This star sounds like a gaseous planet without any solid mass. Where is the line between a cold brown dwarf and a rogue planet?
arbitrary size threshold
A star is a star, a planet is an inert non-star. The gas giant planets in our system probably have some kind of solid core, albiet small. Neptune’s is larger than Uranus (had to work in the latter, because you just know someone wants to get smart with it), although Uranus is larger. Jupiter’s rocky core exceeds the size of the Earth, but the total mass of Jupiter is 318 x Earth masses.