Astronomy:55637 Uni

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55637 Uni
Uni (center) and its moon Tinia (bottom left), imaged by the Hubble Space Telescope on 26 August 2005
Discovery[1]
Discovered bySpacewatch (291)
Discovery siteKitt Peak National Obs.
Discovery date30 October 2002
Designations
Designation
(55637) Uni
Pronunciation/ˈjuːnaɪ/
Named afterUni
2002 UX25
Minor planet categoryCubewano (MPC)[2]
Extended (DES)[3]
Hot Classical Kuiper belt object[4]: 10 [5]: 13 
Orbital characteristics[1]
Epoch 5 May 2025 (JD 2460800.5)
Uncertainty parameter 0
Observation arc33.35 yr (12,182 days)
Earliest precovery date12 October 1991
|{{{apsis}}}|helion}}49.291 AU
|{{{apsis}}}|helion}}36.716 AU
43.003 AU
Eccentricity0.1462
Orbital period282.01 yr (103,005 days)
Average Orbital speed4.54 km/s
Mean anomaly309.49°
Mean motion0° 0m 12.24s / day
Inclination19.400°
Longitude of ascending node204.57°
|{{{apsis}}}|helion}}≈ 5 September 2066[6]
±3 days
275.27°
Known satellites1 (Tinia)
Physical characteristics
Mean diameter571±53 km[7][lower-alpha 1]
610±53 km[7][lower-alpha 1]
Mass(1.25±0.03)×1020 kg[8]
Mean density0.80±0.13 g/cm3[9]
0.075 m/s2
0.227 km/s
Rotation period14.382±0.001 h[10]
Albedo0.139±0.023[7]
0.122±0.02[7]
Physics45.4 K (perihelion)[4]: 10 
Spectral type
CO2-type ("double-dip")[11]
B–V=1.007±0.043[12]
V−R=0.540±0.030[12]
V−I=1.046±0.034[12]
Apparent magnitude19.8[13]
Absolute magnitude (H)3.87±0.02[10]
4.0[1]


55637 Uni (provisional designation 2002 UX25) is a large 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.[14]

It was discovered on 30 October 2002, by the Spacewatch program.[15] Spitzer Space Telescope results estimate it to be approximately 659 kilometers (409 miles) in diameter. 2 occultations in 2026 measured diameters of 571 kilometers (355 miles) and 610 kilometers (379 miles).[7][lower-alpha 1] It has one known moon, named Tinia, which was discovered in 2005.

History

Discovery

Template:Section needs expansion

The 1.8 meter Spacewatch telescope that was used to discover Uni on 30 October 2002

Uni was discovered by the Spacewatch program on 30 October 2002.[15] It has been observed 212 times with precovery images dating back to 1991.[1]

Numbering and naming

Uni was numbered (55637) by the Minor Planet Center on 16 February 2003 (M.P.C. 47763).[16] On 1 September 2025, the object was named after Uni (/ˈjuːnaɪ/), the ancient goddess of marriage, fertility, family, and women in Etruscan religion and myth, and was the patron goddess of Perugia.[17] She is identified as the Etruscan equivalent of Juno in Roman mythology, and Hera in Greek mythology.[18] As the supreme goddess of the Etruscan pantheon, she is part of the Etruscan trinity, an original precursor to the Capitoline Triad,[19] made up of her husband Tinia, the god of the sky, and daughter Menrva, the goddess of wisdom.

Its moon was named together with Uni on 1 September 2025 after Tinia, the Etruscan sky god and husband of the Etruscan goddess of love and fertility Uni.[17]

Orbit and classification

Uni (vmag 19.9) as viewed with a 24" telescope

Uni orbits the Sun at a distance of 36.7–49.3 Astronomical units (AU) with a semi-major axis or average orbital distance of 43.0 AU once every 282.0 years.[1] Its orbit has an eccentricity of 0.146 and an inclination of 19.4° with respect to the ecliptic.[1] It will reach its perihelion of 36.7 AU on 5 September 2066 with an uncertainty rate of 3 days.[6] As of 2020, Uni is 40 AU from the Sun.[13]

Uni is located in the classical region of the Kuiper belt 39–48 AU from the Sun, and is thus classified as a typical member of a classical Kuiper belt object (also known as a "cubewano").[5] This classical population is most concentrated in a range between 42 and 47 AU.[20] The high orbital inclination of Uni makes it a dynamically "hot" member of the classical Kuiper belt, separating it from the flat "cold" classical objects that have tilts under 5°.[5][20] The hot classical Kuiper belt objects are believed to have been scattered by Neptune's gravitational influence into their dynamically excited paths during the Solar System's early history.[21]: 230  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]

Physical characteristics

Size and rotation

History of size estimates for Uni
Year Uni + Tinia diameter (km) Uni diameter (km) Method References
2007 681.2+115.6
−114.0 		— Thermal (Spitzer) Stansberry u. a.[22]
2009 680+106
−108 		— Thermal (Spitzer) Brucker and u a.[23]
2010 681 — Assumption Tancredi[24]
2013 705 — Thermal (Spitzer) Mommert u. a.[25]
2013 697.2+23.0
−24.5 		665±29 Thermal (NEATM) Fornasier u. a.[26]
2013 692±23 664 Assuming same albedos Brown[8]
2014 697±35 670±34 Thermal (NEATM) Vilenius u. a.[27]
2017 698±40 659±38 Thermal (ALMA) Brown u. a.[9]
2017 695+30
−29 		— Thermal (ALMA) Lellouch u. a.[28]
2018 704 — Assumption Brown[29]
2026 — 571±53 km[lower-alpha 1]
610±53 km[lower-alpha 1] 		Stellar occultation Rommel[7]

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).[30] The light-curve amplitude is ΔM = 0.21±0.06.[10]

Uni has an absolute magnitude of about 3.87±0.02. 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.[22] 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.[8] Using an improved thermophysical model slightly different sizes were obtained for Uni and Tinia: 659 km and 230 km, respectively.[9]

2 stellar occultations from 2026 derived albedos of 0.139±0.023 or 0.122±0.02 and diameters of 571±53 km or 610±53 km for Uni respectively,[lower-alpha 1][7] and diameters of 180.8±0.6 km or 193.0±1.0 km for Tinia.[7]

Surface and spectrum

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.[26] Uni has a B–V color index of 1.007±0.043.[12] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300, in spite of similar brightness and orbital elements.

Unlikely dwarf planet

Uni has a measured low density of 0.82±0.11 g/cm3.[8] 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 highly unlikely to be dwarf planets.[31]

Composition

With a density of 0.82±0.11 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.[14] 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 other large KBOs, it would have to be exceptionally porous; this low density initially surprised astronomers.[14] However, studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is quite strong 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 interstitial spaces.[32]

Density comparison
Material Density
(g/cm3) 		References
Settled snow 0.2–0.3 [33]
Slush/firn 0.35–0.9 [33]
Uni 0.71–0.93 [8]
Glacier ice 0.83–0.92 [33]
Tethys 0.984 [34]
Liquid water 1 [33]

Satellite

Uni has one known moon, which is named Tinia, after the Etruscan sky god and husband of Uni. Tinia was discovered in August 2005. Its orbital period is 8.309±0.0002 days, at a distance of 4770±40 km, indicating a total system mass of (1.25±0.03)×1020 kg.

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Under the assumption of equal albedos

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "JPL Small-Body Database Browser: 55637 (2002 UX25)". Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2055637. 
  2. ↑ 2.0 2.1 "MPEC 2009-C70 :Distant Minor Planets (2009 FEB. 28.0 TT)". Minor Planet Center. 2009-02-10. https://www.minorplanetcenter.net/mpec/K09/K09C70.html. 
  3. ↑ 3.0 3.1 3.2 Marc W. Buie. "Orbit Fit and Astrometric record for 55637". SwRI (Space Science Department). http://www.boulder.swri.edu/~buie/kbo/astrom/55637.html. 
  4. ↑ 4.0 4.1 Brunetto, R.Expression error: Unrecognized word "etal". (March 2025). "Spectral Diversity of DiSCo's TNOs Revealed by JWST: Early Sculpting and Late Irradiation". The Astrophysical Journal Letters 982 (1): L8. doi:10.3847/2041-8213/adb977. Bibcode: 2025ApJ...982L...8B. 
  5. ↑ 5.0 5.1 5.2 Vilenius, E.; Kiss, C.; Mommert, M.; Müller, T.; Santos-Sanz, P.; Pal, A. et al. (19 Jan 2014). ""TNOs are Cool": A survey of the trans-Neptunian region X. Analysis of classical Kuiper belt objects from Herschel⋆ and Spitzer observations". Astronomy & Astrophysics 564. doi:10.1051/0004-6361/201322416. Bibcode: 2014A&A...564A..35V. 
  6. ↑ 6.0 6.1 JPL Horizons Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is 3-sigma.)
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Rommel, Flavia L.; Proudfoot, Benjamin; Ortiz, José L.; Morales, Nicolás; Gómez-Limón, José María; Holler, Bryan J.; Ragozzine, Darin; Santana-Ros, Toni et al. (2026-07-01). "Stellar Occultations by Trans-Neptunian Object Satellites: Predictions and Observations". The Planetary Science Journal 7 (7): 184. doi:10.3847/PSJ/ae84c6. ISSN 2632-3338. 
  8. ↑ 8.0 8.1 8.2 8.3 8.4 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. doi:10.1088/2041-8205/778/2/L34. Bibcode: 2013ApJ...778L..34B. 
  9. ↑ 9.0 9.1 9.2 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. doi:10.3847/1538-3881/aa6346. Bibcode: 2017AJ....154...19B. 
  10. ↑ 10.0 10.1 10.2 "(55637) 2002 UX25". http://www.johnstonsarchive.net/astro/astmoons/am-55637.html. 
  11. ↑ Pinilla-Alonso, Noemí et al. (December 2024). "A JWST/DiSCo-TNOs portrait of the primordial Solar System through its trans-Neptunian objects". Nature Astronomy 9 (2): 230–244. doi:10.1038/s41550-024-02433-2. Bibcode: 2025NatAs...9..230P. https://hal.science/hal-04961629v1/file/s41550-024-02433-2.pdf. 
  12. ↑ 12.0 12.1 12.2 12.3 Hainaut, O. R.; Boehnhardt, H.; Protopapa, S. (October 2012). "Colours of minor bodies in the outer solar system. II. A statistical analysis revisited". Astronomy & Astrophysics 546: 20. doi:10.1051/0004-6361/201219566. Bibcode: 2012A&A...546A.115H. https://www.aanda.org/articles/aa/pdf/2012/10/aa19566-12.pdf. 
  13. ↑ 13.0 13.1 "AstDys (55637) 2002UX25 Ephemerides". Department of Mathematics, University of Pisa, Italy. https://newton.spacedys.com/astdys/index.php?pc=1.1.3.0&n=2002UX25. 
  14. ↑ 14.0 14.1 14.2 Cowen, Ron (2013). "Astronomers surprised by large space rock less dense than water". Nature News. doi:10.1038/nature.2013.14135. 
  15. ↑ 15.0 15.1 Marsden, Brian G. (2002-11-01). "MPEC 2002-V08 : 2002 UX25". IAU Minor Planet Center. Harvard-Smithsonian Center for Astrophysics. https://www.minorplanetcenter.net/mpec/K02/K02V08.html. 
  16. ↑ "MPC/MPO/MPS Archive". Minor Planet Center. https://www.minorplanetcenter.net/iau/ECS/MPCArchive/MPCArchive_TBL.html. 
  17. ↑ 17.0 17.1 "WGSBN Bulletin (Vol. 5, No. 20)". IAU. 2025-09-01. p. 6. https://www.wgsbn-iau.org/files/Bulletins/V005/WGSBNBull_V005_020.pdf. 
  18. ↑ de Grummond, Etruscan Myth, Sacred History and Legend, pp. 78–84
  19. ↑ Ryberg, Inez Scott (1931). "Was the Capitoline Triad Etruscan or Italic?". The American Journal of Philology 52 (2): 145–156. doi:10.2307/290109. 
  20. ↑ 20.0 20.1 Malhotra, Renu (2019). "Resonant Kuiper belt objects: a review". Geoscience Letters 6 (1). doi:10.1186/s40562-019-0142-2. ISSN 2196-4092. PMID 32215241. Bibcode: 2019GSL.....6...12M. 
  21. ↑ Lykawka, Patryk Sofia; Tadashi, Mukai (July 2007). "Dynamical classification of trans-neptunian objects: Probing their origin, evolution, and interrelation". Icarus 189 (1): 213–232. doi:10.1016/j.icarus.2007.01.001. Bibcode: 2007Icar..189..213L. 
  22. ↑ 22.0 22.1 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". in M. Antonietta Barucci. The Solar System Beyond Neptune. University of Arizona press. pp. 161–179. ISBN 978-0-8165-2755-7. Bibcode: 2008ssbn.book..161S. http://www.lpi.usra.edu/books/ssbn2008/7017.pdf. 
  23. ↑ Brucker, M.J.; Grundy, W.M.; Stansberry, J.A.; Spencer, J.R.; Sheppard, S.S.; Chiang, E.I.; Buie, M.W. (2009). "High albedos of low inclination Classical Kuiper belt objects". Icarus (Elsevier BV) 201 (1): 284–294. doi:10.1016/j.icarus.2008.12.040. ISSN 0019-1035. 
  24. ↑ "Physical and dynamical characteristics of icy “dwarf planets” (plutoids)". https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1743921310001717. 
  25. ↑ M. Mommert u. a.: "Remnant planetesimals and their collisional fragments: Physical characterization from thermal-infrared observations (2013)". https://refubium.fu-berlin.de/handle/fub188/2283. 
  26. ↑ 26.0 26.1 Fornasier, S. 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. doi:10.1051/0004-6361/201321329. Bibcode: 2013A&A...555A..15F. 
  27. ↑ Vilenius, E.; Kiss, C.; Müller, T.; Mommert, M.; Santos-Sanz, P.; Pál, A.; Stansberry, J.; Mueller, M. et al. (2014). ""TNOs are Cool": A survey of the trans-Neptunian region". Astronomy & Astrophysics 564: A35. doi:10.1051/0004-6361/201322416. ISSN 0004-6361. http://adsabs.harvard.edu/abs/2014A%26A...564A..35V. Retrieved 2026-09-25. 
  28. ↑ Lellouch, E.; Moreno, R.; Müller, T.; Fornasier, S.; Santos-Sanz, P.; Moullet, A.; Gurwell, M.; Stansberry, J. et al. (2017). "The thermal emission of Centaurs and trans-Neptunian objects at millimeter wavelengths from ALMA observations". Astronomy & Astrophysics (EDP Sciences) 608: A45. doi:10.1051/0004-6361/201731676. ISSN 0004-6361. 
  29. ↑ studio7designs.com, fullahead.org -. "Astronomer Mike Brown". https://web.gps.caltech.edu/~mbrown/dps.html. 
  30. ↑ 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.
  31. ↑ 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) DOI: 10.1016/j.icarus.2018.12.037,
  32. ↑ "The Mutual Orbit, Mass, and Density of Transneptunian Binary". 7 April 2019. http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf. 
  33. ↑ 33.0 33.1 33.2 33.3 "Typical densities of snow and ice (kg/m3)". http://www.sciencelearn.org.nz/Contexts/Icy-Ecosystems/Looking-closer/Snow-and-ice-density. 
  34. ↑ Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2
  35. ↑ 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)". Icarus 334: 30–38. doi:10.1016/j.icarus.2018.12.037. Bibcode: 2019Icar..334...30G. http://www2.lowell.edu/users/grundy/abstracts/preprints/2019.G-G.pdf. Retrieved 2026-03-30. 

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