The big planet[overachieving planet or the failed star?]

Gas giant planet,Brown Dwarf and Dwarf Star,such as Jupiter,Gliese 229B and Ogle-TR-122B is made from same ingredients:90%hydrogen+10%Helium.It also comes from heat quality of star.

Let me show you the information of The overachieving planet/Failed Star;

Jupiter

Temperature:-145*C[229*F]

Mass:1 mass of Jupiter

Despite from it's own size,this planet isn't particularly dense,beside it doesn't have enough mass to force the atoms that make it up to fuse them together.It is pretty cold and cool place.

GLIESE 229B

Temperature:730*C[1,346*F]

Mass:40 Jupiter masses

This big Brown Dwarf is far more massive than Jupiter.Gravitational force for extra mass will be enough to make deuterium atoms to fuse from it's core-generating heat for short time.

Ogle TR 122B

Temperature:2,500*C[4,500*F]

Mass:96 Jupiter mass

-Ogle TR 122B is more than 2x massive as Gliese.It can creates it's enough core to pressure for Hydrogen fusion to take place and can create heat for millions of years.