Xandar (//) is a fictional planet appearing in American comic books published by Marvel Comics. The planet is depicted as being in the Tranta system in the Andromeda galaxy. It is best known as the home world of the Nova Corps, an intergalactic police task force. Xandar is also the home planet of Firelord and Air-Walker, former Heralds of Galactus as well as the super-villain Supernova.
The first appearance of Xandar was made in Nova Vol 1 #1 (1976) by writer Marv Wolfman and artist John Buscema. The 2014 film Guardians of Galaxy introduced it to the Marvel Cinematic Universe as an alien planet where the major plot took place.
Blown apart in an attack by the Luphomoid Zorr, Xandar is saved from complete destruction by the timely intervention of the Watcher, who assists the Xandarians in creating a network of interconnected domed cities out of the largest remaining pieces of the planet.
The planet is later nearly destroyed by the Skrulls. Later still Nebula destroyed the planet and the Nova Corps. The Nova Corps were resurrected by Richard Rider aka Nova who used the Nova Force power to reboot the Xandarian Worldmind computer which proceeded to clone the dead Xandarians including Adora their Queen.
The war-torn planet is, once again, completely destroyed, and the Nova Corps decimated at the hands of Annihilus. Rider stands as the last surviving Nova Corps member following the massacre. The collective hive mind which resided in Xandar's core, an entity known as the Xandarian Worldmind, also survived and is now housed within Rider's newly modified uniform, which has also been enhanced to assist him in containing the full measure of the Nova Force.
For a short time, the living planet known as "Ego" became a temporary home for the corps, known as "New Xandar." This was short lived, as it was revealed Ego's intelligence was gradually returning and was attempting to corrupt the newly re-formed Nova Corps.
The Xandarians are a peaceful human-like race. Suzerain Adora was their Queen. Her husband Tanak Valt was Nova Prime. Adora once enlisted the Fantastic Four's help to end the war with the Skrulls. 
The Nova Corps was originally a space militia and exploration group for the planet Xandar. It consisted of 500 soldiers ranging in rank from Corpsman up to Centurion and its leader Centurion Nova Prime. The source the Corps' power is a nearly limitless energy field called the Nova Force, generated by the Xandarian Worldmind.  There are many notable individuals who used to be members of the Corp, including Peter Quill, Rocket Raccoon, and Samuel Alexander. 
The Worldmind is a supercomputer located in the bowls of Xandar, made up of the collective intelligence of the deceased Nova Corps members and the Xandar people. Actual brains are stored within the computer. Its purpose is to preserve the knowledge and history of Xandar and to serve as the custodian of the Nova Force.