We Might Need To Redefine 'Planet' Again | Gizmodo Australia: At present, there's confusion regarding whether the biggest planets really are planets, or whether they're actually ultra-light stars. The working border set by the IAU is that a star should be massive enough for heavy hydrogen, called deuterium, to undergo nuclear fusion. That's around 13 times the mass of Jupiter. But researcher Kevin Schlaufman from Johns Hopkins points out that this isn't a clear cutoff point - and sometimes objects smaller than that definition just float around on their own, not orbiting any star as a typical planet would. He suggests there's a better way to define maximum planet size, and proposes an update to the definition based on how planets form.
"Objects more massive than [around 10 times the mass of Jupiter] should not be thought of as planets," he writes in the study, which is slated for publication in The Astrophysical Journal.
"Objects more massive than [around 10 times the mass of Jupiter] should not be thought of as planets," he writes in the study, which is slated for publication in The Astrophysical Journal.