55637 Uni
A large, low-density Kuiper belt object with a moon.
55637 Uni (originally designated 2002 UX25) is a sizable object in the Kuiper belt, orbiting the Sun beyond Neptune. It attracted brief scientific interest after measurements revealed an unusually low density of roughly 0.82 g/cm³—making it one of the largest solid bodies in the Solar System that is less dense than water.
The Spacewatch program discovered Uni on 30 October 2002. Observations have since recorded it 212 times, with precovery images extending back to 1991. The Minor Planet Center assigned it the number 55637 on 16 February 2003. On 1 September 2025, it was named after Uni, the Etruscan goddess of marriage, fertility, family, and women, and the patron deity of Perugia. She corresponds to the Roman Juno and Greek Hera, and as the supreme goddess of the Etruscan pantheon, she formed part of a trinity with her husband Tinia and daughter Menrva—a precursor to the Capitoline Triad. Its moon, discovered in August 2005 and announced on 22 February 2007, was named Tinia on the same date, after the Etruscan sky god and Uni’s husband.
Uni orbits the Sun at distances between 36.7 and 49.3 AU, with an average orbital distance of 43.0 AU and a period of 282.0 years. Its orbit has an eccentricity of 0.146 and an inclination of 19.4° relative to the ecliptic. It will reach perihelion (36.7 AU) on 5 September 2066, with a 3-day uncertainty. As of 2020, it was 40 AU from the Sun. Located in the classical Kuiper belt region (39–48 AU), it is classified as a cubewano by the Minor Planet Center, though the Deep Ecliptic Survey (DES) labels it as scattered-extended. Its high inclination makes it a dynamically "hot" classical object, unlike the flat "cold" population with tilts under 5°. The DES, using a 10-million-year integration based on observations through October 2009, gives it a minimum perihelion distance of 36.3 AU.
Variability in visual brightness suggests a rotation period of either 14.38 or 16.78 hours, depending on the light-curve model, with an amplitude of 0.21 ± 0.06 magnitudes. Uni has an absolute magnitude of about 3.87 ± 0.02. Combined thermal data from the Spitzer and Herschel space telescopes indicate an effective diameter of 692 ± 23 km and an albedo of 0.107 (+0.005, –0.008).
Quick Facts
- Background
- #C2E0FF
- Discoverer
- Spacewatch (291)
- Discovered
- 30 October 2002
- Discovery Site
- Kitt Peak National Obs.
- Earliest Precovery Date
- 12 October 1991
- Mpc Name
- (55637) Uni
- Pronounced
- ˈ · j · uː · n · aɪ
- Named After
- Uni
- Mp Category
- Cubewano (MPC) / Extended (DES) / Hot Classical Kuiper belt object1013
- Epoch
- 5 May 2025 (JD 2460800.5)
- Uncertainty
- 0
- Observation Arc
- 33.35 yr (12,182 days)
Facts from the source article.
Lore & Background
Uni was discovered by the Spacewatch program on 30 October 2002. It has been observed 212 times with precovery images dating back to 1991. It was numbered by the Minor Planet Center on 16 February 2003 and named on 1 September 2025 after Uni, the Etruscan goddess of marriage, fertility, family, and women, identified with Juno and Hera. Its moon was named Tinia after the Etruscan sky god and husband of Uni.
Uni orbits the Sun at a distance of 36.7–49.3 AU with a semi-major axis of 43.0 AU once every 282.0 years. Its orbit has an eccentricity of 0.146 and an inclination of 19.4°, classifying it as a dynamically hot classical Kuiper belt object. It will reach perihelion on 5 September 2066. The object has a red featureless spectrum in visible and near-infrared, with a negative slope in the K-band possibly indicating methanol compounds.
Uni has a measured low density of 0.82 ± 0.11 g/cm³, implying it has never compressed into a fully solid body or collapsed into hydrostatic equilibrium, making it highly unlikely to be a dwarf planet. Its low density suggests exceptional porosity, possibly because at low temperatures ice is strong enough to support porosity without warming and deforming during formation.
Reader's Guide
Uni's significance lies in its unexpectedly low density, which challenged assumptions about the composition and internal structure of mid-sized trans-Neptunian objects. With a density less than water, it is one of the largest known solid objects in the Solar System with such a low density. This finding initially surprised astronomers, as objects of its size in the Kuiper belt often contain a fair amount of rock and are denser. Studies suggest that at the low temperatures beyond Neptune, ice can support significant porosity even in objects larger than Uni, particularly if rock is present, implying that Uni failed to warm sufficiently during formation to deform the ice and fill interstitial spaces. Its moon Tinia, discovered in 2005, allowed measurement of the system mass and further characterization. The object's high orbital inclination and classification as a hot classical Kuiper belt object indicate it was likely scattered by Neptune's gravitational influence early in Solar System history. Two stellar occultations in 2026 provided refined diameter and albedo measurements, contributing to ongoing understanding of these distant bodies.
Did You Know?
- Uni has a density of 0.82 g/cm³, less than water.
- Its moon Tinia was discovered in 2005 using the Hubble Space Telescope.
More in Trans-Neptunian Objects 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced