IAU is well known for coming up with shitty arbitrary classifications about nomenclature that many astronomers don't agree with. They are wrong here because they don't take into account post-Cassini/Juno understanding of gas giant morphology. The IAU definition is outdated and highly misleading.

Copied from another reply I gave in this thread:

I've seen 13 MJ argued as a boundary, but it's selected somewhat arbitrarily and based around idealized models of Deuterium fusion, which has never been observed, and which is a process these brown dwarves would only undergo for a brief flash in their early life. Deuterium isn't abundant enough for its fusion to significantly alter the stellar morphology that has already become established for objects larger than Saturn. Saturn is our solarsystem's example of an object that does not fit cleanly into one side or the other of a mass-based binary classification scheme for determining a hard boundary between "planet" and "star". To understand what is a planet vs what is a star, study Saturn.

And if you want more, check out what I said last time this meme was posted.

As someone who worked as an astrophysicist for 9 years, I assure you that the question of "what is a planet?" is a nuanced discussion with a lot of diverse opinions and no clear answer that gets endlessly debated by students as they learn that these definitions aren't as cut and dry as irresponsible science communicators made it seem during the disastrous and highly politically motivated demotion of Pluto to dwarf planet.

Based on what criteria?

Jupiter is large enough for the hydrogen to become a plasma and dissolve the rocky "planetary" core that was once at the center. Morphologically, it has passed the transition from planet to star. Saturn appears to be somewhere along that transition and is harder to cleanly classify.

Morphologically, Jupiter is a star.

I've seen 13 MJ argued as a boundary, but it's selected somewhat arbitrarily and based around idealized models of Deuterium fusion, which has never been observed, and which is a process these brown dwarves would only undergo for a brief flash in their early life. Deuterium isn't abundant enough for its fusion to significantly alter the stellar morphology that has already become established for objects larger than Saturn. Saturn is our solarsystem's example of an object that does not fit cleanly into one side or the other of a mass-based binary classification scheme for determining a hard boundary between "planet" and "star". To understand what is a planet vs what is a star, study Saturn.

Anti-Mindvirus