Which planet became Starkiller Base?

Question

In The Force Awakens, we see the First Order using a new superweapon -- Starkiller Base -- to siphon up the power of stars and use it to destroy multiple planets within that same solar system at a time.

The superweapon portion of this resembles a large Death Star trench, but the rest of the weapon is clearly an icy planet, complete with life (trees, at the very least, and a breathable atmosphere).

So, going by the logic that this has to have been a superweapon using a planet as the base structure, is there any canon evidence (perhaps in the blueprint in the movie or in the novelization) as to which planet it was?

Answer 1

No canon evidence right now.

• WGA script calls it "Starkiller Base" only

  EXT. STARKILLER BASE - DAY
  A pair of TIE fighters roar towards a magnificent PLANET, frozen WHITE. This is the STARKILLER BASE, a natural planet that has been hacked and modified into something sinister.

• Novelization offers no name

  The fleet of Star Destroyers stood off the white world. Spectacular and isolated, with a mean surface temperature varying from merely cold to permanently arctic, the planet had been altered: its mountains tunneled into, its glaciers hacked, and its valleys modified until it no longer resembled its original naturally eroded form. Those who had remade it had renamed it.

• The Visual Dictionary merely calls it "Starkiller Base" on Galaxy Map

  

Answer 2

This answer to another question related to the Base provides a link to a canon map that implies the planet was named Ilum.

https://scifi.stackexchange.com/a/111804/57506

Answer 3

Unknown, but clues point toward Ilum

To the best of my knowledge, the name of the planet on which Starkiller Base was built has not yet been established in canon. However, substantial clues point toward Ilum, the planet on which Jedi younglings gathered lightsaber crystals.

1. As mentioned in another answer, the location of Starkiller Base on new canon maps corresponds roughly to the location of Ilum on Legends maps.

2. Starkiller Base was discovered by Palpatine (a.k.a. Darth Sidious), using secret maps from the Jedi temple.

   During the height of the Galactic Empire, Palpatine demanded deep space exploration of the Unknown Regions, using ancient navigational routes uncovered in the captured Jedi temple as the backbone of such expeditions. It was in this manner that the Empire secured a small, frozen world with a natural abundance of energy-focusing kyber crystals. The Empire mined these crystals to create focusing arrays for the Death Stars’ superlasers. When the First Order fled into the Unknown Regions, it turned this world into the heart of its holdings. A massive engineering project transformed it into Starkiller Base, a weapons platform of terrifying destructive power and range.

   _{Star Wars: Complete Locations}

3. Ilum was rich in kyber crystals. So was Starkiller Base. As mentioned by Ahsoka Tano, there are other planets with kyber crystals:

   She followed their structure, ordered and regular, searching for other sources in the galaxy. She knew there must be more crystals somewhere. Master Yoda had never said as much, but he had certainly hinted at it. It was, after all, a very big galaxy.

   _Ahsoka

   However, they can’t be that numerous, since kyber crystals are a very rare resource. Still, at least one other planet (Lothal) had some.

4. Both planets are icy. Of course, no trees were shown on Ilum, but let’s not fall into the “single-biome planet” trap: we only saw one small part of Ilum up close in The Clone Wars, and other areas could be lightly forested. Besides (as discussed below), Starkiller Base was subject to extensive Imperial and First Order mining, which could certainly have changed the climate.

5. Ilum was extensively mined by the Empire to produce crystals for the Death Star:

   Great chunks of the planet had been carved away, exposing rock and lava that boiled up from the planet’s core. With no real hope, Ahsoka scanned the usual landing site.

   _Ahsoka

   So too, of course, was Starkiller Base, as discussed in (2).

None of this adds up to a definitive answer, of course. We know Palpatine mined other worlds for kyber crystals (such as Lothal), and there could easily have been other icy worlds with kyber crystals. Still, it does seem to make it a lot more likely that it is Ilum.

Answer 4

Ilum

According to the Rise of Skywalker Visual Dictionary, the Jedi planet was transformed into the Starkiller Base, chosen because of its kyber crystal core

FIERCE MACHINE

Properly energized, kyber crystals create containment fields that hold and amplify power to incredible levels. The Empire controlled several kyber-rich worlds, funneling their resources into the Tarkin Initiative's Death Star development. One unique world in the Unknown Regions, Ilum, had a kyber crystalline core. Most of its more easily accessible deposits were scoured, but there were even larger crystals buried deeper beneath the surface. The First Order continued its excavations and gradually transformed Ilum — a revered Jedi world since antiquity — into an instrument of unfathomable destruction.

Also confirmed in various other canon sources including the Star Wars: Battles that Changed the Galaxy factbook.

LOCATION: STARKILLER BASE (ILUM), UNKNOWN REGIONS

Deep in the Unknown Regions of the galaxy, the First Order turned the planet Ilum into a terrifying superweapon. This small, frigid world, once known as a sacred site for the Jedi Order, was transformed into a weapon that dwarfed the destructive power of the Death Stars that came before—a weapon that could win a war with a single shot.