Changes in planetary orbit? Forgive a novice and her naive curiosity, but I was wondering about planetary orbit and the aging of planets.
As a planet ages, does its transit around its sun/star decrease or increase in speed?
Does the speed and proximity of a planet's orbit around the star, have more to do with the age and type of the star, and less to do with the age of the planet and its distance from the star?
Also, is it possible for a planet to be pushed or pulled closer to its star, into a closer orbit, for example by increased gravitational pull of the star, or the planet being knocked off orbit by a comet or asteroid, or even a shockwave from an implosion -- one far enough away so that no debris comes near, but near enough that the shockwave is powerful enough to knock the planet.
I realise such an occurance would probably cause catastrophic upset on the planet, but if it remained relatively intact, could it find itself on a new path around its star??
Or could the comet/asteroid/shockwave hit the planet from the opposite direction of its transit, perhaps glancing it at such an angle as to cause it to slow in pace around the star, or to turn faster or slower on its own axis?
And would its poles be reversed?
Living in a world with sci-fi like Doctor Who, Eureka, Stargate, Star Trek, X-files, Fringe.. it makes you wonder what is theoretically possible. Often the explanations they give sound far-fetched, yet plausible.
Just as every myth evolves from a truth, you tend to wonder how much truth exists within these stories.
Hope one of you can shine a light on my wonderings and find a kernel of truth among them.
 A: You've asked a very entertaining question, and the answer is not simple. 
Let's ignore collisions for the moment. The "purest" effect, that is, the one which involves no change on the part of the planet or its sun, is the effect of tidal bulges in the sun. Just as the earth, for instance, is not a perfect sphere due to tidal forces, so the sun is not a perfect sphere, due to tidal forces caused by the earth. The resulting bulge in the sun lags behind the planet, and essentially acts as a brake on the planet. Over time, the planet will gradually lose velocity, and will eventually fall into the star. For most planetary systems, the effect will take a very, very, long time, since the planet is much smaller than the sun, and far away. 
But there's another factor to consider. Any star produces a "solar wind" which causes it to lose mass. The amount lost per year is small, but it never stops. The result is that, over billions of years the planet's orbit will grow larger as the gravitational attraction to the sun diminishes.
Finally, for stars like our sun, stellar evolution will eventually cause the star to become a red giant. If the diameter of the star exceeds the orbital distance of the planet, of course, the planet will be vaporized. Even it if doesn't, the tidal bulge will become much more effective in slowing the planet, and depending on details of the planet's orbit may or may not cause the planet to drop into the star before the star shrinks to red dwarf status.
In the case of the earth, according to http://arxiv.org/pdf/0801.4031v1.pdf that is exactly what will happen to the earth in (roughly) 7.59 billion years. It's notable that if the earth's orbit were 15% larger it would be safe. But just before the sun reaches peak diameter tidal forces conspire to slow the earth down and it plunges (will plunge)into the sun.
As for other considerations, such as explosions, impacts and shock waves, the answer is that they can have an effect, but generally not much. Basically, if the impact or whatever were big enough to make a major change in the planet's orbit, the planet would cease to exist, and would be replaced by a great big debris field. To some degree this would recondense into a smaller planet with a different orbit, but it wouldn't be the original one. Just as a thought experiment, though, if the earth were to hit another earth-sized body exactly head on, and the other body were in an identical orbit but going the other way, and the two planets fused instead of turning into a massive debris field, the resulting fused body would drop straight into the sun.
As for a planet ageing, for earth-types the answer is, not much. It's true that our kind of planet can lose volatiles such as water and air (and do so at a very low rate), but the total effect is miniscule. We are, after all, mostly rock and iron, and those just don't go anywhere. For gas giants like Jupiter, if they are close in they can get their gasses blown off until there is nothing left, or only the non-gas core. However, any such loss will be at right angles to the orbital motion (for more-or-less circular orbits) and will have virtually no effect the orbital motion of the planet.
A: Over long periods gravitational interactions between planets can have a very large effect on their orbits - especially if the planet you are interacting with is Jupiter.  Orbits can become "adjusted" into periods which are in resonance with Jupiter's period.
It is also possible for a planet to be moved to a completely different part of the Solar System by such interactions, and this may have happened in its early history.
In general gravitational systems are not stable.  Planets can be ejected completely by a close gravitational interaction with each other.
Orbits around binary or multiple stars will be more complicated, and probably more liable to catastrophic events such as ejection from the system into interstellar space.  The same will be true for planets in globular clusters, where many stars are nearby.
WhatRoughBeast is right that a sudden event with enough energy to change the orbit will also disrupt the planet, or at least boil its surface.  For example a nearby supernova might cause the plane to jet itself into a new orbit by boiling off the surface facing the supernova.
Things are very different for comets.  Their hypothesized orbits in the Oort cloud are so slow that they can be easily knocked into another orbit - for example one that enters the inner solar system.  They are so weakly bound by their own gravity that they can be broken up by near-misses with planets.
