Jupiter Facts



  • Jupiter is the biggest planet in the Solar System – twice as heavy as all the other planets put together.
  • Jupiter has no surface for a spacecraft to land on because it is made mostly from helium gas and hydrogen. The massive pull of Jupiter’s gravity squeezes the hydrogen so hard that it is liquid.
  • Towards Jupiter’s core, immense pressure turns the hydrogen to solid metal.
  • The Ancient Greeks originally named the planet Zeus, after the king of their gods. Jupiter was the Romans’ name for Zeus.
  • Jupiter spins right round in less than ten hours, which means that the planet’s surface is moving at nearly 50,000 km/h.
  • Jupiter’s speedy spin makes its middle bulge out. It also churns up the planet’s metal core until it generates a hugely powerful magnetic field, ten times as strong as the Earth’s.
  • Jupiter has a Great Red Spot – a huge swirl of red clouds measuring more than 40,000 km across. The scientist Robert Hooke first noticed the spot in 1644.
  • Jupiter’s four biggest moons were first spotted by Galileo in the 17th century. Their names are Europa, Callisto and Ganymede.
  • Jupiter also has 17 smaller moons – Metis, Adastrea, Amalthea, Thebe, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope as well as five recent discoveries.
  • Jupiter is so massive that the pressure at its heart makes it glow very faintly with invisible infrared rays. Indeed, it glows as brightly as four million billion 100-watt light bulbs. But it is not quite big enough for nuclear reactions to start, and make it become a star.
  • The Galileo space probe reached Jupiter and its moons in the year 1995.
  • The Galilean moons are the four biggest of Jupiter’s moons. They were discovered by Galileo, centuries before astronomers identified the other, smaller ones.
  • Ganymede is the biggest of the Galilean moons – at 5268 km across, it is larger than the planet Mercury.
  • Ganymede looks hard but under its shell of solid ice is 900 km of slushy, half-melted ice and water.
  • Callisto is the second biggest, at 4806 km across.
  • Callisto is scarred with craters from bombardments early in the Solar System’s life.
  • Io is the third biggest, at 3642 km across.
  • Io’s surface is a mass of volcanoes, caused by it being stretched and squeezed by Jupiter’s massive gravity.
  • The smallest of the Galilean moons is Europa, at 3138 km across.
  • Europa is covered in ice and looks like a shiny, honey-coloured billiard ball from a distance.
  • A crater called Valhalla on Callisto is so big it makes the moon look like a giant eyeball.