The most massive planet in our solar system is Jupiter. A gas giant of 67 moons, the majority of which are roughly 6 miles or less in diameter. In 2011, NASA launched a spacecraft to orbit and observe Jupiter, but it will not reach the planet until late 2016.
Laying 483 billion miles from the Sun, it takes nearly 12 years for Jupiter to complete one full orbit around the Sun, and the Jovian day lasts about 10 hours. The planet is primarily composed of gas and liquid matter, and actually lacks a well-defined solid surface. However, it is speculated that the planet has a solid inner core that is roughly the size of Earth.
For hundreds of years a giant storm has raged on Jupiter, which is known as the Great Red Spot, and is visible through a telescope. At its widest, the storm is three times the diameter of the Earth. The Great Red Spot was first observed in 1635.
Although Saturn is better known for its rings, Jupiter also has a faint planetary ring system made of dust. It is speculated that the main ring is likely made of material from the moons Adrastea and Metis.
Of its 67 moons, the 4 largest can be visible from Earth with binoculars. These moons—Io, Europa, Ganymede, and Callisto—are known as the Galilean moons because Galileo Galilei discovered them himself.
Ganymede is not only the largest moon in our solar system, but it is actually larger than both Mercury and Pluto. Furthermore, it has at least one ocean between layers of ice. Europa’s frozen crust may be hiding a liquid ocean that holds twice as much water as Earth does. Since Callisto has low reflectivity, it is surmised that the moon is made up of a dark, colorless rock. Finally, Io has volcanoes that spew sulphur. This volcanic activity is caused by Jupiter’s immense gravity, which causes tides in Io’s solid surface that rise roughly 300 feet high.