The Mystery Of Mirkheim

The following propositions are facts within Poul Anderson's History of Technic Civilization:

planetary systems do not condense around giant stars;

Beta Centauri is a giant star with planets because, while it was condensing, its nebula retarded, then captured, a group of rogue planets;

van Rijn says somewhere that scientists are still trying to figure out how the red giant Betelgeuse has planets;

Mirkheim was a planet of fifteen hundred Terrestrial masses orbiting a star as bright as a hundred Sols.

So how did Mirkheim exist? We are told how it was found:

"From the known distribution of former supernovae, together with data on other star types, dust, gas, radiation, magnetism, present location and concentrations, the time derivatives of these quantities: using well-established theories of galactic development, it is possible to compute with reasonable probability the distribution of undiscovered dark giants within a radius of a few hundred parsecs....The most you can learn is the likelihood (not the certainty) of a given type of object existing within such-and-such a distance of yourself, and the likeliest (not the indubitable) direction."
-Poul Anderson, David Falkayn: Star Trader (New York, 2010), p. 653.

But the "...given type of object..." has to be possible, however improbable, in the first place. Surely Astrocenter would not have been able to compute the probability of a condensing giant star capturing several rogue planets? So how was Mirkheim's probability computed?