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Until recently, it was thought that should the size of Jupiter exceed 86,881.4 miles in diameter or the mass 318 times greater than Earth's, it would become a dwarf star, not a planet.

WASP-17b is twice as wide as Jupiter but only half as dense, and that's likely because of its gravitational proximity to its star, which is close enough to be categorized as a "Hot Jupiter".

A really mind-boggling discovery is HD 106906b. Its mass exceeds Jupiter's by eleven times. Which raises the question: How can we be sure that HD 106906b is a planet as opposed to a wandering dwarf star?

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    $\begingroup$ How can something that is only 11 Jupiter masses be called a star? It would need to have at least 75 Jupiter masses before standard hydrogen fusion could be possible (and that is the definition of a star). The information in your first paragraph is incorrect. $\endgroup$ – ProfRob Feb 6 '17 at 20:29
  • $\begingroup$ @RobJeffries 20:36 youtube.com/watch?v=pypDWDTdiy4 $\endgroup$ – JohnWDailey Feb 6 '17 at 21:30
  • $\begingroup$ "If Jupiter were much bigger...". To be more accurate - about 75 times more massive. $\endgroup$ – ProfRob Feb 6 '17 at 23:14
  • $\begingroup$ With all due respect, where'd you get the info on the 75? $\endgroup$ – JohnWDailey Feb 7 '17 at 1:23
  • $\begingroup$ You could have a look at Burrows et al. (1997) arxiv.org/abs/astro-ph/9705201 a canonical piece of work. The plots from p49 onwards show that the dividing line between stars and brown dwarfs is just above 0.07 masses (73 Jupiter masses). $\endgroup$ – ProfRob Feb 7 '17 at 6:56
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Judging from the distance to its parent star, this could easily be a brown dwarf that formed together with the more massive star in a progenitor cloud.

However as this object is directly imaged, it's orbital properties are unknown. A high eccentricity together with semimajor-axis of a few tens to 100 AU, if found, would speak for the co-formation scenario. The discovery paper mentions that formation is unlikely to have happened from the protoplanetary disc, as well that it is co-moving with the host star, a.k.a. physically associated.

Also remember, that the $11\pm2 M_J$ mass of the planet is not measured directly, but derived from a model, that associates the observed luminosity to a mass, assuming a formation and atmospheric structure mechanism. Thus, it might have to be corrected upwards or downwards, emphasizing its origin as fragment of the original parent cloud.
This means there's a possibility to not have been formed in the way normal planets are formed.

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