1° Forum di Astronomia Amatoriale Italiano

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,[27] 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.[28][29]

The accretion disk of the protostar IRAS 16293-2422 has parts rotating in opposite directions. This is the first known example of a counterrotating accretion disk. When its planets form, the inner planets will orbit in the opposite direction to the outer planets.[30]

WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[31] A second such planet was announced just a day later: HAT-P-7b.[32]

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]

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.[33]

All eight planets in the 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. The reason for Uranus's unusual axial tilt is not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.[6]

It is unlikely that Venus was formed with its present slow retrograde rotation which takes 243 days to rotate. 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 which create a retrograde torque. Venus' 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 other alternative hypotheses have been proposed to explain Venus' retrograde rotation, such as collisions or it having originally formed that way.[a] Mercury is closer to the Sun than Venus but Mercury is not tidally locked because it has entered a spin-orbit resonance due to the eccentricity of its orbit. The rotation of Earth and Mars is also affected by tidal forces with the Sun but they haven't reached an equilibrium state like Mercury and Venus because they are further out from the Sun where tidal forces are weaker. The gas giants of the solar system are too massive and too far from the Sun for tidal forces to slow down their rotations.[7]

Nel caso specifico non lo so, ma il concetto e' che tutti ruotano e "rivoluzionano" nella stessa direzione a meno che non gli accada qualcosa di brutto (leggi collisione). Poi, nel caso di "cattura", ogni combinazione diventa possibile, presumo.

in realtà sarebbe molto più macchinoso buttare a mare la meccanica e ammettere una violazione della conservazione del momento angolare (fenomeno mai osservato altrove e non riproducibile in laboratorio)