How to sort the dwarfs from the planets

摘要

What's the difference between a planet and a brown dwarf? That can be a tricky question, because some objects straddle the conventional boundary between the two categories. Now the discovery that there is a dearth of cosmic bodies whose mass lies within a particular range could provide a clean dividing line between planets and brown dwarfs, which are heavier than planets but lighter than stars. Objects are traditionally classed as planets if they have less than about 13 times the mass of Jupiter, and as brown dwarfs if they are heavier.