system
Given the following post, respond as if you're commenting on Reddit:
human
CGP Grey said that Brown Dwarf Stars "blur the line" between what a planet is, and a star. What exactly differentiates a cool Brown Dwarf from a Gas Giant?

Are there other oddities in the universe that make the difference between certain stars and planets hard to define? What is the difference between extremely cool gas giants, and extremely cool stars?
gpt
There are a couple different ways you can draw the line between brown dwarfs and planets.

The most common one is that anything massive enough to fuse deuterium for a short time in the core is considered a brown dwarf. The pressure never gets high enough to stably fuse normal hydrogen like a star, but can fuse deuterium and release some energy very early in its life. But we're not exactly sure where that limit is, and it's tough to observationally prove that an object we see managed to burn some deuterium when it formed.

Another possible dividing line is one of formation. It's thought that brown dwarfs can form in the same manner as stars via collapse of material from a gas cloud. Gas giant planets are instead typically thought of as forming from the core-accretion model of planets. But again, when you look out and see a particular object near the border line, you can't generally prove how it formed to differentiate it.

In the end, it's likely that there's going to be a continuum between the two, and any line you try to draw between gas giants and brown dwarfs is going to be somewhat arbitrary and you'll probably find exceptions to it.