Jupiter is primarily composed of hydrogen with a quarter of its mass being helium. It is also considered a failed star as it didn’t have enough mass to start the nuclear reaction at the core but nearly big as a star.

It has a diameter of 142,984 km at its equator (1/10 of Sun). The average density of Jupiter, 1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. 

Jupiter’s upper atmosphere is about 88–92% hydrogen and 8–12% helium and there are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur.

Jupiter’s mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun’s surface and both Jupiter and Sun revolve around this common point. If we consider the barycenter of Earth and Sun, it lies inside the sun.