Astrophysics (Index)About

retrograde orbit

(orbit opposite the usual direction)

A retrograde orbit is an orbit the opposite of the usual direction. Orbits are typically in the same general direction, perhaps with some small inclination, and in the same direction as the host star's rotation. The presumed reason is that the whole system began from a spinning cloud which naturally formed into bodies circling in the direction the cloud was spinning, a consequence of the conservation of angular momentum. The term prograde orbit indicates an orbit that is not retrograde. Given a reference plane, orbital inclinations of retrograde orbits are generally characterized as outside the -90° to +90° interval.

Neptune's moon, Triton has a retrograde orbit, which is considered evidence that it is a captured minor planet. Among the early extra-solar planet discoveries where retrograde hot Jupiters, hot Jupiters with retrograde orbits.

Retrograde rotation is rotation of the planet or moon in the opposite of the expected direction. Venus's rotation is retrograde.

Further reading:

Referenced by pages:
irregular moon
Moon formation
planetary migration
retrograde hot Jupiter
Rossiter-McLaughlin effect (RM effect)