Planets were celestial bodies of varying size, large enough to be rounded into a spherical shape by their own gravity, but not massive enough to cause thermonuclear fusion. Most planets existed as part of a solar system. Some planets possessed an atmosphere, and could support life.
There were a number of highly unusual planets in the universe, including Magla, in reality an immense amoeba with a crust-like shell, (TV: Destiny of the Daleks) and the artificial planet Arcadia. (PROSE: Deceit)
Some planets were artificially accelerated through the interference (intentional or not) of other life forms. Normally in these cases large starships provided the initial large mass which began the process of planetary formation. Such a fate befell a Minyan ship, the P7E, by accident (and nearly also happened to another Minyan vessel, the R1C), (TV: Underworld) and the vessel carrying the last of the Racnoss by design. (TV: The Runaway Bride)