Why can't our Sun be a binary with Jupiter as a T or Y dwarf? I just learned about Brown Dwarfs, they are "failed" stars, they narrowly missed the stellar mass mark. I learned that Y Dwarfs have temperature as low as 80 Fahrenheit (The first one found by WISE observatory), why aren't those not called a Jupiter-like planets without a host star? Why do we call them a star and designate a separate category for them (T or Y)?

Ok suppose T and Y dwarfs are okay, let's leave them alone. But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? Sun-Jupiter's Barycenter is just outside the Sun, we could call them in orbit with each other correct?
But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun?


There are two reasons: One is that Jupiter is too small to have ever undergone fusion of any sort. To qualify as a brown dwarf, an object needs to be large enough to have undergone deuterium fusion in its core. This requires a mass of at least 13 Jupiter masses. The other is that Jupiter formed by planetary formation mechanisms rather than by gravitational collapse.

There is debate on what to call objects of less than 13 Jupiter masses that formed by gravitational collapse. Are these sub brown dwarfs or free-floating gas giants, and does it even matter? There is also debate on what to call objects above 13 Jupiter masses that formed by planetary formation mechanisms as opposed to by gravitational collapse. Are these brown dwarfs or hyper planets, and once again, does it even matter?

In any case, Jupiter is not a brown dwarf.