perfect design thanks ventolin cost uk Jupiter, the fifth planet from the sun, is the largest planet in our solar system and has more than 60 known moons. Europa is the smallest of Jupiterâs four largest moons, known as the Galilean satellites after Galileo Galilei, who first observed them with his newly invented telescope in 1610. It is slightly smaller than Earthâs moon and is believed to be made of silicate rock with an iron core. Its atmosphere contains mostly oxygen. Scientists theorize that one place life could exist on Europa is around hydrothermal vents on its ocean floor.