(612095) 1999 OJ4

Lore & Background

1999 OJ4 was first observed at Mauna Kea Observatory on 18 July 1999. Its orbit characterizes it as a classical Kuiper Belt object, or cubewano, with a nearly circular orbit and low inclination, placing it in the cold population. As a result, it is likely reddish in color.

The system has one moon, discovered by the Hubble Space Telescope on 5 October 2013. The moon orbits 3306±17 km away from the primary, completing one orbit every 84.1147±0.0050 days with an eccentricity of 0.3683±0.0038. At about 72 km, it is nearly the same size as 1999 OJ4. From the surface of 1999 OJ4, the moon would have an apparent diameter of roughly 8.11°, over fourteen times the apparent size of the Sun from Earth.

Reader's Guide

The significance of (612095) 1999 OJ4 lies in its nature as a binary system in the cold classical Kuiper belt. Its nearly equal-sized components—the primary at 75 km and the moon at 72 km—make it a rare example of a near-symmetric binary among trans-Neptunian objects. The moon's orbit, with a period of 84.1147 days and an eccentricity of 0.3683, provides valuable data for understanding the formation and dynamical evolution of such systems. As a cold classical cubewano, its low-inclination, nearly circular orbit suggests it has remained relatively undisturbed since the early Solar System, offering clues to primordial conditions. The moon's large apparent size from the primary's surface—over fourteen times that of the Sun from Earth—highlights the dramatic sky of this distant world. The system's discovery and characterization, including the moon's detection by the Hubble Space Telescope in 2013, underscore the importance of high-resolution observations in revealing the complexity of the Kuiper belt.

Did You Know?

• Its moon is just 3 kilometers smaller than the primary, at about 72 km in diameter.
• The moon orbits 3306±17 km away from the primary with an orbital period of 84.1147±0.0050 days.
• From the surface of 1999 OJ4, the moon would appear over fourteen times the size of the Sun as seen from Earth.