I would like to know how you can tell what a planet's core is made of.
The answer is part observation, and part intelligent guesswork.
In order to find out what's inside a planet, we can't just drill a hole. Even on Earth we can only penetrate a few miles into the crust! We have to rely on indirect methods.
Gravity: By putting a spacecraft in orbit around a planet, we can observe how the spacecraft's orbit is affected by the planet's gravity. This lets us determine its density profile (how dense the different layers of the planet are). Trajectories that pass closer to a planet are more sensitive to the planet's interior structure.
Oblateness: No planet is a perfect sphere. They all bulge a bit at the equator due to the centrifugal force caused by their rotation. The amount of the bulge is called the planet's oblateness, and how strong the oblateness is also tells us something about the density profile of the planet.
Seismology: Using seismometers on the surface of the Earth and the Moon (from the Apollo missions), and soon Mars (with InSight, launching in 2018), we can measure tremors passing through the planet by measuring earthquakes (or moonquakes or marsquakes!) at the surface. Quakes travel through the planet as waves, and these waves travel at different speeds through different materials, and can also be reflected by the boundaries between layers. By tracking the path of seismic waves through a planet, we can tell what layers of a planet are high and low density, and which layers are is solid and which are molten.
Now, here's where the guesswork comes in. Based on the density profile of the planet, we can guess what the planet is made of. Rock has a lower density than metal, for example. And different types of metal have different densities. While making these guesses, we have to keep in mind that the deeper you go into a planet's interior, the higher the temperature and pressure, and this can affect the density of the planet as well. And of course we keep in mind what kind of building blocks there are to make planets from. It would be a little silly to conclude that the center of the Moon is made of gold, for example, because we know from studying the composition of meteorites and the crusts of the planets that gold is fairly rare in the Solar System.
So this is how we can make fairly good guesses at what the interiors or Mercury, Venus, Earth, the Moon, and Mars are like. We can also use these methods to infer the interior structure of some of the better-studied moons in the outer Solar System, Io, Europa, Ganymede, and Callisto.
For gas giants, it's a little tougher because they don't tend to have nice distinct layers like the rocky planets. Also, they are so huge that the pressures and temperatures in their deep interiors are much greater than we can simulate in the laboratory, so we must rely on theoretical predictions rather than actual data to say how the substances they are made of act under those circumstances. Still, we can determine that Jupiter and Saturn are composed almost entirely of hydrogen and helium, and that most of the heavier stuff, including rocky materials and metals, must have sunk down to their cores. But is the core solid, in any real sense of the word? And what exactly is the core's composition? It's very difficult to say, because we have a hard time understanding how materials behave when they are so hot and so dense.
Currently (September 2016), there are spacecraft at both Jupiter and Saturn that will soon probe the interiors of those planets by orbiting such that they will pass just a few thousand kilometers above the cloud tops - close enough for fine measurements of both planets' gravity. NASA's Juno probe arrived at Jupiter on July 4, 2016, and its measurements of Jupiter's gravity will provide valuable information on Jupiter's interior. The NASA-ESA-ASI Cassini spacecraft has been orbiting Saturn since 2004, and in early 2017, for Cassini's Grand Finale, its orbit will be shifted so that it passes between Saturn and its inner rings.
This page was last updated on September 5, 2016.