True, but if Jupiter was a dwarf star, we would consider it a binary star system even if its barycenter with the Sun remained within the Sun's surface. (Not tha

True, but if Jupiter was a dwarf star, we would consider it a binary star system even if its barycenter with the Sun remained within the Sun's surface. (Not that that would be possible in practice since even the least massive stars are more than 50x Jupiter's mass). That is, binary planet means external barycenter AND both objects must be planetary in nature. If the objects are small enough the term binary asteroid could come into play. https://en.wikipedia.org/wiki/Binary_asteroid https://en.wikipedia.org/wiki/Binary_asteroid