291 AU
Perihelion 36.716 AU
43.003 AU
Eccentricity 0.1462
282.01 yr (103,005 days)
Average orbital speed
4.54 km/s
309.49°
0° 0m 12.24s / day
Inclination 19.400°
204.57°
≈ 5 September 2066 [4]
±3 days
275.27°
Known satellites 1: Tinia
Physical characteristics
665±29 km [5]
659±38 km [6]
Mass (1.25±0.03)×1020 kg [7]
Mean density
0.82±0.11 g/cm3
(assuming equal densities
for primary and satellite) [7]
0.80±0.13 g/cm3 [6]
0.075 m/s2
0.227 km/s
14.382±0.001 h [8]
Albedo 0.107+0.005
−0.008 [5]
0.1±0.01 [6]
Temperature ≈ 43 K
Spectral type
 B–V=1.007±0.043 [9]
V−R=0.540±0.030 [9]
V−I=1.046±0.034 [9]
19.8 [10]
3.87±0.02, [8] 4.0 [1]

    55637 Uni (provisional designation 2002 UX25) is a large trans-Neptunian planetoid 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. [11] It was discovered on 30 October 2002, by the Spacewatch program. [12]

    Contents

    Uni has an absolute magnitude of about 4.0, [1] and Spitzer Space Telescope results estimate it to be about 660 km in diameter. [6] The low density of this and many other mid-sized TNOs implies that they have never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets. [13]

    There is one known moon, Tinia, discovered in 2005.

    Numbering and naming

    Uni was numbered (55637) by the Minor Planet Center on 16 February 2003 ( M.P.C. 47763). [14] On 1 September 2025, the object was named after Uni, the Etruscan goddess of love and fertility. [15]

    Classification

    Uni (vmag 19.9) as viewed with a 24" telescope UX25-LB1-2009Nov19-06UT.jpg
    Uni (vmag 19.9) as viewed with a 24" telescope

    Uni has a perihelion of 36.7  AU, [1] which it will next reach in 2065. [1] As of 2020, Uni is 40 AU from the Sun. [10]

    The Minor Planet Center classifies Uni 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). [16] The light-curve amplitude is ΔM = 0.21±0.06. [8]

    The analysis of combined thermal radiometry of Uni 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
    −0.008    . [17] 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. [7] Using an improved thermophysical model slightly different sizes were obtained for Uni and Tinia: 659 km and 230 km, respectively. [6]

    Uni 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. [5] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300 , in spite of similar brightness and orbital elements.

    Composition

    With a density of 0.82 g/cm3, assuming that the primary and satellite have the same density, Uni is one of the largest known solid objects in the Solar System that is less dense than water. [11] 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; [7] this low density thus astonished astronomers. [11] 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 Uni, 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. [18]

    Density comparison
    MaterialDensity
    (g/cm3)    		Notes
    Settled snow0.2–0.3 [19]
    Slush/firn 0.35–0.9 [19]
    Uni0.82 [7]
    Glacier ice0.83–0.92 [19]
    Tethys 0.984 [20]
    Liquid water1 [19]

    See also

    References

    1. 1 2 3 4 5 6 7 "JPL Small-Body Database Browser: 55637 (2002 UX25)" (2025-02-17 last obs.). Jet Propulsion Laboratory . Retrieved 16 August 2025.
    2. 1 2 "MPEC 2009-C70 :Distant Minor Planets (2009 FEB. 28.0 TT)". Minor Planet Center. 10 February 2009. Retrieved 5 July 2011.
    3. 1 2 3 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. 1 2 3 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.
    6. 1 2 3 4 5 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 .
    7. 1 2 3 4 5 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.
    8. 1 2 3 "(55637) 2002 UX25". www.johnstonsarchive.net. Archived from the original on 28 June 2012. Retrieved 21 May 2020.
    9. 1 2 3 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.
    10. 1 2 "AstDys (55637) 2002UX25 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 12 December 2020.
    11. 1 2 3 Cowen, Ron (2013). "Astronomers surprised by large space rock less dense than water". Nature News. doi:10.1038/nature.2013.14135. S2CID   123788849.
    12. ↑ Marsden, Brian G. (1 November 2002). "MPEC 2002-V08 : 2002 UX25". IAU Minor Planet Center. Harvard-Smithsonian Center for Astrophysics. Retrieved 5 July 2011.
    13. ↑ 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,
    14. ↑ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 24 February 2018.
    15. ↑ "WGSBN Bulletin (Vol. 5, No. 20)" (PDF). IAU. 1 September 2025. p. 6.
    16. ↑ 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.
    17. ↑ 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.
    18. ↑ "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.
    19. 1 2 3 4 "Typical densities of snow and ice (kg/m3)". Archived from the original on 1 January 2014. Retrieved 21 May 2020.
    20. ↑ Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2
    21. ↑ Grundy, W. M.; Noll, K. S.; Buie, M. W.; Benecchi, S. D.; Ragozzine, D.; Roe, H. G. (December 2019). "The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)" (PDF). Icarus. 334: 30–38. Bibcode:2019Icar..334...30G. doi:10.1016/j.icarus.2018.12.037. S2CID   126574999.
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