Jupiter, the largest planet in the solar system, might seem utterly alien from the perspective of an Earthling. It has no immediate solid surface, but rather is a gas giant, largely comprised of hydrogen and helium. Its atmosphere is so dense that huge storms many times the size of Earth, such as the one contained in its Giant Red Spot, persist for centuries.
But despite its bizarre composition compared to Earth, Jupiter-like planets are actually exceedingly common in the galaxy. Indeed, astronomers observe gas giants of similar makeup everywhere they look. It is believed that the reason that Jupiter-like worlds are so common is because most solar systems like ours form in a similar way. Our solar system started life as a rotating, disk-shaped cloud of gas and dust; at its central point, some gas slowly accreted into a star (our sun); other bits of gas and dust accreted into small rocks that became large rocks, which eventually accrued into planets.
