Therefore, its known narrative elements are not a part of the Doctor Who universe as we, on this Wiki, choose to define it. It may have been the basis for a similar story in another medium, however — and that story may indeed be valid.
Ishmael's Star was an unproduced P.R.O.B.E. film written by Mark Gatiss. It would have been the third and final P.R.O.B.E. film, serving as a prequel to The Zero Imperative and The Devil of Winterborne, about how Liz Shaw retired from Cambridge. It was going to be about "a war criminal who had some plan about a comet". Gatiss had Jeffrey Holland in line for the titular character.