The registration point isn’t measured in meters. It’s a percentage of the dimensions, with 0.5 being the exact (geometric) center of the object, 0.0 being the bottom near left (I think) and 1.0 being the upper far right (again, I think).
Granted, you could create some kind of orrery by having a bunch of planets with their registration point set to that they were all at the center of the overall set of planets, and then setting all of them to rotate, this wouldn’t be a very flexible solution. First, you wouldn’t be able to have a planet spin independently of it’s position, so the same face would always be pointing towards the sun. Second, you wouldn’t be able to have moons that rotated around the planet (well, you could, but the calculations to do so would be hideously complex and ugly).
A much better solution would be to use entity parenting. Create the sun entity, and then for each planet, create it and set it’s parent entity ID to the sun, and give it localRotation and localPosition properties to position it in the sun’s reference frame. Moons could then have a given planet parent entity, and similarly move relative to them.
If you want them to be in motion (“orbiting”) the sun at individual rates, it’s a little trickier. Either you could have a script running (ideally an AC script) which is constantly updating the positions, or you could do a little bit more with the parent child relationships and creating invisible entities to serve the purpose for each individual planet orbit.