The brilliant planet Jupiter dazzles anyone with a clear sky. Roman observers named Jupiter after the patron deity of the Roman state following Greek mythology, which associated it with the supreme god, Zeus. But when Galileo turned his telescope skyward in 1610, Jupiter took on new significance. Galileo discovered the planet’s four principal moons — and witnessed the first clear observation of celestial motions centered on a body other than Earth.

Astronomers recognized Jupiter as the largest planet in the solar system long before any spacecraft provided detailed exploration. The planet’s mammoth size — 88,846 miles (142,984 kilometers) at the equator — holds 2.5 times the mass of all the other planets combined. This makes Jupiter the most dominant body in the solar system after the Sun. The planet’s volume is so great that 1,321 Earths could fit inside it.

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Jupiter is a magnificent example of a gas giant planet. It has no solid surface and is composed of a small rocky core enclosed in a shell of metallic hydrogen, which is surrounded by liquid hydrogen, which, in turn, is blanketed by hydrogen gas. By count of atoms, the atmosphere is about 90 percent hydrogen and 10 percent helium.