Planet Profile Jupiter

The largest planet has a lot to tell us

- Ian Evenden

Fifth in the eight-planet line-up of our Solar System, Jupiter also happens to be the largest, and by quite some distance. The mass of this gigantic ball of gas is two-and-a-half times that of all the other planets put together, and you could fit 11.2 Earths within its radius. While there’s likely a rocky core somewhere under the enormous gaseous atmosphere, scientists can’t be sure whether it’s solid or not, but gravitational measurements suggest it could make up as much as 15 per cent of Jupiter’s mass.

What is known is that Jupiter is contracting, and this generates more heat than the planet receives from the Sun, warming the huge number of moons that orbit around it. It also has a faint ring system, too thin to be seen from Earth with any but the largest telescopes and first spotted by the Voyager 1 probe in 1979.

Jupiter plays a major role in many theories of the formation of our Solar System. In one, known as the grand tack hypothesis, Jupiter formed at 3.5 AU – 1 AU is the Earth-Sun distance – before plunging inward towards the Sun until it reached 1.5 AU, then reversing course and moving out again, stopping at its current distance of 5.2 AU. It crossed the asteroid belt twice, scattering rocks in all directions and contributing to the low mass of the belt today. It may also have caused rocky planets orbiting closer to the Sun to crash into the star’s surface. This answers questions such as why Mars is so small – Jupiter’s presence limited the material available for its formation – and why there are no large planets orbiting close to the Sun, as we see in other solar systems.