Retrograde motion
Encyclopedia
Retrograde motion is motion in the direction opposite to the movement of something else, and is 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.
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. The nature of the collapse is explained by the principle called conservation of angular momentum. In 2010 the discovery of several hot jupiter
s with backward orbits called into question the theories about the formation of planetary systems. This can be explained by noting that stars and their planets do not form in isolation but in star cluster
s and when the protoplanetary disk
from one star collides or steals material from another star's disk this can result in retrograde motion of a disk and the resulting planets.
indicates whether the object's orbit is direct 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 our solar system
, inclination of the planets is measured from the ecliptic plane, which is the plane of Earth
's orbit around the Sun
. The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination between -90 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 direct nor retrograde. An object with an inclination between 90 degrees and 270 degrees is in a retrograde orbit.
indicates whether the object's rotation is direct 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 direct nor retrograde. An object with an axial tilt between 90 degrees and 270 degrees is rotating in the opposite direction to its orbital direction.
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.
or from west to east through the Trade wind
easterlies.
will orbit the planet in the same direction as the planet is rotating. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it will 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. The retrograde orbits of a planet's satellites are said to be irregular. Prograde orbits are said to be regular.
In our 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. 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. As Saturn has a more even mix of retrograde/prograde moons, however, the underlying causes appear to be more complex.
Comets from the Oort cloud
are much more likely than asteroids to be retrograde. Halley's Comet has a retrograde orbit around the Sun.
The first Kuiper belt
object discovered to have a retrograde orbit is 2008 KV42
. The dwarf planet
Pluto
has retrograde rotation, not orbit. Pluto's axial tilt is approximately 120 degrees.
s 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.
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,, 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.
In one study more than half of all the known hot Jupiter
s had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits.
WASP-17b
was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating. A second such planet was announced just a day later: HAT-P-7b
.
than in the galactic disk. The Milky Way
's outer halo has many globular clusters with a retrograde orbit and with a retrograde or zero rotation. The halo consists of two distinct components. The stars in the inner halo mostly have prograde orbits around the galaxy, while stars in the outer halo favour retrograde orbits.
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.
s can pull material out of galaxies and create small satellite galaxies in either prograde or retrograde orbits around larger galaxies.
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.
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.
. 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. Building on work by Chris Reynolds, astrophysicist of the University of Maryland, College Park, astrophycisists David Garofalo of the Jet Propulsion Laboratory, Daniel A. Evans of the Harvard Smithsonian Center for Astrophysics and Rita M. Sambruna of NASA's Goddard Space Flight Center, 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.
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of one body about another body or about some other point, or the rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
of a single body about its axis, or other phenomena such as precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
or nutation
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
of the axis. In reference to celestial systems, retrograde motion usually means motion which is contrary to the rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
of the primary
Primary (astronomy)
A primary is the main physical body of a gravitationally-bound, multi-object system. This body contributes most of the mass of that system and will generally be located near its center of mass....
, that is, the object which forms the system's hub.
Formation of celestial systems
When a galaxyGalaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
or a planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
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. The nature of the collapse is explained by the principle called conservation of angular momentum. In 2010 the discovery of several hot jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s with backward orbits called into question the theories about the formation of planetary systems. This can be explained by noting that stars and their planets do not form in isolation but in star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
s and when the protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
from one star collides or steals material from another star's disk this can result in retrograde motion of a disk and the resulting planets.
Inclination
A celestial object's inclinationInclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
indicates whether the object's orbit is direct or retrograde. The inclination of a celestial object is the angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
between its orbital plane
Orbital plane
Orbital plane may refer to:*Orbital plane *In anatomy, it refers to a specific area of the maxilla...
and another reference frame such as the equatorial plane of the object's primary. In our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, inclination of the planets is measured from the ecliptic plane, which is the plane of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's orbit around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination between -90 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 direct nor retrograde. An object with an inclination between 90 degrees and 270 degrees is in a retrograde orbit.
Axial tilt
A celestial object's axial tiltAxial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
indicates whether the object's rotation is direct or retrograde. Axial tilt is the angle between an object's rotation axis and a line perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to its orbital plane
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...
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 direct nor retrograde. An object with an axial tilt between 90 degrees and 270 degrees is rotating in the opposite direction to its orbital direction.
Earth and the planets
All eight planets in our 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 axes in this same direction. The exceptions—the planets with retrograde rotation—are VenusVenus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
and Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
. 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.
Within the atmosphere
Retrograde motion, or retrogression, within the Earth's atmosphere refers to weather systems which move from east to west through the WesterliesWesterlies
The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical...
or from west to east through the Trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...
easterlies.
Moons and rings
If formed in the gravity-field of a planet as the planet is forming, a moonNatural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
will orbit the planet in the same direction as the planet is rotating. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it will 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. The retrograde orbits of a planet's satellites are said to be irregular. Prograde orbits are said to be regular.
In our solar system, many of the asteroid-sized moons have retrograde orbits whereas all the large moons except Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
(the largest of Neptune's moons), have prograde orbits. The particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe
Phoebe (moon)
Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart...
.
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
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...
, 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. As Saturn has a more even mix of retrograde/prograde moons, however, the underlying causes appear to be more complex.
Asteroids, comets, and Kuiper belt objects
Asteroids usually have a direct orbit. By February 2011, astronomers have identified a mere 36 asteroids in retrograde orbits. The retrograde asteroids may be burnt-out comets.Comets from the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
are much more likely than asteroids to be retrograde. Halley's Comet has a retrograde orbit around the Sun.
The first Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
object discovered to have a retrograde orbit is 2008 KV42
2008 KV42
', nicknamed Drac , is a trans-Neptunian object orbiting the Sun in retrograde motion and almost perpendicular to the ecliptic: it has a 104-degree inclination. This odd orbit suggests that may have been perturbed inwards from the Oort cloud...
. The dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
has retrograde rotation, not orbit. Pluto's axial tilt is approximately 120 degrees.
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.Exoplanets
Stars and planetary systems tend to be born in star clusterStar cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
s rather than forming in isolation, so one protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
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.
Retrograde motion may also result from gravitational interactions with other celestial bodies in the same system (See Kozai mechanism
Kozai mechanism
In celestial mechanics, the Kozai mechanism, or the Lidov-Kozai mechanism, causes a periodic exchange between the inclination and eccentricity of an orbit...
.) or a near-collision with another planet,, 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.
In one study more than half of all the known hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits.
WASP-17b
WASP-17b
WASP-17b is an exoplanet in the constellation Scorpius that is orbiting the star WASP-17. Its discovery was announced on 11 August 2009. It is the first planet discovered to have a retrograde orbit, meaning it orbits in a direction counter to the rotation of its host star. This discovery changed...
was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating. A second such planet was announced just a day later: HAT-P-7b
HAT-P-7b
HAT-P-7b is an extrasolar planet discovered in 2008. It orbits very close to its parent star GSC 03547-01402, and is both larger and more massive than Jupiter. Due to the extreme heat it receives from its star, the dayside temperature is predicted to be 2730 K...
.
Stars
Stars with a retrograde orbit are more likely to be found in the galactic haloGalactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
than in the galactic disk. The Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
's outer halo has many globular clusters with a retrograde orbit and with a retrograde or zero rotation. The halo consists of two distinct components. The stars in the inner halo mostly have prograde orbits around the galaxy, while stars in the outer halo favour retrograde orbits.
The nearby Kapteyn's Star
Kapteyn's Star
Kapteyn's Star is a class M1 red dwarf star about 13 light years from Earth in the southern constellation of Pictor. With a magnitude of nearly 9 it is visible through binoculars or a telescope.-History:...
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.
Satellite galaxies
Close-flybys and mergers of galaxies within galaxy clusterGalaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
s can pull material out of galaxies and create small satellite galaxies in either prograde or retrograde orbits around larger galaxies.
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.
Counter-rotating bulges
NGC 7331NGC 7331
NGC 7331 is a spiral galaxy about away in the constellation Pegasus. It was discovered by William Herschel in 1784...
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.
Central black holes
The center of a spiral galaxy contains at least one supermassive black holeSupermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
. 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. Building on work by Chris Reynolds, astrophysicist of the University of Maryland, College Park, astrophycisists David Garofalo of the Jet Propulsion Laboratory, Daniel A. Evans of the Harvard Smithsonian Center for Astrophysics and Rita M. Sambruna of NASA's Goddard Space Flight Center, 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.
Further reading
- How large is the retrograde annual wobble?, N. E. King, Duncan Carr Agnew, 1991.