Galileo’s most remarkable discovery happened when he pointed his telescope at the planet Jupiter. He found that Jupiter had moons! No one had conceived of the possibility that other planets could have moons that orbit the planet, just as Earth’s moon orbits Earth.
Jupiter is the largest planet in the solar system – over ten times the size of Earth in diameter. Like the sun, it is an enormous ball of hydrogen and helium gas with smaller amounts of other elements held together by its own gravity. Although hydrogen and helium are the two lightest elements, Jupiter has so much of them that its mass is greater than the rest of the planets combined, and yet is only 0.1% the mass of the sun. The pressure of the gases increases as we dive deeper into Jupiter, and at some point, they are forced into a liquid state. But Jupiter has no solid surface.