Why do Jupiter’s distant moons go backwards?

Hill spheres, stability and capture — the physics behind Harpalyke’s reversed orbit.

Published 2026-10-23

Most of Jupiter’s outer moons, including Harpalyke, orbit in the opposite direction to the planet’s spin.^[1]

A planet’s zone of control

A planet’s Hill sphere is the region where its gravity dominates over the Sun’s. A moon must stay inside it to remain bound. Jupiter’s Hill radius is about 53 million km.^[5]

Retrograde is more stable

At large distances from a planet, retrograde orbits stay stable over a wider region than prograde ones.^[5] Harpalyke orbits at about 21 million km — well within Jupiter’s Hill sphere but far enough out that this matters.^[3][5]

But not the whole story

This stability was thought to explain Jupiter’s many retrograde moons, but Saturn has a more even mix, so the reasons are more complicated.^[5]

Captured, not born there

Ananke-group moons are thought to be fragments of a captured asteroid rather than bodies that formed with Jupiter.^[2]

Numbered citations link to the sources list on the Harpalyke overview page.