Monoetius

Monoetius is the companion object of 617 Patroclus. Monoetius lies beyond the orbit of Jupiter in the Sol system, and is a member of a group of objects known as the Trojans. The object has a density of only 0.8 grams per cubic centimeter with a diameter of 70 miles, which implies it is a comet composed of water ice.

Patroclus and Monoetius are the only known binary objects around Jupiter. The pair orbit around each other while floating 750 million kilometers from Jupiter on one of the gas planet's two so-called Lagrange points. At these points, the gravitational field of Jupiter and Sol are perfectly balanced, and objects can be captured and brought to relative rest. Jupiter has two Lagrange points, one in front and the other behind as the planet orbits its star.

The binary system is thought to have formed 650 million years after the formation of Sol system.