Jupiter is composed mostly of gas. This enormous planet radiates twice as much heat as it absorbs from the Sun. It also has an extremely strong magnetic field. The planet is slightly flattened at its poles and it bulges out a bit at the equator.

It takes Jupiter 9.8 Earth hours to revolve around its axis (this is a Jovian day). It takes 11.86 Earth years for Jupiter to orbit the Sun once (this is a Jovian year).

Jupiter's diameter is 88,700 miles (142,800 km). This is a little more than 11 times the diameter of the Earth. Jupiter is so big that all the other planets in our Solar System could fit inside Jupiter (if it were hollow).

Jupiter's mass is about 318 times the mass of the Earth, but the gravity on Jupiter is only about 2.54 times of the gravity on Earth. This is because Jupiter is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared). A 100-pound person would weigh 254 pounds on Jupiter.

In 1610, Galileo first discovered the four largest moons of Jupiter, Io (which is volcanically active), Europa, Ganymede (the largest of Jupiter's moons), and Callisto; these moons are known as the Galilean moons. Ganymede is the largest moon in the Solar System.

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