So far we have been fixated on the effects of gravity in outer space, with some amazing findings. But gravity has some surprising effects on the formation of the solar system as well, which not surprisingly have been overlooked by that army of astronomers out there.
Einstein started it.† He reckoned that if the gravitational force of the sun could reduce the velocity of light in its vicinity, then it could do the same for any ponderable object (i.e. planet) within its proximity. To prove it, he solved his equations for the motion of the planet mercury as it orbits the sun. Astronomers have long known that the perihelion of mercuryís orbit advances a small but measurable amount more than could be explained by Newtonís theory of gravity. But Einsteinís equations predicted the exact value measured over the years!† This is one of the major tests which have been used to verify Einsteinís General Theory of Relativity. It is what I call Gravity Drag!
Figure 1- Illustration (greatly exaggerated) of the advance of the perihelion of the planet mercury, predicted exactly by Einsteinís theory that the sunís gravity field would slightly slow the planetís velocity in its vicinity.
But Einstein stopped there. He never took his investigation to the general case of what gravityís effect on the other planets would be. This is probably because Einstein usually limited his results to occurrences which could be verified by experiment, and such an effect would not be measureable for the other planets due to their distance from the sun.
But let us look at it anyway.† For centuries, astronomers have puzzled over the observation that most planetary orbits are nearly circular, and in general are in the same plane as the earth.† This is peculiar, since if they were captured from outer space, their orbits should be either hyperbolic or highly elliptical, and in random planes.
Figure 2 - Illustrating the orbits of the planets around the sun.† Note that most are nearly circular and in the same plane as the earth.
It can be shown that a very small but constant deceleration of the velocity of a planet as a function of the gravitational field through which it travels (greatest near the sun) will gradually convert a hyperbolic orbit to an elliptical orbit, and gradually to a circular orbit.† This is a very small effect, and would take a very long time to developómany billions of years. But time is abundant in our universe!† And remember, the velocity reduction effect (Shapiro effect or gravity dtag) is a long-range effect, varying by the logarithm of the distance from the sun.† The same effect would tend to normalize the plane of the orbits to be perpendicular to the rotational axis of the sun.
Thus a very small but cumulative reduction in the orbital velocity of planets as they pass near the gravitational field of the sun can explain the current orbits of the planets.† This results completely from Einsteinís observation that any object passing through a gravitational field will experience a reduction in velocity!
Taking it one step further, this small but ever present force would create small torque forces on an orbiting planet or moon. The side nearest the sun or planet (in the case of a moon or satellite) would have slightly more deceleration forces than the opposite side, creating a torque moment on the orbiting body.† The net effect would be that the orbiting object would spin up or down until it reached equilibrium with the forces.† That point would be when its rotation rate matches the rotation rate of the object being orbited.† That is, when it presents the same face at all times, as does the moon toward the earth and mercury toward the sun.
So we see that the deceleration of an object passing through a gravitational field, as pointed out by Einstein, can explain how our solar system appears today. Make no mistake. This is a long and very slow processómany billions of years, and possibly not observable in our lifetime (although astronomers have not looked for it), but it surely happened.