Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black).

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.

## 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 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.[5] 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 tilt 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.

## 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 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.

### Within the 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

The orange moon is orbiting in the opposite direction.

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. 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.[6]

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.[7] 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.[8]

## 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.[9]

Comets from the Oort cloud are much more likely than asteroids to be retrograde.[9] Halley's Comet has a retrograde orbit around the Sun.[10]

The first Kuiper belt object discovered to have a retrograde orbit is 2008 KV42.[11] The dwarf planet Pluto has retrograde rotation, not orbit. Pluto's axial tilt is approximately 120 degrees.[12]

## 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.[13]

## 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,[14] 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.[15][16]

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]

WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[17] A second such planet was announced just a day later: HAT-P-7b.[18]

## Stars

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[19] and with a retrograde or zero rotation.[20] 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.[21]

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.[22]

## 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.[23]

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.[24][25]

### 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.[26]

### Central black holes

The center of a spiral galaxy contains at least one supermassive black hole.[27] 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.[28][29]

## References

1. ^ Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. Retrieved 10 October 2009.
2. ^ a b Turning planetary theory upside down
3. ^ a b Stars that steal give birth to backwards planets, New Scientist, 23 August 2011
4. ^ a b A natural formation scenario for misaligned and short-period eccentric extrasolar planets, Ingo Thies, Pavel Kroupa, Simon P. Goodwin, Dimitris Stamatellos, Anthony P. Whitworth, 11 Jul 2011
5. ^ McBride, Neil; Bland, Philip A.; Gilmour, Iain (2004). An Introduction to the Solar System. Cambridge University Press. p. 248. ISBN 0-521-54620-6.
6. ^ Encyclopedia of the solar system. Academic Press. 2007.
7. ^ Mason, John (22 July 1989). "Science: Neptune's new moon baffles the astronomers". NewScientist. Retrieved 10 October 2009.
8. ^ Chaos-assisted capture of irregular moons, Sergey A. Astakhov, Andrew D. Burbanks, Stephen Wiggins & David Farrelly, NATURE |VOL 423 | 15 MAY 2003
9. ^ a b Hecht, Jeff (1 May 2009). "Nearby asteroid found orbiting Sun backwards". NewScientist. Retrieved 10 October 2009.
10. ^ Halley's Comet
11. ^ Hecht, Jeff (5 September 2008). "Distant object found orbiting Sun backwards". NewScientist. Retrieved 10 October 2009.
12. ^ http://www.daviddarling.info/encyclopedia/P/Pluto.html
13. ^ Javaraiah, J. (12 July 2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Monthly Notices of the Royal Astronomical Society 362 (2005): 1311–1318. arXiv:astro-ph/0507269. Bibcode 2005MNRAS.362.1311J. doi:10.1111/j.1365-2966.2005.09403.x.
14. ^ Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. Retrieved 7 September 2010.
15. ^ Tilting stars may explain backwards planets, New Scientist, 01 September 2010, Magazine issue 2776.
16. ^ Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems, Dong Lai, Francois Foucart, Douglas N.C. Lin
17. ^ D. R. Anderson et al. (2009). "WASP-17b: an ultra-low density planet in a probable retrograde orbit". arXiv:0908.1553 [astro-ph.EP].
18. ^ Second backwards planet found, a day after the first, New Scientist, 13 August 2009
19. ^ Kravtsov, V. V. (2001). "Globular clusters and dwarf spheroidal galaxies of the outer galactic halo: On the putative scenario of their formation". Astronomical and Astrophysical Transactions 20 (1): 89–92. Bibcode 2001A&AT...20...89K. doi:10.1080/10556790108208191. Retrieved 13 October 2009.
20. ^ Kravtsov, Valery V. (2002). "Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?". Astronomy & Astrophysics 396: 117–123. arXiv:astro-ph/0209553. Bibcode 2002A&A...396..117K. doi:10.1051/0004-6361:20021404.
21. ^ Carollo, Daniela; Timothy C. Beers, Young Sun Lee, Masashi Chiba, John E. Norris, Ronald Wilhelm, Thirupathi Sivarani, Brian Marsteller, Jeffrey A. Munn, Coryn A. L. Bailer-Jones, Paola Re Fiorentin, Donald G. York (13 December 2007). "Two stellar components in the halo of the Milky Way". Nature 450 (7172): 1020–5. arXiv:0706.3005. Bibcode 2007Natur.450.1020C. doi:10.1038/nature06460. PMID 18075581. Retrieved 13 October 2009.
22. ^ http://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html
23. ^ Making Counter-Orbiting Tidal Debris - The Origin of the Milky Way Disc of Satellites, M. S. Pawlowski, P. Kroupa, and K. S. de Boer
24. ^ Cain, Fraser (22 May 2003). "Galaxy Orbiting Milky Way in the Wrong Direction". Universe Today. Retrieved 13 October 2009.
25. ^ Lockman, Felix J. (2003). "High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit?". The Astrophysical Journal Letters 591 (1): L33–L36. arXiv:astro-ph/0305408. Bibcode 2003ApJ...591L..33L. doi:10.1086/376961.
26. ^ Prada, F.; C. Gutierrez, R. F. Peletier, C. D. McKeith (14 March 1996). "A Counter-rotating Bulge in the Sb Galaxy NGC 7331". arXiv:astro-ph/9602142 [astro-ph].
27. ^ D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http://relativity.livingreviews.org/Articles/lrr-2005-8/
28. ^ "Some black holes make stronger jets of gas". UPI.com. 1 June 2010. Retrieved 1 June 2010.
29. ^ Atkinson, Nancy (1 June 2010). "What's more powerful than a supermassive black hole? A supermassive black hole that spins backwards.". The Christian Science Monitor. Retrieved 1 June 2010.