Have the planets changed since they were first discovered & documented?

Well it's hard to say for most of the planets -- the "classical" planets (those that are visible to the naked eye) have been known about since before recorded history (in fact, the word planet is derived from the ancient greek astēr planētēs, or "wandering star"), but until Galileo pointed his telescope at Jupiter, no recordable data was available from any of them.

But in the sense of Jupiter, it has changed noticeably - not just since the recent comet impact (which may have resulted in the Southern Equatorial Belt fading almost totally away for a few years - (it is reviving now) - which was a VERY strange sight. Jupiter has always had those two major bands of brown clouds. Seeing it without one was very odd.

But most notable over the long term is the gradual disintegration of Jupiter's most famous feature - The Great Red Spot. In Galileo's time it was brick red and MUCH larger than it is currently. In fact, it was noticeably larger and redder during Voyager's flyby than it is today. It is fading very rapidly over the past 2 decades (compared to the fact it's been a stable anticyclonic storm for at LEAST 400 years - who knows how much longer it was there prior to its first observation). Now it's more of a rusty red/orange color than the bright red "splotch" it used to be even in amateur scopes.

Just a few years ago Saturn had a massive storm break out, a bizarre storm that almost appeared to be a hurricane with a tail -- a "dragon storm" i heard it dubbed, that actually seemed to create a "wake" in the clouds as it went through them. One of the most interesting things I've gotten to watch at a telescope was this storm's obvious development and (for lack of a better term) rampage across the Saturn as it slowly seemed to expand almost all the way around the planet.

Mars goes through seasons as well, Martian summer is usually marked by ferocious sandstorms approaching Dune-worthy levels, and its icecaps shrinking. In the winter the caps expand again.

Uranus and Venus are both pretty one note, observationally. Uranus is a pretty monochromatic blue ball of methane and hydrogen/helium, and Venus is just white CO2 clouds and hellishly hot year round.

Neptune has only JUST finished it's first "year" since it was discovered*, and it's so far out that it doesn't really have major seasonal changes -- though bizarrely the plant has the fastest winds in the entire solar system (Including a massive cyclonic "Dark Spot" like Jupiter's GRS). What drives these winds is currently unknown, as it does not receive enough heat from the sun to drive its atmosphere like that.

But hey, curiosity's good. Only way to learn things is to ask and read about them.

you're the two incorrect. The moon isn't pulling the Earth everywhere, and the Earth isn't being pulled into outer area. The moon's gravity motives tides on earth, and those tides reason friction between the sea and the seafloor. And this friction is amazingly gradually slowing the Earth's rotation (via some seconds a century). The regulation of conservation of momentum states that the full momentum of a gadget (the rotation of the Earth and the moon and the orbits of the two products around the centre of gravity) has to stay the comparable. So if the Earth is slowing down, then some thing has to alter to shield momentum. that must be the moon rushing up (which it won't be ready to do) or the moon shifting extra beneficial from the Earth (which it could do, and is doing). The Earth/Moon gadget is held in orbit via the solar's gravity, and the only way the Earth must be pulled into outer area became via yet another super merchandise's gravity (and there is not any super merchandise everywhere close to via to tug the Earth everywhere). The moon actually does not have the mass or gravity to tug the Earth everywhere (its the Earth that retains the moon in orbit).

Yes, the planets are constantly changing.

The ones with atmospheres have changes similar to weathering on Earth.

The planets without atmospheres are unprotected from meteors, and so their surface gets new craters every day.

The sun's planets will not change enough in our lifetime to make a non-habitable planet become habitable. But if we continue to make irresponsible decisions, maybe Earth itself will become non-habitable.

Right now, the Kepler satellite is discovering planets around other "suns". Concentrating on these planets is the best hope for finding habitable planets in the next 100 years.

adaviel is right, Jupiter has changed from the shoemaker meteor. To be precise about what changed though, there are now much higher concentrations of water in the upper atmosphere of the southern hemisphere.

On Saturn's moon Titan, as the seasons change so do the areas that get the most wet from the methane rain. It could be possible that locations of lakes have changed over the course of the time we've been able to peer into its atmosphere of it, or it could happen soon.

Yes. There have been extensive changes in the atmosphere of Jupiter over the past century and a quarter, during which Jupiter has been studied in detail. Dust storms are reported in Mars with regularity. Temporary storms have formed and disappeared in the atmospheres of Saturn and Neptune. Just this week NASA released videos of changes in Saturn's polar regions.

Jupiter has changed noticably since comet Shoemaker-Levy hit it

People have changed the climate and environment of Earth, but I guess that doesn't count