The map that leads to Treasure Planet once belonged to Captain Flint, and given to Jim Hawkins by Billy Bones from John Silver.
It apparently could only be opened by someone possessing a certain quality, shared by Captain Nathaniel Flint and Jim Hawkins, as the latter had simply been fiddling with it and activated it by accident.
When John Silver tried to open it, he couldn't, forcing him to bring Jim along. The Map also turned out to be a key to the very center of Treasure Planet and to the portal system Flint had used to pillage and raid ships.