By the power of truth, I, while living, have conquered the universe.
Given that the "leak" says that Sigma 7 is the planet that John is going to, it couldn't be in the Sigma Octanus system (as you suggest), as there's only 4 planets in that system and you'd think that humanity would have examined these other planets in detail - especially since there's 17 million people on Sigma Octanus IV.
The only thing that we really have to go off is what we see in The Return, but perhaps the details of this short story have been retconned?