The International Astronomical Union (UAI) defined in a provisional way the exoplaneta concept in the year 2003. According to the same one, the planets out of the Solar system must orbit about a star or remnant of star (white dwarf or neutrons star) and have a mass lower than 14 masses of Jupiter. Due to his limited mass, temperatures and thickness do not reach it in his interiors sufficiently high as to fuse deuterium, an isotope of the hydrogen composed by a proton and a neutron, or any other chemical element. Therefore, they do not produce energy from this type of source. 
The system exoplanetario of the star HR8799, composed by at least three exoplanetas. The image was obtained by the telescope Keck and special skills that allow to heighten the contrast and eliminate almost the whole sheen of the central star. His planets have masses between 7 and 10 times that of Jupiter and orbit at big distance of the star (15, 40 and 70 astronomical units). Credits: Christian Marois, Bruce Macintosh, Keck Observatory.
According to the same resolution of the UAI, the substellar objects, with masses superior to the previous ones, but that do not fuse hydrogen, they must be named brown dwarfs. On the other hand, the isolated objects of planetary mass, with mass below the limit of 14 masses of Jupiter, must be named brown subdwarfs or any other name that is appropriate, except planet.
Of course, these definitions might modify according to our knowledge advance. Some investigators think that the brown sub-dwarf expression is not very guessed right, and that other terms would be more suitable as IPMOs, planemos, oriones, or xebarcos.
Glossary: "100 basic concepts of Astronomy”
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