This beautiful, mountainous and cultured planet is located in the Core Worlds. It's day is 18 Standard hours long and has a year of 364 days. 

It had long been Allied with the Old Republic and the Rebels, leading to it's complete destruction by the Death Star. 

(WoG Canon) It was rebuilt and remade as before in our Galaxy, at great expense, and became the seat of government for the Emperor and Supreme Chancellor. Unfortunately, it was blown up once again in the KR War (Kouhan Reformers) and yet again rebuilt and so is now Alderaan III.