It used to be thought that it was a 'failed star', but you’d need at least 75-85 Jupiter masses to get fusion started it turns out so no, it wasn't close to being a star.

No, Jupiter is a planet not a star. Brown dwarfs start out well over a dozen times more massive than Jupiter. Planets form from a proto-planetary disk, as Jupiter did (you can tell because it is in the same orbital plane as the other planets, the same as the original plane of the disk).

99.9% the mass of the Solar System is in the Sun. 0.1% of the mass of the Solar System is in Jupiter. It's very big for a planet in our Solar System but it is very small compared to even the smallest stars.

Related question(s):

Do stars ever look like Jupiter/gas-giant planets at some point during their formation?

Can planets like Jupiter "pull in" enough matter to "ignite" and become stars in the future?

Not impossible, but rather unlikely. According to TFA, the partner star was much further out. Also, gas giant planets Jupiter's size are fairly common I thought (this knowledge may be outdated).

No: https://www.reddit.com/r/askscience/comments/gnq8t/is_it_tru...

The question itself is weird; planets don't really "fail" to become something else. They simply accumulate mass during accretion, and either that mass (and other factors) is sufficient for ignition and sustained fusion, or not.

It would be a little bit like asking if Neptune is a failed Jupiter or Mars was a failed Earth (hmm, perhaps a bad example).

Jupiter is far, far too small to have been a candidate for a brown dwarf.

It does emit more radiation than it absorbs, which is star-like.

Don't think so, it's too small.

Well what do you know, Jupiter doesn't actually orbit the sun! Not properly anyway

http://www.iflscience.com/space/forget-wha-you-heard-jupiter...