55637 Uni

55637 Uni (provisional designation ) 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.

It was discovered on 30 October 2002, by the Spacewatch program. Spitzer Space Telescope results estimate it to be approximately in diameter. 2 occultations in 2026 measured diameters of and . It has one known moon, named Tinia, which was discovered in 2005.

History Discovery

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

Numbering and naming Uni was numbered (55637) by the Minor Planet Center on 16 February 2003 (). On 1 September 2025, the object was named after Uni (), the ancient goddess of marriage, fertility, family, and women in Etruscan religion and myth, and was the patron goddess of Perugia. She is identified as the Etruscan equivalent of Juno in Roman mythology, and Hera in Greek mythology.de Grummond, Etruscan Myth, Sacred History and Legend, pp. 78–84 As the supreme goddess of the Etruscan pantheon, she is part of the Etruscan trinity, an original precursor to the Capitoline Triad, 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.

Orbit and classification

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. Its orbit has an eccentricity of 0.146 and an inclination of 19.4° with respect to the ecliptic. It will reach its perihelion of 36.7 AU on 5 September 2066 with an uncertainty rate of 3 days.JPL Horizons Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is 3-sigma.) As of 2020, Uni is 40 AU from the Sun.

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"). This classical population is most concentrated in a range between 42 and 47 AU. 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°. 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. The Minor Planet Center classifies Uni as a cubewano while the Deep Ecliptic Survey (DES) classifies it as scattered-extended. The DES using a 10 My integration (last observation: 2009-10-22) shows it with a minimum perihelion (qmin) distance of 36.3 AU.

Physical characteristics Size and rotation + History of size estimates for Uni Year Uni + Tinia diameter (km) Uni diameter (km) Method References 2007 — Thermal (Spitzer) Stansberry u. a. 2009 — Thermal (Spitzer) Brucker and u a. 2010 — Assumption Tancredi 2013 — Thermal (Spitzer) Mommert u. a. 2013 Thermal (NEATM) Fornasier u. a. 2013 Assuming same albedos Brown 2014 Thermal (NEATM) Vilenius u. a. 2017 Thermal (ALMA) Brown u. a. 2017 — Thermal (ALMA) Lellouch u. a. 2018 — Assumption Brown 2026 — Stellar occultation Rommel

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). The light-curve amplitude is ΔM = .

Uni has an absolute magnitude of about . The analysis of combined thermal radiometry of Uni from measurements by the Spitzer Space Telescope and Herschel Space Telescope indicates an effective diameter of and albedo of . 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. Using an improved thermophysical model slightly different sizes were obtained for Uni and Tinia: 659 km and 230 km, respectively.

2 stellar occultations from 2026 derived albedos of or and diameters of or for Uni respectively, and diameters of or for Tinia.

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. Uni has a B–V color index of . It is redder than Varuna, unlike its neutral-colored "twin" , in spite of similar brightness and orbital elements.

Unlikely dwarf planet Uni has a measured low density of . 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.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 ()', Icarus (forthcoming, available online 30 March 2019) DOI: 10.1016/j.icarus.2018.12.037,

Composition With a density of , 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. 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. 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. + Density comparison Material Density(g/cm3) References Settled snow 0.2–0.3 Slush/firn 0.35–0.9 Uni 0.71–0.93 Glacier ice 0.83–0.92 Tethys 0.984 Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2 Liquid water 1

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 days, at a distance of , indicating a total system mass of .

55637 Uni

Bron en licentie