Jupiter is the largest planet in our solar system and the fifth planet from the Sun, orbiting at a distance of 778 million kilometres (482 million miles). With a diameter of 141,000 kilometres (87,000 miles) it is about 11 times the diameter of Earth and given that volume is proportional to the cube of the diameter it has over 1,000 times the volume of Earth i.e. you could fit 1,000 Earths inside Jupiter.
Jupiter can be seen by the naked eye, though you have to know where to look. Excellent views can be seen through a telescope.
Jupiter is made primarily of hydrogen and helium gas stretching tens of thousands of kilometres towards what is maybe an inner rocky or icy core. Although Jupiter is made mainly of gas, the gas is extremely dense and gets denser and denser the closer you get to the planet's core. Deep within the planet the pressure is so dense that hydrogen gas is transformed into a liquid state, a process that releases energy which powers some of the powerful storms on Jupiter. Unlike Earth, there's no clear boundary between a thin atmosphere and a rocky planet, most of Jupiter is a swirling violent mass of gas with storms raging constantly. One of the most recognisable is the great red spot which is a huge anticyclonic storm (40,000 kilometres (25,000 miles) across and 14,000 kilometres from north to south) that has been raging for centuries. The Jovian storms are far more powerful than even the most extreme storms that occur on Earth. However, the storms are created and governed by the same laws of physics that govern storms on our home planet.
Jupiter, because of its size and mass, has an important gravitational influence in the solar system. Jupiter's gravitational pull can affect the orbit of passing celestial bodies which are either pulled towards Jupiter and captured, or it can change the orbit of the passing body so that it is slung out of the solar system entirely, or so that its orbit is altered - which could be away from or towards planet Earth.
Jupiter has 63 moons. The four largest moons were first observed by Galileo in 1610. Galileo, using his telescope in January 1610, noticed small points of light appearing and disappearing from view. He realised that these points of light were bodies orbiting Jupiter. His discovery was of the four largest moons of Jupiter - Io, Europa, Ganymede and Callisto.
Io is closest to Jupiter, orbiting at about 422,000 kilometres (261,000 miles) (similar distance of Earth's moon from Earth). Io orbits Jupiter once every 1.77 days travelling at over 62,000 kilometres per hour. Io has a surface temperature of - 155 degrees Celsius - but beneath the surface it is far from cold. Through the Voyager and Galileo space probes scientists have discovered that Io has an internal heat source. Given its size (about the size of our moon) it should have lost its heat to space a long time ago but the -155 degrees Celsius surface temperature is skin deep, because the rocky moon is in fact a boiling ball of molten. The source of the internal heat is the gravitational tug of war between Jupiter and the large Jovian moons of Europa and Ganymede. In the time that Io orbits Jupiter once it passes Europa twice and Ganymede four times. Io's orbit is influenced therefore not just by Jupiter but by Europa and Ganymede. The effect of this gravitational interplay is that Io is squashed and stretched making it a boiling hot lump of rock. Io is the most volcanic place so far discovered in our solar system, with giant lakes of lava and volcanoes that erupt 'big style'. The volcanic activity of Io gives it its yellow surface colour which is frozen sulphur.
Discovered in 1610 by Galileo, Europa circles Jupiter once every 3.5 days at a distance of approximately 670,000 kilometres. Europa is considered at the current time to be one of the places most likely to harbour some form of alien life. Europa is an icy moon about the size of our moon. Europa's surface is a smooth icy surface with a temperature of -160 degrees Celsius. Covering the smooth icy surface are cracks which scientists believe are caused by liquid water oceans underneath the icy shell.
Discovered in 1610 by Galileo, Ganymede circles Jupiter once every 7 days at a distance of approximately 1.1 million kilometres. Ganymede is the largest moon in the solar system - with a diameter of 5,260 kilometres (3,200 miles) it is larger than the planet Mercury. Ganymede is made of rock and ice and has a surface temperature is -160 degrees Celsius.
Discovered in 1610 by Galileo, Callisto circles Jupiter once every 16.7 days at a distance of 1.9 million kilometres. Callisto is the third largest moon in the solar system (Ganymede is the largest, Saturn's moon Titan is the second largest). Callisto is made of rock and ice and has a surface temperature of -155 degrees Celsius.