You'll need to progress the MSQ. Without giving too much away, the world we play on is actually called 'The Source'. It is one of several shards that the primal, Hydaelyn created to seal away Zodiark. Each of those shards functions as a 'world' in its own right. The role of the Ascians is to engineer what they call "The Rejoining", and a calamity needs to take place on both the source as well as one of the shards in order to rejoin that shard to the source. There are 13 of these shards in total, with the source being one of them, and the 13th (World of Darkness) was lost to the void. You will also note that the summoning of the primal form of Bahamut lead to the 7th calamity. This should let you know how many shards are actually left.

The planet in question isn't even referred to as a planet. It is called a star. The role Hydaelyn and Zodiark play on this star is all explained in the ShB MSQ.