(278361) 2007 JJ43

is a large trans-Neptunian object (TNO) orbiting the Sun near the outer edge of the Kuiper belt.

Its discovery images were taken in 2007, and its absolute magnitude of 4.5 is one of the twenty brightest exhibited by unnamed TNOs. It has a diameter of roughly 504 to 610 km, making it a possible dwarf planet.

History Discovery

was discovered by the Palomar Mountain telescope with Megan Schwamb, Michael E. Brown, and David L. Rabinowitz on 14 May 2007, using discovery images taken by the 1.2-meter Samuel Oschin telescope at the Palomar Observatory in California. Its discovery was also a part of the Palomar Distant Solar System Survey, the survey aimed to find objects in the region around Sedna, farther than 50 AU from the Sun. The survey was designed to detect the orbital movements of distant objects that were at least 1,000 AU from the Sun. Its discovery was further announced on 5 July 2007. The object has precovery images back to 10 May 2002 by Palomar Mountain (NEAT).

Further observations The observation arc of begins with an observation on 10 May 2002 by Palomar Mountain (NEAT). Following this, has been observed by various telescopes, including the Las Campanas Observatory, European Southern Observatory, and Palomar Mountain. As of September 2026, 209 observations of this object had been recorded for over a period of 20.94 years.

Provisional designation and numbering The object was officially given the provisional designation of by the Minor Planet Center on 5 July 2007. The provisional designation of indicates its discovery date, the first letter represents the first half of May and the second letter and numbers (43) mean that it is the 1085th object discovered during that half-month. was the minor planet number 278361 by the IAU. As of 2026, has not been officially given any name by the International Astronomical Union.

Orbit and classification Orbital characteristics orbits the Sun at a distance of 40.4–56.3 AU with an semi-major axis or average orbital distance of 48.3 AU once every 335.9 years (for reference, Neptune's orbit is at 30 AU). Its orbit has a relatively high eccentricity of 0.164 and an inclination of 12.04° with respect to the ecliptic. , it is about 40.5 AU from the Sun.

Classification

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 mostly concentrated in a range between 42 and 47 AU. The high orbital inclination of makes it a dynamically "hot" member of the classical Kuiper belt, separating it from the "cold" classical Kuiper Belt objects that have inclinations under 5°. The hot classical Kuiper belt objects are believed to have been scattered by the gravitational influence of Neptune into their dynamically excited orbital paths during the Solar System's early history. However, a 2015 study found that is likely in a 2:1 resonance with Neptune.

Physical characteristics Size, mass, and density

Assuming it has a typical cluster average geometric albedo, this would make it roughly the same size as Ixion (about 697 km diameter). In a 2015 paper in the Astrophysical Journal Letters, András Pál and colleagues estimate a diameter of of . A following study in 2021 further refined its diameter down to , assuming a median geometric albedo, making it very similar in size to Saturn's moon Enceladus (504.2 km in diameter). is not known to have any natural satellites or moons, which means there is currently no way to measure its mass and density, thus, its mass and density are unknown as of 2026.

The large size of it makes it a possible dwarf planet. However, based on its size of or , it belongs to the class of "mid-sized" TNOs between in diameter which scientists proposed in 2019. These mid-sized TNOs are believed to represent the transition between small, low-density TNOs and large, high-density dwarf planets. Planetary scientists have hypothesized that mid-sized TNOs in the lower range of 400–1000 km may have highly porous and unheated interiors, because TNOs in the lower range of this zone (such as Uni and Gǃkúnǁʼhòmdímà) have been found to have low densities around .

Surface and spectra Composition and spectrum A paper released on 28 June 2015 by Gourgeot et al. using the instrument X-Shooter at the Paranal Observatory, covering 300 to 2480 nm shows that ‘s spectra is mostly flat. and can be considered to be a blue-red object. They detected no absorption bands of icy elements, and their models show crystalline water ice is likely not on the surface of , with no more than a concentration of 6.5%. They also proposed a surface composition of 50% complex organics. Observations covering about one-third of the entire surface of show no differences, showing its surface is mostly homogeneous. A following 2021 study measured a surface composition of (water), silicates, and organics.

Color and albedo has color indices of: B-V of , V-R of , and R-I of . A later study led by Susan D. Benecchi and Scott S. Sheppard shows that it has color indices of: g'-r' of , g'-i' of , and r'-i' of . A earlier paper by only Scott S. Sheppard in 2010, shows that has color indices of: g-r of , a g-i of , a r-i of , a B-R of , a V-R of , a R-I of , and a B-I of . András Pál and colleagues assumed a cluster average geometric albedo of for . A 2021 study by Fernández Valenzuela refined its albedo to by assuming the median albedos of their samples.

Rotation rotates on its axis once every 12.1 hours, with a light curve amplitude of approximately 0.10, indicating an approximate spheroidal shape.

Lack of moons Observations by Mike Brown in 2012, using the W. M. Keck Observatory, suggest that does not have a significantly sized companion.

(278361) 2007 JJ43

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