By scientific definition, a planet is any object orbiting a sun with an empty path and sufficient gravity to not tear apart or deform. However, the definition of "planet" used in Transformers context is a bit different. ANYTHING with gravity can be a planet, and oddly, they all seem to be about the same.
Major known planets
Over the course of the many continuities of Transformers, a lot of planets have been encountered, most of them sustaining life. Here are some of the more well-known planets prominently mentioned. It should be noted in some continuities, they are vastly different in atmosphere and gravitational pull, while in others, they're invariably JUST LIKE EARTH.
- Earth - Most likely, you're on it.
- Cybertron - The homeworld of the Transformers. According to which source you use, it is either much smaller than earth, about equal to the Moon in size, or much larger, about equal to Saturn. Regardless of the differences in gravity either size would cause, humans can walk on it with no problems (in most, maybe all, continuities). It may or may not also house a God and turn into a gigantic robot of unspeakable and awesome power.
- Nebulos - Home to human-like aliens who can combine with larger robots through special armor and cybernetic implants.
- Master - Kinda the same, but the aliens are robots or something. Crazy, we know, but only in the Japanese Headmasters anime.
- Junkion - AKA the intergalactic junk yard. Looks more like a single continent than a whole planet (and probably wouldn't be considered a planet either, at least in real life).
- Velocitron, Jungle Planet, and Gigantion - the three colony worlds of Cybertron. Way in the past, these planets were to be made into homes for Cybertronians, but the space bridge project failed and most of them forgot about it (except for the Gigantions who just kept on mindlessly building). Gigantion in particular appears to be much larger than Earth but still has Earth-like gravity (that, or the human kids are incredibly strong), and all of them have an Earth-like atmosphere (or, the kids the Autobots picked up DON'T BREATHE).
Some planets are known to be capable of transforming. The two most commonly known are Unicron and Primus. Both were given life at the beginning of time by the One, the sentient core of the universe, or in one case, a puddle of glowing food-goo. Whatever the origins, they are beyond legendary, with their stories heard throughout the universe, and not only by Transformers. The planet Junkion was SUPPOSED to transform, but they probably forgot when they came back from commercial break.