What are Gas giants?

A gas giant (sometimes also known as a jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extra solar gas giants have been identified orbiting other stars. Planets above 10 Earth masses are termed giant planets. Lower-mass gassy planets are sometimes called "gas dwarfs".

Objects large enough to start deuterium fusion (above 13 Jupiter masses for solar composition) are called brown dwarfs and these occupy the mass range between that of large gas giant planets and the lowest mass stars. The 13 Jupiter mass (MJ) cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, especially on the amount of helium and deuterium present. The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.