How close did Jupiter come to being a “no foolin\” star, thus giving us a binary solar system?
Not even close. If Jupiter were about 13 times bigger, it MAY (stress – MAY) have experienced deuterium fusion for a few thousand years, then died. Significantly larger – 65 times or so, would fuse deuterium and lithium, but again, only for a very short time. However, Brown dwarfs have convective surfaces and interiors, meaning the layers are constantly churning and mixing due to heat. Under about 13 times Jupiter’s size, the body is separated into layers and considered a planet. The tiniest stars capable of full fusion are, at a minimum, about 100 times larger than Jupiter. However, and this is the weird part, we’re talking about mass, and the main differentiator is how that mass reacts and what forces are in play. Which means Jupiter, the Brown Dwarfs, and the tiniest smallest stars are all about the same physical size, just massively different densities.
