Chris Peterson wrote:
Mercury doesn't fall into the Sun because it's in orbit. Bodies in orbit never fall into the object they are orbiting, unless something else takes energy away from them. In fact, the solar wind does just that, although from a practical viewpoint the effect is insignificant. But all the planets are moving through the solar wind, which creates a miniscule amount of drag, and therefore results in a (very, very) slow spiraling in towards the Sun. If the Solar System had a much longer lifetime, and the Sun remained in its current state, the planets would eventually end up hitting the Sun.
The "pushing" effect of the solar wind is insignificant.
If the body is in a nearly circular orbit around the Sun, and the solar wind is flowing radially outward from the Sun, then the force imparted by the solar wind on the body should also be directed radially outward and in fact should
add to the energy of the orbit, not
subtract from it. This would cause the orbit to spiral outwards, not inwards.
Now if the solar wind were not flowing outward with such great speed (relative to the tangential orbital velocity of the body in orbit) there
would be a drag effect which would remove energy from the orbit and cause eventual orbital decay and inward spiraling.
Having said that I see that there must be a drag effect regardless of the speed difference between the solar wind and the orbital velocities. I can't say which would dominate with any certainty, but Chris apparently believes that the radial "pushing" is "insignificant", and also that the "drag" is "miniscule". I wonder which would win in the long run?
The reason that the solar wind causes orbital decay for bodies orbiting a planet is that the solar wind is not radially aligned, and so the perturbance is half the time net outward, and half the time net inwards causing the orbit to become increasingly eccentric until it intersects either the atmosphere or the planet itself.