How big does a planet or moon have to be to have a molten core?

Alexander Banks

The structure of the core itself is an interesting topic as the majority of objects have both an inner and outer core. The inner core is generally solid, this is due to the immense pressure forcing the core together. The outer core tends to be molten, this molten core is what is responsible for an object’s magnetic field. Almost every planet or moon in our Solar System is thought to have some form of core. However, these cores vary in size depending on the object. Rocky objects, such as the smaller planets and moons, tend to have cores that are roughly a third of their radius, while the gas giants have proportionally lower core sizes although this is relative. So, while Jupiter’s core makes up only around 3 to 15 per cent of its mass, it is still 12 to 45 times the mass of the Earth. As objects get smaller they are less likely to be separated into distinct structures but it is thought that the large asteroid Vesta may have a core at its centre. Vesta is the second-most massive object in the asteroid belt (and the second-largest asteroid). If objects are much smaller than this it is thought that there is not an opportunity for the distinct layered structures of cores to form, although further investigation of the asteroids in the Asteroid Belt needs to be carried out to confirm this.

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