How to Sort the Dwarfs from the Planets

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. Uncertainties in the measurement of mass make it hard to classify borderline objects this way. But when Johannes Sahlmann of the Geneva Observatory in Switzerland and colleagues surveyed brown dwarfs and planets orbiting stars, they found a dearth of objects between 25 and 45 times Jupiter's mass, but plenty of objects outside this range.