A planetary core was the innermost portion of a planet.
Earth's core[]
When Earth formed billions of years ago, the Secret Heart became its core. (TV: The Runaway Bride [+]Russell T Davies, Doctor Who Christmas Special 2006 (BBC One, 2006).)
Around 40,000 BC, the Euterpians planned to use Earth's core to refuel their mothership, but it was destroyed by solar flares before they could do so. (PROSE: Invasion of the Cat-People [+]Gary Russell, Virgin Missing Adventures (Virgin Books, 1995).)
In 1984, the Dominators attempted to acquire Earth's core to fuel their fleet, but were defeated by UNIT. (COMIC: The Fires Down Below [+]John Peel, DWM backup comic stories (Marvel Comics, 1982).)
In 2007, the Torchwood Institute drilled a borehole to Earth's core under the Thames Flood Barrier in an attempt to free the Secret Heart. (TV: The Runaway Bride [+]Russell T Davies, Doctor Who Christmas Special 2006 (BBC One, 2006).)
During their invasion of Earth, the Daleks attempted to remove the core and replace it with a power system that would allow them to pilot the planet anywhere in the universe, but they were defeated by the First Doctor. (TV: The Dalek Invasion of Earth [+]Terry Nation, Doctor Who season 2 (BBC1, 1964).) During their second invasion thirty years later, they again attempted to remove the core and for the same purpose, but they were defeated by the Eighth Doctor. (AUDIO: Lucie Miller [+]Nicholas Briggs, Eighth Doctor Adventures (Big Finish Productions, 2011)., To the Death [+]Nicholas Briggs, Eighth Doctor Adventures (Big Finish Productions, 2011).)
Iris Wildthyme visited the Earth's core. While there, she encountered reptile people and a "dashing old gentleman". (PROSE: From Wildthyme with Love [+]Paul Magrs, Iris Wildthyme (Snowbooks Ltd, 2013).)
Cores of other planets[]
Earth's was not the only planetary core the Daleks wanted. They mined the core of Arkheon in an attempt to acquire the Arkheon Threshold, (PROSE: Prisoner of the Daleks [+]Trevor Baxendale, BBC New Series Adventures (BBC Books, 2009).) mined the core of Galacton to fuel the Dalek Seventh Fleet. (PROSE: Mission to Galacton [+]Justin Richards, Doctor Who Files (2006).) During the Last Great Time War, the Daleks attempted to replace every planetary core in the Null Zone with hyperdrives so they could fire all the planets at Gallifrey. (AUDIO: The Heart of the Battle [+]Nicholas Briggs, Only the Monstrous (The War Doctor, Big Finish Productions, 2015).)
The Kastrians' power source was drawn from the core of the planet Kastria. This was one of Eldrad's gifts to his people. (TV: The Hand of Fear [+]Bob Baker and Dave Martin, Doctor Who season 14 (BBC1, 1976).)
The core of Lucifer was made of heavy elements which were the remains of the native Angels. (PROSE: Lucifer Rising [+]Jim Mortimore and Andy Lane, Virgin New Adventures (Virgin Books, 1993).)
The core of Spiridon was an allotrope of ice. (TV: Planet of the Daleks [+]Terry Nation, Doctor Who season 10 (BBC1, 1973).)
Among the exhibits in the Pentaquoteque Gallery were paintings made from metal from a giant gas planet's core and held in stasis. (COMIC: The Arts in Space [+]Nick Abadzis, Doctor Who: The Tenth Doctor (Titan Comics, 2014).)
The Omega Arsenal on Gallifrey was kept near the planet's core. (PROSE: A Brief History of Time Lords [+]Steve Tribe, BBC Books (2017).)
The Skaro of N-Space had an icy core, known as the Ice Heart of Skaro. (COMIC: The Dalek Trap [+]Brad Ashton and Terry Nation, The Dalek Outer Space Book (Dalek annuals, Panther Books, 1966).) In the "unforeseeable future", the Daleks had lifts in Zone X that led to the very heart of their world. When Skaro imploded in this age, the First Doctor and Susan Foreman briefly sought to take refuge at Skaro's core to ride out the collapse. (COMIC: The Test of Time [+]Tim Quinn and Dicky Howett, The Doctor Who Fun Book stories (Target Books, 1987).) In the Unbound Universe, Skaro's core held a vast store of dimensional energy, making the planet a target of the Quatch. (AUDIO: Masters of War [+]Eddie Robson, Doctor Who Unbound (Big Finish Productions, 2008).)
At the core of Alvega resided the Controller of the Amarylls. When the Daleks invaded the planet, the natives' mastery of the environment proved too much for the invaders. A single Dalek flying saucer burrowed underground with the intent to destroy the Controller. When the last Dalek drone of its crew did so, Alvega's core collapsed, destroying the planet. (COMIC: The Amaryll Challenge [+]unclear authorship, The Daleks comics (City Magazines, 1965).)