Jupiter outweighs every other planet, moon, asteroid, and comet in the Solar System combined — by a factor of 2.5.
Its composition is nearly identical to the Sun's: the vast majority of its mass is hydrogen and helium. In essence, Jupiter is a star that never quite made it. To ignite nuclear fusion, it would have needed roughly 80 times more mass.
Had that happened, we'd have two suns in our sky.