: Does anyone know if planetary orbits naturally decay or expand ?

::I would venture a guess something like this:

For an orbit to expand, the planet would need to be subjected to some force of acceleration to it's orbital velocity. Since no natural force I know of does this, an expansion seems unlikely. Artificial Earth satellites are subjected to this on occasion, like the Space Shuttle itself, so and expansion in orbit is the result of an intent to do so.

For planets however, the opposite must generally be true, a decay if you will over time, albeit a very slow one since there IS miniscule matter in "space" that would eventually slow a planet's momentum until the orbit collapses.

I believe in our case, the decay of Earth's orbit about the Sun is so slow as not be of consequence since the life cycle of the Sun itself will cause it to swell to a red giant, engulfing the planets out thru Mars before the orbits decay to the point of collapse. This to occur in 5-8 billion years if I recall correctly.