Jupiter

Jupiter is the fifth planet from The Sun, and the largest planet in The Solar System. It takes 11.86 Earth Years for it to complete an orbit, and travels at an average speed of 29,236 mph.

Size
Jupiter is the second largest object in The Solar System (the first being the Sun). It has a diameter of 142,984km, whereas Earth's is only 12,756km. However, it is no where near the size of the sun (that having a diameter of over a million kilometers). Scientists believe (although they do not know for sure) that the core of Jupiter could be the size of around 10-15 Planet Earth's.

Discovery
Jupiter is the fourth brightest object in the sky, which means we can sometimes see it from the surface of Earth. When we had no electrics, there wasn't a problem of 'Light Pollution'. This meant that during the night, we could see planets and stars much clearer than we can today. We didn't need the aid of a telescope to discover Jupiter, as we just needed to look up when it was visible to us. So Jupiter doesn't really have a discoverer, as people could see it even back in the Prehistoric times.

Moons
Jupiter has an astonishing 64 moons. There is a list of them here. The four main moons of Jupiter are: Io, Europa, Ganymade, and Callisto (they are currently known as the Galilean Moons). They were discovered in 1610 by Galileo Galilei, and they convinced him that the Geocentric Model of the Solar System was incorrect, and that Nicolaus Copernicus's theory was true. However, when he spoke out his new beliefs, people became angry as this went against everything they had been taught, and Galileo was arrested and jailed for the remainder of his life.