Three potential explanations
How did it form in such challenging conditions?
Planet formation is robust
These theories look to account for the intense amounts of radiation emitted by the huge main star in the system, since this creates an environment in which planets shouldn’t really be able to form. But maybe planets can form and flourish in such environments. Perhaps, as Janson says, planet formation is possible in “more diverse environments than we had imagined”.
Formed close and booted out
A standard theory for the formation of gas giants is core accretion. An icy core forms from collisions between planetesimals, then gas in the protoplanetary disc is attracted. This could have happened with b Centauri b, forming close to the binary star before being forced by gravitational interactions to move to its present location.
Gravitational instability
Another theory suggests the core accretion happened close to its present location. This supposes that the planet hasn’t moved.
The mass of the protoplanetary disc will have been so great that part of it collapsed in on itself, creating a planet that orbits the star. It’s an unusual way to form a gas giant, but it’s possible this could happen.