(

provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...

) is a trans-Neptunian object that orbits the Sun in the

Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...

beyond

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

. 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; as of August 2024, the object has yet to be named. has an absolute magnitude of about 4.0, and

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...

results estimate it to be about 681 km in diameter. The low density of this and many other mid sized TNOs implies that they have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets.

Numbering and naming

This minor planet was numbered (55637) by the

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...

on 16 February 2003 (). , it has not been named.

Classification

has a perihelion of 36.7 AU, which it will next reach in 2065. As of 2020, is 40 AU from the Sun. The

classifies as a

cubewano A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axe ...

while the

Deep Ecliptic Survey The Deep Ecliptic Survey (DES) is a project to find Kuiper belt objects (KBOs), using the facilities of the National Optical Astronomy Observatory (NOAO). The principal investigator is Robert L. Millis. Since 1998 through the end of 2003, the sur ...

(DES) classifies it as scattered-extended. The DES using a 10 My integration (last observation: 2009-10-22) shows it with a minimum perihelion (''q_min'') distance of 36.3 AU. It has been observed 212 times with

precovery In astronomy, precovery (short for pre-discovery recovery) is the process of finding the image of an object in images or photographic plates predating its discovery, typically for the purpose of calculating a more accurate orbit. This happens mos ...

images dating back to 1991.

Physical characteristics

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 Δ''M'' = . The analysis of combined thermal radiometry of from measurements by the ''

'' and ''

Herschel Space Telescope The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched until the launch of the James Webb Space Teles ...

'' indicates an effective diameter of and albedo of 0.107. 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 UX25 and its satellite: 659 km and 230 km, respectively. 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 Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...

compounds on the surface. It is redder than Varuna, unlike its neutral-colored "twin" , in spite of similar brightness and orbital elements.

Composition

With a density of 0.82 g/cm³, assuming that the primary and satellite have the same density, 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 others large KBOs, it would have to be exceptionally porous, which was believed to be unlikely given the compactability of water ice; this low density thus astonished astronomers. Studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is brittle and can support significant porosity in objects significantly larger than , 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 pore spaces.

Satellite

The discovery of a minor-planet moon was reported in IAUC 8812 on 22 February 2007. The satellite was detected using the Hubble Space Telescope in August 2005. The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5. It orbits the primary in days, at a distance of , yielding a system mass of . The eccentricity of the orbit is . This moon is estimated to be in diameter. Assuming the same albedo as the primary, it would have a diameter of 190 km, assuming an albedo of 0.05 (typical of other cold, classical KBOs of similar size) a diameter of 260 km.

References

External links

MPEC 2002-V08

Astronomers surprised by large space rock less dense than water
Ron Cowen, ''Nature'', 13 November 2013

Bob Yirka, ''Phys.org'', 14 November 2013 * * {{DEFAULTSORT:055637 Classical Kuiper belt objects Discoveries by the Spacewatch project Possible dwarf planets Binary trans-Neptunian objects 20021030