- They're massive enough to be rounded by their own gravity.
- They're not massive enough to cause thermonuclear fusion.
- And have cleared their neighboring region of planetesimals.
The appearance of a planet is never constant, and they tend to change depending on various factors: like temperature, surface, etc.
Sentient planets, as remonstrated by Planet Janet, apparently have the ability to change their appearance at will. However, since only Janet has shown this ability, it might be rare or not commonly seen in other sentient planets.
Average planets seen in the series. Most of them have the ability to support life in it, though it is never specified how.
Planets that have some level of sentience, like the ability to speak and understand their inhabitants. Seeing or traveling to this planets is rare in the galaxy.