Why is jupiter made of the same elements as the sun but is only considered a planet

The ancient Greeks counted the Earth's Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited.       Jupiter is composed of gases — hydrogen and helium, mostly — all ... the largest planet in the solar system, the gas giant just doesn't have ... But unlike the sun, it lacks the necessary amount to begin fusion, the process that fuels a star. ... to 80 times more massive than it is at present to be considered a star.