So there are just a couple of reasons why it's increasingly believed that there must be a significant number of wandering planets moving through space - and why it makes sense that some of these may be captured by other stars.
I can't find the article that talks about it, but from modelling of solar system formation and evolution, some think that there are probably more planets out in interstellar space - 'Rogue Planets' - than there are in orbit around stars. This was backed up by the discovery of some potential candidates a few years ago. Modern estimates of Jupiter-sized rogue planets give supposedly their number at about 0.25 times the number of stars in the galaxy. Of course then, you have to add 'Earth-sized' and smaller planets? (Surely, as they are lower mass and assuming that astronomical objects fits some reasonable power law - don't see why not - there must be much more of them.) As you've stated, Brian, I do think that a great deal of these rogue planets were indeed formed in solar systems but then wrenched out by processes - some of which you mention.
Unfortunately for any idea that a rogue planet might have been gravitationally captured by a star system....while it may be possible, as I've said many times before on this topic, SPACE IS VERY BIG
. In four billion years the Andromeda galaxy will collide with out galaxy and when these ~1 trillion stars hits our ~300 million...yep, there are expected to be no direct collisions. Similarly for a rogue planet to just hit a star system is going to be extremely rare. Furthermore the planet must move with a velocity very close to that of its capturer. Also, it must involve a third body (either another planet or star bound to its capturer) to absorb, into its orbit, the orbital energy and angular momentum released by the capture. It all adds up to, 'well the universe is such a big place, so it must have happened
somewhere, but it's so rare we should not rely on this explanation to explain virtually anything!'
As for this star and planet - the star is described as a red dwarf with a mass half that of our sun, hence it's still main sequence and likely to remain so for many billions of years. So the star should not have lost mass and that cannot be an explanation for said Jupiter sized object to be in it's system.
I believe the standard theory they are referring to is the Solar Nebular Disk Model and my guess is that there probably is a correlation between the mass of the proto-star accretion disk, the mass of star it produces, and how much 'stuff' is left over for planetary formation. In this case it seems said planet has taken a much higher proportion of the 'stuff'. Note however what sort of figures we're dealing with here: Here on Sol, Jupiter is ~0.1% the mass of the Sun. In this system, this big planet is ~0.15% the mass of the red dwarf. Doesn't look much at all!
The problem with all current theories and models of solar system formation is that we've only really had one example to theorise about and measure properly, and it is only in the past few decades that we've been able to start to pick out what other systems do. As each new observation adds to the zoo of star systems perhaps we'll have a better chance at correctly unpicking how solar systems
can form and evolve.
