<font color="yellow">One day the IAU may well be wrestling with this question. The question being "At what point is it called a planet or brown dwarf". There may even be some category between planet and brown dwarf. I consider dwarf planets to be planets, just smaller in size. Jupiter and Saturn have always been referred to as giants...or gas giants. The Sun is classified as a yellow dwarf star but its definetely a star. <br /><br />Two criteria will probably be used to define a star. Does it sustain nuclear fusion at its core. And does that fusion cause the star to emit its own light rather than reflect light as planets do? There was a scientist a couple years ago who suggested planets, namely Earth, may have fusion going on deep within their cores. Just not enough to cause the planet to ignite and become a star. <br /><br />It sounded plausible to me and could be the key to bridging planets and stars, but it is just theory at this point. </font><br /><br />If you read my definition of a planet (in another thread), I do think it should be limited by:<br />small enough mass to not sustain fusion (about <13 mass of Jupiter)<br /><i>and</i><br />composed primarily of "normal matter" (i.e. not degenerate matter, or a black hole, or neutron matter, or antimatter or anything other exotic substance we may discover in the future).<br /><br />As the mass of a planet increases, but still remains less than 13x Jupiter, some of the matter in the core become degenerate. Interestingly, this actually causes the diameter of the body to decrease. Somewhere about 5x to 10x Jupiter mass, the majority of the body's mass is degenerate matter. Brown dwarfs have a majority of degenerate matter.<br /><br />My definition that planets are composed primarily of "normal matter" would exclude brown dwarfs and other weird objects from being planets. Interestingly, this rather subtle but important issue is ignored by all the people who love to debate the definition of a planet but dwell o <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>