OK, I'm making up a fictional planet (this is something of a follow up to my earlier thread). The planet is supposed to orbit Alpha Centauri B at .72-.73 AU, be about .76 Earth masses, and have an Earthlike moon.

According to this paper the stability region for satellites of a planet is .48 times the Hill sphere for prograde orbits. This Hill sphere calculator gives a Hill sphere of roughly 1 million km for my planet, which should translate into a stability region of ~480,000 km. So going by that an Earthlike moon should be fine.

However, I tried simulating the arrangement with Gravity Simulator and I got a highly unstable orbit.

I'm inclined to trust the paper (it does focus on gas giant satellites but I don't see why this situation should essentially be any different), but my knowledge of orbital mechanics is nonexistant. Does anybody who knows more about the subject have any advice on this?

Thanks.