Final answer:
Evidence for Jupiter's composition includes hydrogen and helium spectral lines and its low density of 1.3 g/cm³, which is inconsistent with a rocky composition. Theories and models suggest Jupiter has a central pressure over 100 million bars, making the light gases behave unlike anywhere on Earth. Additionally, Jupiter's similarity to the Sun in chemical makeup, with hydrogen becoming metallic liquid due to extreme pressure, supports the conclusion of its primary composition.
Step-by-step explanation:
The evidence that Jupiter is primarily composed of hydrogen and helium, rather than rock, is twofold. First, the presence of hydrogen and helium lines in Jupiter's spectrum indicates that these gases exist in its atmosphere. Second, the density of Jupiter, which is about 1.3 grams per cubic centimeter, is too low for a planet of its size to be made up of rocks or metals alone. The high mass of Jupiter leads to the intense compression of these gases in its center, causing hydrogen and helium to behave in ways they don't on Earth. Despite being made mostly of light gases, Jupiter's immense pressure turns the hydrogen into a metallic liquid, which contributes to its overall density.
Moreover, theoretical models predicting Jupiter's central pressure exceed 100 million bars with a central density of about 31 g/cm³; this compaction of hydrogen and helium accounts for its density. Also, astronomers conclude that the only possible materials that could construct a planet with the observed masses and densities of Jupiter and Saturn are these light gases. While we cannot directly observe the deep interiors of these planets, these indicators provide strong support for their composition.
Additionally, the fact that Jupiter and Saturn share a chemical makeup similar to the Sun, primarily hydrogen and helium, reinforces that these are not terrestrial or rocky planets. The notion that Jupiter could be termed a 'gas planet' is misleading as the pressures within cause hydrogen to become a liquid rather than a gas. Therefore, it is more accurate to reference Jupiter as a 'liquid planet' considering the state of the majority of its hydrogen.