What is the name of the planet made entirely of a city in The Force Awakens?

The story behind the answer

In The Force Awakens, the planet obliterated by Starkiller Base—and seat of the New Republic—is Hosnian Prime, depicted with vast urban skylines. It’s often confused with Coruscant, the classic city-planet and prequel-era capital, but Coruscant doesn’t appear in the film. Alderaan was destroyed decades earlier in A New Hope, and Corellia is an industrial shipbuilding world best known from Solo.

Context: the New Republic used a rotating capital, and at the time of TFA it was based in the Hosnian system; destroying the system erased both the Senate and much of the Republic fleet in one strike. The filmmakers chose a new capital to avoid revisiting Coruscant and to emphasize a narrative break from the prequels, which is why the scene can mislead viewers expecting the familiar ecumenopolis.