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Why do Jupiter and Saturn contain mostly hydrogen and helium?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

Jupiter and Saturn contain mostly hydrogen and helium because they formed in the outer regions of the solar nebula, far enough from the Sun that temperatures were low enough for water to condense into ice.1

In these cooler regions, accreting bodies could gather significantly more solid material than those forming closer to the Sun. Once a forming planet reached approximately 10 times the mass of Earth, its gravitational field became strong enough to capture and retain large amounts of hydrogen and helium directly from the surrounding nebular gas. Because hydrogen and helium are the most abundant elements in the universe, these planets were able to grow into massive, gas-rich bodies.12

While the inner planets remained small and rocky due to higher temperatures that prevented the condensation of volatile substances, Jupiter and Saturn successfully underwent this gravitational collapse, accumulating their vast gaseous envelopes during the early stages of the solar system's formation.123