Retrograde motion is in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can refer to the orbit of one body about another body or about some other point, or to the rotation of a single body about its axis, or other orbital parameters 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. 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.

## 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 often 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 beyond 90 degrees up to 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 center. 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 beyond 90 degrees up to 270 degrees has a retrograde rotation relative 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 Earth's north pole. Six of the planets also rotate 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.

## 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. 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.

Within the Hill sphere, the region of stability for retrograde orbits at a large distance from the primary is larger than the region for prograde orbits at a large distance from the primary. This was thought to explain the preponderance of retrograde moons around Jupiter, however Saturn has a more even mix of retrograde/prograde moons so the reasons are more complicated.

## Asteroids, comets, and Kuiper belt objects

Asteroids usually have a direct orbit. By 1 May 2009, astronomers had identified a mere 20 asteroids in retrograde orbits. The retrograde asteroids may be burnt-out comets.

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. Pluto's axial tilt is approximately 120 degrees.

## The Sun

The sun's motion about the center 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

Astronomers have discovered some exoplanets with retrograde orbits. WASP-17b is the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating. HAT-P-7b also has a retrograde orbit. The retrograde motion may be the result of gravitational interactions with other celestial bodies (See Kozai mechanism.) or a collision with another planet. It has been found that several hot jupiters have backward orbits and this calls into question the theories about the formation of planetary systems. By combining new observations with the old data it was found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars, and six exoplanets in this study have retrograde motion.

## 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 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.[16]

## Galaxies

The center of a spiral galaxy is at least one supermassive black hole. A prograde black hole spins in the same direction as its disk. A retrograde black hole spins in the opposite direction to the disk. A retrograde black hole spews jets that are much more powerful than the jets from a prograde black hole, which may have no jet at all. The retrograde black hole shoots more powerful jets because the gap between it and the inner edge of its disk is greater than the gap between the prograde black hole and the inner edge of its disk. The greater gap is presumed to provide more room for the build-up of the magnetic fields which fuel the jets. (This presumption is known as the "Reynold's conjecture" after the theoretical astrophysicist Chris Reynolds of the University of Maryland, College Park.)

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.

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.[21][22]

## References

1. Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html. Retrieved 10 October 2009.
2. Turning planetary theory upside down
3. https://www.newuniverse.co.uk/Axial_tilt.html
4. Encyclopedia of the solar system. Academic Press. 2007.
5. Mason, John (22 July 1989). "Science: Neptune's new moon baffles the astronomers". NewScientist. https://www.newscientist.com/article/mg12316742.600-science-neptunes-new-moon-baffles-the-astronomers.html. Retrieved 10 October 2009.
6. Chaos-assisted capture of irregular moons, Sergey A. Astakhov, Andrew D. Burbanks, Stephen Wiggins & David Farrelly, NATURE |VOL 423 | 15 MAY 2003
7. Hecht, Jeff (1 May 2009). "Nearby asteroid found orbiting sun backwards". NewScientist. https://www.newscientist.com/article/dn17073-nearby-asteroid-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
8. Halley's Comet
9. Hecht, Jeff (5 September 2008). "Distant object found orbiting Sun backwards". NewScientist. https://www.newscientist.com/article/dn14669-distant-object-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
10. https://www.daviddarling.info/encyclopedia/P/Pluto.html
11. Javaraiah, J. (12 July 2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Royal Astronomical Society, Monthly Notices (Royal Astronomical Society) 362 (2005): 1311–1318. https://arxiv.org/abs/astro-ph/0507269v1. Retrieved 11 October 2009.
12. Grossman, Lisa (13 August 2009). "Second backwards planet found, a day after the first". NewScientist. https://www.newscientist.com/article/dn17613-second-backwards-planet-found-a-day-after-the-first.html. Retrieved 10 October 2009.
13. Kravtsov, V. V. (1 June 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 (2001): 89–92. doi:10.1080/10556790108208191. https://images.astronet.ru/pubd/2008/09/28/0001230622/89-92.pdf. Retrieved 13 October 2009.
14. Kravtsov, Valery V. (28 August 2002). "Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?". Astronomy & Astrophysics (EDP Sciences) 396 (2002): 117–123. doi:10.1051/0004-6361:20021404. https://www.aanda.org/articles/aa/full/2002/46/aa2635/aa2635.html. Retrieved 13 October 2009.
15. 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. doi:10.1038/nature06460. https://stromlo.anu.edu.au/news/media_releases/nature06460.pdf. Retrieved 13 October 2009.
16. https://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html
17. D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." https://relativity.livingreviews.org/Articles/lrr-2005-8/
18. "Some black holes make stronger jets of gas". UPI.com. 1 June 2010. https://www.upi.com/Science_News/2010/06/01/Some-black-holes-make-stronger-jets-of-gas/UPI-80711275420355/. Retrieved 1 June 2010.
19. 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. https://www.csmonitor.com/Science/Cool-Astronomy/2010/0601/What-s-more-powerful-than-a-supermassive-black-hole-A-supermassive-black-hole-that-spins-backwards. Retrieved 1 June 2010.
20. Prada, F.; C. Gutierrez, R. F. Peletier, C. D. McKeith (14 March 1996). "A Counter-rotating Bulge in the Sb Galaxy NGC 7331". arXiv.org. https://arxiv.org/abs/astro-ph/9602142v2. Retrieved 10 October 2009.
21. Cain, Fraser (22 May 2003). "Galaxy Orbiting Milky Way in the Wrong Direction". Universe Today. https://www.universetoday.com/2003/05/22/galaxy-orbiting-milky-way-in-the-wrong-direction/. Retrieved 13 October 2009.
22. Lockman, Felix J. (2 June 2003). "High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit?". The Astrophysical Journal (The American Astronomical Society) 591 (1 July 2003): L33-L36. https://www.iop.org/EJ/article/1538-4357/591/1/L33/17178.web.pdf. Retrieved 13 October 2009.