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 star system. Some planets possessed an atmosphere and could support life.
Overview
Planets generally orbited stars, but rogue planets (COMIC: The Rogue Planet) and wandering planetoids were not uncommon. (TV: Revenge of the Cybermen, The Tenth Planet) Perihelion was when a planet or other body came closest to its star. (HOMEVID: The Zero Imperative)
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 Secret Heart, the vessel carrying the last of the Racnoss, by design. (TV: The Runaway Bride)
As noted by Professor Alistair Gryffen, planets would continually move through space across their history. Thus, in the year 50000, the Earth would be located far away in the galaxy from the place it held in the 21st century. (TV: The Bounty Hunter)
Dwarf planets were a smaller type of celestial body, not considered to be actual planets. (AUDIO: The Anachronauts, The Bounty of Ceres)
Statistics
The New Dalek Empire used a magnetron to steal 27 planets, including the "lost moon of Poosh" across time and space, with twenty-four being taken from 2009. (TV: The Stolen Earth) They were returned to their rightful places by the Children of Time. (TV: Journey's End)
The Daleks gained control of over seventy planets in Ninth Galactic System and forty more in the constellation of Miros in the five hundred years up to 4000. (TV: Mission to the Unknown)
According to the Ninth Doctor, the Fourth Great and Bountiful Human Empire encompassed a million planets. (TV: The Long Game)
The Rally of the Twelve Galaxies encompassed 94 planets. (TV: The Ghost Monument)