Bigger moons like Europa and Ganymede typically orbit in the same direction as the rotation of their planet, in what astronomers call a prograde motion. That is not surprising, because these moons likely formed out of a disk of dust and gas that was spinning in the same direction as the planet as the solar system took form.
But the immense gravity of Jupiter could also capture other passing objects, and those space rocks sometimes end up orbiting the other way, in what’s known as retrograde motion.
Two of the new moons have prograde orbits, matching an inner group of moons that are thought to be fragments of a larger moon that shattered. Nine fall among three swarms of more distant retrograde moons, likely remnants of three larger, captured moons.
The 12th moon is an oddity. It moves among the retrograde moons yet orbits in a prograde direction. “It’s going down the highway the wrong direction.”
It is important to note that astronomers do not clearly understand exactly why some planets (or moons) move in retrograde motion, but one possible answer is explained in the Urantia Book, where it states that, as our solar system was being formed, another nearby-system known as Angona was traveling close enough to create a gravitational tug, thus, resulting in a slight loss if its own stellar material, some of which was capture by the gravity of Jupiter and Saturn, and makes this curious statement:
“The powerful gravity pull of Jupiter and Saturn early captured most of the material stolen from Angona as the retrograde motion of certain of their satellites bears witness.”
All of the solar system material derived from the sun was originally endowed with a homogeneous direction of orbital swing, and had it not been for the intrusion of these three foreign space bodies, all solar system material would still maintain the same direction of orbital movement. As it was, the impact of the three Angona tributaries injected new and foreign directional forces into the emerging solar system with the resultant appearance of retrograde motion. Retrograde motion in any astronomic system is always accidental and always appears as a result of the collisional impact of foreign space bodies. Such collisions may not always produce retrograde motion, but no retrograde ever appears except in a system containing masses which have diverse origins.:
In summary, the Urantia Book authors are once more vindicated by a new scientific discovery, and in this case, even provides an answer to astronomers with regard to understanding why there is sometimes retrograde motion among our solar neighbors.
And there is this report from 2004 that hypothesizes just such an collusion described in the paragraph above from Paper 57 of the Urantia Book.
The icy planet Uranus is an odd place. It spins on an axis almost perpendicular to its orbit, with one pole pointed straight at the sun for much of the year. It’s also colder than expected and has an unusually-shaped magnetic field. One theory for how Uranus became such an oddball in our space neighborhood involves a massive impact strong enough to tip a young planet onto its side.
In research published this week in the Astrophysical Journal, a group of researchers ran the numbers on such a collision and simulated what the results might be if a planet one, two, or three times the size of the Earth were to strike Uranus in the early days of our solar system. Jacob Kegerreis, one of the authors of the report, joins the program to talk about the research and what it might tell us about planetary formation elsewhere in the universe.