Retrograde and prograde motion
Retrograde motion in astronomy is, in general,
In the
Most low-inclination
Formation of celestial systems
When a
Orbital and rotational parameters
Orbital inclination
A celestial object's
Axial tilt
A celestial object's
Solar System bodies
Planets
All eight planets in the
The reason for Uranus's unusual axial tilt is not known with certainty, but the usual speculation is that it was caused by a collision with an Earth-sized protoplanet during the formation of the Solar System.[6]
It is unlikely that Venus was formed with its present slow retrograde rotation, which takes 243 days. 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 that create a retrograde torque. Venus's 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 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 alternative hypotheses have been proposed to explain Venus's retrograde rotation, such as collisions or it having originally formed that way.[a]
Despite being closer to the Sun than Venus,
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.[9] Pluto and its moon Charon are tidally locked to each other. It is suspected that the Plutonian satellite system was created by a massive collision.[10][11]
Natural satellites 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 either 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.[12]
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.[13] 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.[14]
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 the same type of rotation as their host planet relative to the Sun because they have prograde orbits around their host planet. That is, they all have prograde rotation relative to the Sun except those of Uranus.
If there is a collision, material could be ejected in any direction and coalesce into either prograde or retrograde moons, which may be the case for the moons of dwarf planet
Asteroids
Some asteroids with retrograde orbits may be burnt-out comets,[16] but some may acquire their retrograde orbit due to gravitational interactions with Jupiter.[17]
Due to their small size and their large distance from Earth it is difficult to
Asteroids with satellites, also known as binary asteroids, make up about 15% of all asteroids less than 10 km in diameter in the
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.[23]
Comets
Kuiper belt objects
Most Kuiper belt objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit was
Meteoroids
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.[29]
Planetary atmospheres
Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general regional direction of airflow, i.e. from east to west against the
Artificial satellites
Artificial satellites destined for low inclination orbits are usually launched in the prograde direction, since this minimizes the amount of propellant required to reach orbit by taking advantage of the Earth's rotation (an equatorial launch site is optimal for this effect). However, Israeli Ofeq satellites are launched in a westward, retrograde direction over the Mediterranean to ensure that launch debris does not fall onto populated land areas.
Exoplanets
Stars and planetary systems tend to be born in star clusters rather than forming in isolation. Protoplanetary disks can collide with or steal material from molecular clouds within the cluster 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,[1] 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.[31][32]
The
WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[34] A second such planet was announced just a day later: HAT-P-7b.[35]
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] One proposed explanation is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.[36]
The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet's rotation rate. During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three dimensions. This results in the axial tilt of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable. Therefore, prograde spin with small axial tilt, common for the solar system's terrestrial planets except for Venus, is not common for terrestrial planets in general.[37]
Stars' galactic orbits
The pattern of stars appears fixed in the sky, insofar as human vision is concerned; this is because their massive distances relative to the Earth result in motion imperceptible to the naked eye. In reality, stars orbit the center of their galaxy.
Stars with an orbit retrograde relative to a
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.[45]
Galaxies
Satellite galaxies
Close-flybys and mergers of galaxies within
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.[47][48]
Counter-rotating bulges
NGC 7331 is an example of a galaxy that has a bulge that is rotating in the opposite direction to the rest of the disk, probably as a result of infalling material.[49]
Central black holes
The center of a spiral galaxy contains at least one supermassive black hole.[50] 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.[51][52][53]
See also
- Artificial satellites in retrograde orbit
- Gravitomagnetic clock effect
- Yarkovsky effect
- Apparent retrograde motion
- Alaska yo-yo, a toy involving simultaneous circular motion of two balls in opposite directions
Footnotes
- ^ Venus's retrograde rotation is measurably slowing down. It has slowed by about one part per million since it was first measured by satellites. This slowing is incompatible with an equilibrium between gravitational and atmospheric tides
References
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Further reading
- Retrograde-rotating exoplanets experience obliquity excitations in an eccentricity-enabled resonance, Steven M. Kreyche, Jason W. Barnes, Billy L. Quarles, Jack J. Lissauer, John E. Chambers, Matthew M. Hedman, 30 Mar 2020
- Gayon, Julie; Eric Bois (21 April 2008). "Are retrograde resonances possible in multi-planet systems?". Astronomy and Astrophysics. 482 (2): 665–672. S2CID 15436738.
- Kalvouridis, T. J. (May 2003). "Retrograde Orbits in Ring Configurations of N Bodies". Astrophysics and Space Science. 284 (3): 1013–1033. S2CID 117212083.
- Liou, J (1999). "Orbital Evolution of Retrograde Interplanetary Dust Particles and Their Distribution in the Solar System". Icarus. 141 (1): 13–28. .
- How large is the retrograde annual wobble? Archived 2012-09-20 at the Wayback Machine, N. E. King, Duncan Carr Agnew, 1991.
- Fernandez, Julio A. (1981). "On the observed excess of retrograde orbits among long-period comets". Monthly Notices of the Royal Astronomical Society. 197 (2): 265–273. .
- Dynamical Effects on the Habitable Zone for Earth-like Exomoons, Duncan Forgan, David Kipping, 16 April 2013
- What collisional debris can tell us about galaxies, Pierre-Alain Duc, 10 May 2012
- The Formation and Role of Vortices in Protoplanetary Disks, Patrick Godon, Mario Livio, 22 October 1999