In Phantasy Star 1, Motavia was the closest planet to the sun, with Palma being the second. When Phantasy Star II came around, they had switched - Palma was now closer to the sun than Motavia without any explanation.
The Compendium book released shortly after PSIV tried to explain this using an event called the Conjunction. It says that, about 450 years before PSII, the three planets came into alignment. Some kind of gravity problem caused Motavia and Palma to switch orbits, physically moving the planets with entire cities being destroyed in the process. The worldships from PSIII were also supposedly created during this time, probably to evacuate the planets on a temporary basis so everyone wouldn't die.
More recently, Sega of Japan's website, in a retrospective look at the series, offered a different explanation. It said that Motavia just had an elliptical orbit, causing it to occasionally pass between Palma's orbit and the star. There was no Conjunction under this explanation. The massive devastation that required evacuating in spaceships didn't happen.
So, my question to you is: which explanation do you prefer? Did the planets really trade places, involving worldships and all, or does Motavia just occasionally slide in front of Palma?