55637 Uni

Discovery

Uni was discovered by the Spacewatch program on 30 October 2002.^[12] 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).^[13] 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.^[14] She is identified as the Etruscan equivalent of Juno in Roman mythology, and Hera in Greek mythology.^[15] As the supreme goddess of the Etruscan pantheon, she is part of the Etruscan trinity, an original precursor to the Capitoline Triad,^[16] 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.^[14]

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).^[17] The light-curve amplitude is ΔM = 0.21±0.06.^[7]

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.^[18] 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 Uni and Tinia: 659 km and 230 km, respectively.^[5]

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.^[8] Uni has a B–V color index of 1.007±0.043.^[9] 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.^[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 highly unlikely to be dwarf planets.^[19]

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.^[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 other large KBOs, it would have to be exceptionally porous; this low density initially surprised astronomers.^[11] 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.^[20]

Discovery

Uni has one known moon, which is named Tinia. Tinia was discovered in August 2005, by observations with the Hubble Space Telescope, and announced on 22 February 2007.^[25]

Naming

Tinia 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.^[14] Tinia (also Tin, Tinh, Tins or Tina) was the sky god and the highest deity in Etruscan religion, equivalent to the Roman Jupiter and the Greek Zeus.^[26]

Orbital characteristics

Tinia completes one complete orbit around Uni every 8.309±0.0002 days, at a distance of 4770±40 km, indicating a total system mass of (1.25±0.03)×10²⁰ kg. Its orbital eccentricity is 0.17±0.03.

Physical characteristics

Tinia was discovered at 0.16 arcseconds from Uni with a difference in absolute magnitude of 2.5.^[25][27] Its diameter is estimated at 210±30 km. Assuming an albedo similar to its primary, Tinia should have a diameter of approximately 190 km; assuming an albedo of 0.05, typical of cool classical KBOs of similar size, it should have a diameter of approximately 260 km.