A retrograde planet or moon orbits its star or planet in the opposite direction then the hosting star (or planet) and majority of planets (or moons).
In Solar System, many outer moons orbiting all four gas giants are orbiting retrograde . Also some asteroids are known to orbit retrograde too . Even some retrograde Centaurs (asteroids orbiting between Jupiter and Neptune) are known to exist . Retrograde comets are common, Halley's Comet is the best example. Usually, retrograde satellites orbit their planets far away, but there is one exception: Triton, which is very close to Neptune. Even more, a small percent of the stars in the Milky Way have retrograde orbits.
The presence of many retrograde celestial bodies in the Solar System suggests that other similar bodies might exist around other stars and planets.
There are a few mechanisms that can explain the retrograde motion.
- . Captured bodies. Majority of the outer moons orbiting giant planets are captured asteroids. Usually, if the motion is prograde, it means that the satellite was an asteroid moving closer to the star, while a retrograde motion suggests the satellite was an asteroid moving further away, usually a captured Kuiper Belt object. However, this is not a rule. When an asteroid comes too close to a planet, its gravitational influence might change its orbit, sending it closer to the sun or ejecting it from the solar system. Usually, in order for an object to be captured by a planet, an auxiliary mechanism is needed. While passing close to the planet, the asteroid might come close enough to a moon. The moon gains momentum, while the asteroid losses it. As a result, the moon might be ejected, might impact with the asteroid or might end-up with a new orbit. The same mechanism can work for a Rogue planet free-floating through the galaxy, that gets very close to a star.
- . Outer bodies. Objects in the Oort Cloud have loose, chaotic orbits. A gravitational influence from outside might send an asteroid into a comet-like trajectory, with a highly elliptical orbit. Such elliptical orbits, with an eccentricity over 0.95, can be easily changed if the object comes close to a giant planet. A slightly change of trajectory might result into a retrograde orbit. Then, several encounters with other planets might decrease the eccentricity, as the orbit remains retrograde. If the object is close enough to the star or planet, its eccentricity slowly decreases to a nearly spherical orbit. It is supposed that Triton had a highly elliptical orbit first, but changed to its current path. This mechanism is supposed to have created the current orbits of retrograde Centaurs.
- . Orbital Plane Changing. If an asteroid comes close enough to Jupiter, but on an angle, its orbit plane can dramatically tilt. This mechanism was used by the Ulysses spacecraft, to fly over the poles of the Sun. If the orbital plane has an inclination of over 90 degrees, the orbit will be retrograde. However, the angle will remain high (between 90 and 110 degrees) and the orbit will not change further, to a similar plane with the planets. This is how retrograde asteroids are supposed to have gained their orbits.
Retrograde orbits are less stable then prograde ones. Slowly, retrograde moons lose momentum as they create tides on the planet. A similar phenomenon occurs with retrograde planets. However, outer moons and outer planets will stay in their orbits for much longer.
Since majority of retrograde moons orbit far away from their planets, the gravity holding them is weak. A small influence from a passing asteroid or resonance with another planet might pull the moons out.
In a system with both prograde and retrograde planets orbiting a star, if the orbits are circular and there is enough space between them, the system is safe for millions of years. However, if the orbits are elliptical and not in the same plane, collisions might occur sooner.
When a planet acquires a new large moon or when a star acquires a large planet, the effect on the system could be catastrophic, mainly if the new object is retrograde. At first, it will have an elliptical orbit that will impact or knock out majority of other planets or moons. Then, when the new planet will gain a more circular orbit, the system will look completely different. For Neptune, it is supposed that when Triton was captured, almost all the existing moons were broken apart, ejected from Neptune or had impacted Triton. From the remaining debris, the inner moons and the rings might have formed. The outer moons around Neptune are supposed to also be captured objects. The only moon that survived in one piece is Nereid, who has an extremely elliptical orbit. Based on what we can learn from Neptune, we can conclude that if we detect retrograde planets orbiting a star, the chance for other planets to exist is very small.
Left alone, in time, all retrograde objects lose momentum and migrate closer to their parent body. As their orbit decreases below the Roche limit, they break apart into a ring of debris. The ring itself further loses altitude, until particles from the ring impact the planet. However, this scenario needs millions of years to happen, too much compared to a human lifetime.
Since retrograde planets and moons are usually captured bodies, their chemical composition should differ from the other planets and moons. If they come from the Kuiper Belt or the Oort Cloud, they should contain higher amounts of water, gasses and other volatiles. Since such bodies are sometimes outside the heliosphere and are more exposed to interstellar environment, on their surface a higher amount of tholins should exist. An outer, retrograde planet, orbiting a star, could be in fact a rocky planet ejected from another solar system. In the same way, a captured planet that came from a system rich in heavy metals will be completely different from natural planets existing around the same sun.
When a new planet or moon is captured into orbit (and especially in a retrograde one), strong gravitational forces will create huge quakes, will reactivate tectonic movement and volcanism. If other bodies are disturbed from their orbits, they might collide with the newly arrived object, creating craters and further activating tectonic movements and volcanism. Even more, if the orbit is close enough, disturbed by other bodies and elliptical, the internal dynamo can start, creating magnetic fields and geological activity.
Terraforming a retrograde planet or moon can be done in the same manner as a prograde one. The technology used should be adapted to the conditions found on the planet. The rotation direction alone cannot influence terraforming methods.
Transport Problems Edit
It is far more difficult for a spacecraft to travel between a prograde and a retrograde planet. Consider that Mars were retrograde. Sending a spaceship from Earth to Mars means that the ship first needs to decelerate from Earth's orbital speed to zero, then to speed-up in the opposite direction. This will result in at least 10 times more fuel consumption then to fly to a prograde Mars.
With current technology, the best way to fly towards a retrograde planet is to fly further away (for example to the Kuiper Belt or beyond), where orbital speed is far smaller. There, the spaceship can more easily decrease its speed and speed-up again in the opposite direction. This means that, sending a probe from a prograde Earth to a retrograde Mars is more difficult and more fuel consuming then sending a probe to Pluto.
If one prograde planet has a retrograde moon, the same problems occur when a spaceship tries to fly from a moon to another. The best way to navigate is to fly outside of the planet's gravitational influence, then to return to the planet, but orbiting in the different direction.
Sending a spacecraft towards a retrograde satellite of a prograde planet will cost the same as sending a spacecraft towards a prograde satellite of the same planet. The only difference is that the ship will enter orbit around the planet in the opposite direction. Again, sending a spaceship from a retrograde moon of a prograde planet towards another prograde planet requires the same costs as sending the same ship from a prograde moon.
Retrograde planets and moons exist throughout the Universe. They are usually captured bodies, with different chemical composition. Terraforming will be possible. The advantage of a different chemical composition can boost the local economy. Still, transport between prograde and retrograde objects will be very hard and challenging, both during terraforming and after.