Although 318 times more massive than Earth, Jupiter would need 80 times more mass for its core to be hot enough to sustain thermonuclear fusion — the creation of helium from four atoms of hydrogen with the release of energy. Fusion generates the energy that allows stars to shine.
Brown dwarfs are not massive enough to sustain fusion, either. These “failed stars,” however, form entirely from interstellar gas. Planets, on the other hand, assemble from the dust and gas of the remnant disk that forms while the star is accreting matter. The two classes may in fact overlap — as of now, we do not know.
James B. Kaler
University of Illinois at Urbana-Champaign
(July 1997 issue)
Additionally: Today, the International Astronomical Union places the dividing line between brown dwarfs and planets at 13 Jupiter masses. This is the minimum mass required to ignite deuterium fusion. (Deuterium is an isotope of hydrogen.)