Question

made of mostly hydrogen and helium with a rocky core

Answer 1

An object made mostly of hydrogen and helium with a rocky core is characteristic of gas giant planets, such as Jupiter and Saturn. These planets have substantial atmospheres composed primarily of hydrogen and helium surrounding a smaller, denser core made up of heavier elements and rocky materials. This core is thought to be surrounded by a thick layer of metallic hydrogen under extreme pressure, which further contributes to their unique characteristics, such as strong magnetic fields and high atmospheric pressures.

If you were referring to a specific context, such as a theoretical exoplanet or a particular celestial body, please provide additional details!