Retrograde motion is motion in the direction opposite to the movement of something else and the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis. In reference to celestial systems, retrograde motion usually means motion which is contrary to the rotation of the primary, that is, the object which forms the system's hub.

In the Solar System, all of the planets and most of the other objects that orbit the Sun, with the exception of many comets, do so in the "prograde" direction, i.e. the same sense as the rotation of the Sun. Also the rotations of most planets are prograde, with the exceptions of Venus and Uranus, which have retrograde rotations. Most satellites of planets revolve around their planets in the prograde sense. (In the case of the satellites of Uranus, this means they revolve in the same sense as Uranus's rotation, which is retrograde relative to the Sun.) There are some satellites which orbit in the retrograde sense, but these are generally small and distant from their planets, except for Neptune's satellite Triton, which is large and close. It is thought that these retrograde satellites, including Triton, are bodies which have been captured into orbit around their planets, having been formed elsewhere.

Formation of celestial systems

When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud.[1] The nature of the collapse is explained by the principle called conservation of angular momentum. In 2010 the discovery of several hot jupiters with backward orbits called into question the theories about the formation of planetary systems.[2] This can be explained by noting that stars and their planets do not form in isolation but in star clusters and when the protoplanetary disk from one star collides with or steals material from another star's disk this can result in retrograde motion of a disk and the resulting planets.[3][4]

Orbital parameters

Inclination

A celestial object's inclination indicates whether the object's orbit is prograde or retrograde. The inclination of a celestial object is the angle between its orbital plane and another reference frame such as the equatorial plane of the object's primary. In the Solar System, inclination of the planets is measured from the ecliptic plane, which is the plane of Earth's orbit around the Sun.[5] The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination between 0 and 90 degrees is orbiting or revolving in the same direction as the primary is rotating. An object with an inclination of exactly 90 degrees has a perpendicular orbit which is neither prograde nor retrograde. An object with an inclination between 90 degrees and 180 degrees is in a retrograde orbit.

Axial tilt

A celestial object's axial tilt indicates whether the object's rotation is prograde or retrograde. Axial tilt is the angle between an object's rotation axis and a line perpendicular to its orbital plane passing through the object's centre. An object with an axial tilt up to 90 degrees is rotating in the same direction as its primary. An object with an axial tilt of exactly 90 degrees has a perpendicular rotation which is neither prograde nor retrograde. An object with an axial tilt between 90 degrees and 180 degrees is rotating in the opposite direction to its orbital direction.

Planets

All eight planets in the Solar System orbit the Sun in the direction that the Sun is rotating, which is counterclockwise when viewed from above the Sun's north pole. Six of the planets also rotate about their axis in this same direction. The exceptions—the planets with retrograde rotation—are Venus and Uranus. Venus's axial tilt is 177 degrees, which means it is spinning almost exactly in the opposite direction to its orbit. Uranus has an axial tilt of 97.77 degrees, so its axis of rotation is approximately parallel with the plane of the Solar System. The reason for Uranus's unusual axial tilt is not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.[6]

It is unlikely that Venus was formed with its present slow retrograde rotation which takes 243 days to rotate. Venus probably began with a fast prograde rotation with a period of several hours much like most of the planets in the solar system. Venus is close enough to the Sun to experience significant gravitational tidal dissipation, and also has a thick enough atmosphere to create thermally driven atmospheric tides which create a retrograde torque. Venus' present slow retrograde rotation is in equilibrium balance between gravitational tides trying to tidally lock Venus to the Sun and atmospheric tides trying to spin-up Venus in a retrograde direction. In addition to maintaining this present day equilibrium, tides are also sufficient to account for evolution of Venus's rotation from a primordial fast prograde direction to its present-day slow retrograde rotation.[7] In the past various other alternative hypotheses have been proposed to explain Venus' retrograde rotation, such as collisions or it having originally formed that way. Mercury is closer to the Sun than Venus but Mercury is not tidally locked because it has entered a spin-orbit resonance due to the eccentricity of its orbit. The rotation of Earth and Mars is also affected by tidal forces with the Sun but they haven't reached an equilibrium state like Mercury and Venus because they are further out from the Sun where tidal forces are weaker. The gas giants of the solar system are too massive and too far from the Sun to have their rotations slowed down by tidal forces.[7]

Dwarf planets

All known dwarf planets and dwarf planet candidates have prograde orbits around the Sun, but some have retrograde rotation. Pluto has retrograde rotation; its axial tilt is approximately 120 degrees.[8] Pluto and its moon Charon are both tidally locked to each other. It is suspected that the Plutonian satellite system was created by a massive collision.[9][10]

Earth's atmosphere

Retrograde motion, or retrogression, within the Earth's atmosphere refers to weather systems which move from east to west through the Westerlies or from west to east through the Trade wind easterlies.

Moons and rings

If formed in the gravity-field of a planet as the planet is forming, a moon will orbit the planet in the same direction as the planet is rotating and is a regular moon. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it can be captured into a retrograde or prograde orbit depending on whether it first approaches the side of the planet that is rotating towards or away from it. This is an irregular moon.[11]

In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except Triton (the largest of Neptune's moons) have prograde orbits.[12] The particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe.

All retrograde satellites experience tidal deceleration to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible.

Within the Hill sphere, the region of stability for retrograde orbits at a large distance from the primary is larger than that for prograde orbits. This has been suggested as an explanation for the preponderance of retrograde moons around Jupiter. Because Saturn has a more even mix of retrograde/prograde moons, however, the underlying causes appear to be more complex.[13]

With the exception of Hyperion all the known regular planetary natural satellites in the Solar System are tidally locked to their host planet, so they have zero rotation relative to their host planet, but have prograde rotation relative to the Sun because they have prograde orbits around their host planet.

If there is a collision, material could be ejected in any direction and coallesce into either prograde or retrograde moons which may be the case for the moons of dwarf planet Haumea although Haumea's rotation direction is not known.[14]

Small solar system bodies

Asteroids

Asteroids usually have a prograde orbit around the Sun. Only a few dozen asteroids in retrograde orbits are known.

Some asteroids with retrograde orbits may be burnt-out comets, [15] but some may acquire their retrograde orbit due to gravitational interactions with Jupiter.[16]

Due to their small size and their large distance from Earth it is difficult to telescopically analyse the rotation of most asteroids. As of 2012, data is available for less than 200 asteroids and the different methods of determining the orientation of poles often result in large discrepancies.[17] The asteroid spin vector catalog at Poznan Observatory[18] avoids use of the phrases "retrograde rotation" or "prograde rotation" as it depends which reference plane is meant and asteroid coordinates are usually given with respect to the ecliptic plane rather than the asteroid's orbital plane.[19]

Asteroids with satellites, also known as binary asteroids, make up about 15% of all asteroids less than 10km in diameter in the main belt and near-Earth population and most are thought to be formed by the YORP effect causing an asteroid to spin so fast that it breaks up.[20] As of 2012, and where the rotation is known, all satellites of asteroids orbit the asteroid in the same direction as the asteroid is rotating.[21]

Most known objects that are in orbital resonance are orbiting in the same direction as the objects they are in resonance with, however a few retrograde asteroids have been found in resonance with Jupiter and Saturn.[22]

Comets

Comets from the Oort cloud are much more likely than asteroids to be retrograde.[15] Halley's Comet has a retrograde orbit around the Sun.[23]

Kuiper belt

Most Kuiper belt objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit is 2008 KV42.[24]

Meteroids

Meteoroids in a retrograde orbit around the Sun hit the Earth with a faster relative speed than prograde meteroids and tend to burn-up in the atmosphere and are more likely to hit the side of the Earth facing away from the Sun (i.e. at night-time) whereas the prograde meteroids have slower closing speeds and more often land as meterorites and tend to hit the sun-facing side of the Earth. Most meteroids are prograde.[25]

The Sun

The Sun's motion about the centre of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde.[26]

Exoplanets

Stars and planetary systems tend to be born in star clusters rather than forming in isolation, so one protoplanetary disk can collide with another or with dense gas clouds and this can lead to disks and their resulting planets having inclined or retrograde orbits around their stars.[3][4] Retrograde motion may also result from gravitational interactions with other celestial bodies in the same system (See Kozai mechanism.) or a near-collision with another planet,[27] or it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disk.[28][29]

The accretion disk of the protostar IRAS 16293-2422 has parts rotating in opposite directions, the first time a counterrotating accretion disk has been found and means that when planets form, the inner planets will orbit in the opposite direction to the outer planets.[30]

WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[31] A second such planet was announced just a day later: HAT-P-7b.[32]

In one study more than half of all the known hot Jupiters had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits.[2]

Stars

The pattern of stars appears fixed in the sky, but that is only because they are so far away that their motion isn't visible to the naked eye; actually, they are orbiting the centre of the galaxy. Stars with a retrograde orbit are more likely to be found in the galactic halo than in the galactic disk. The Milky Way's outer halo has many globular clusters with a retrograde orbit[33] and with a retrograde or zero rotation.[34] The halo consists of two distinct components. The stars in the inner halo mostly have prograde orbits around the galaxy, whereas stars in the outer halo typically have retrograde orbits.[35]

The nearby Kapteyn's Star is thought to have ended up with its high-velocity retrograde orbit around the galaxy as a result of being ripped from a dwarf galaxy that merged with the Milky Way.[36]

Galaxies

Satellite galaxies

Close-flybys and mergers of galaxies within galaxy clusters can pull material out of galaxies and create small satellite galaxies in either prograde or retrograde orbits around larger galaxies.[37]

A galaxy called Complex H, which was orbiting the Milky Way in a retrograde direction relative to the Milky Way's rotation, is colliding with the Milky Way.[38][39]

Counter-rotating bulges

NGC 7331 is an example of a galaxy which has a bulge that is rotating in the opposite direction to the rest of the disk, probably as a result of infalling material.[40]

Central black holes

The center of a spiral galaxy contains at least one supermassive black hole.[41] A retrograde black hole – one whose spin is opposite to that of its disk – spews jets much more powerful than those of a prograde black hole, which may have no jet at all. Scientists have produced a theoretical framework for the formation and evolution of retrograde black holes based on the gap between the inner edge of an accretion disk and the black hole.[42][43]