Planet

From TARDIS Index File, the free Doctor Who reference.

Planets are, simply speaking, large spherical masses in space. Planets generally orbit stars, but rogue planets are not uncommon. Their size, composition and features can vary greatly, with such planets such as Voga (a planet largely made of gold) (DW: Revenge of the Cybermen) and Barcelona (a planet with noseless dogs). The exact definition of a planet varies between cultures.

There are a number of highly unusual planets in the universe, including Magla, in reality an immense amoeba with a crust-like shell (DW: Destiny of the Daleks), and Mondas, the twin world of Earth (DW: The Tenth Planet), which for part of its existence had a planetary drive system.

The creation of a number of planets was 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 the Minyan ship the P7E by accident (and nearly also happened to another Minyan vessel, the R1C) (DW: Underworld), and the vessel carrying the last of the Racnoss by design (DW: The Runaway Bride)

Earth scientists have long argued over the proper definition of a planet. An attempt at reaching consensus in the mid-2000s resulted in Pluto, previously known as the ninth planet, losing its status as a full planet, though that didn't stop industrial development of the world (DW: The Sun Makers).