Jupiter

Jupiter is the largest planet in our solar system — more than twice the combined mass of all other planets, moons, and asteroids combined. The giant gas planet is composed almost entirely of hydrogen and helium with traces of methane, ammonia, and other gases, with no defined solid surface. Atmospheric pressure increases with depth until the gases gradually transition into a liquid metallic hydrogen ocean tens of thousands of miles deep, surrounding a possible rocky core.

Jupiter has been one of the most observed objects in the sky for as long as humans have looked up. The planet shines brighter than every star except the Sun at maximum brightness and is easily visible to the naked eye almost any night when above the horizon. The four largest moons — Io, Europa, Ganymede, Callisto — are visible in small binoculars and were discovered by Galileo in 1610, providing one of the first decisive observational arguments that not everything in the solar system orbited Earth.

The Great Red Spot, an enormous anticyclonic storm in Jupiter's southern hemisphere, has been observed continuously for at least 350 years. The storm is larger than Earth, with winds exceeding 400 mph, and has been shrinking slowly over the past century. The cause of the spot's reddish color remains debated.

Jupiter's moons are remarkable in their own right. Europa's icy surface conceals a global subsurface ocean believed to contain more liquid water than all of Earth's oceans combined — a leading candidate for extraterrestrial life in our solar system. Io is the most volcanically active body in the solar system, with massive sulfur volcanoes driven by tidal heating from Jupiter's enormous gravity. Ganymede is the largest moon in the solar system — larger than the planet Mercury.

Jupiter's strong gravity has profoundly shaped solar system evolution. The planet ejected smaller bodies into deep space throughout the early solar system and continues to deflect comets and asteroids from inner solar system trajectories — Jupiter has been called Earth's "outer guardian" because it intercepts many objects that would otherwise threaten the inner planets.