55637 Uni
Classical Kuiper belt object
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It was discovered on 30 October 2002, by the Spacewatch program. 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). 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 (M.P.C. 47763). 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. 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.
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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. 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
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 = 0.21±0.06.
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. 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 0.139±0.023 or 0.122±0.02 and diameters of 571±53 km or 610±53 km for Uni respectively, and diameters of 180.8±0.6 km or 193.0±1.0 km 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 1.007±0.043. 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. 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.
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. 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.
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.