How do we determine what's in the core of stars/planets?

[mace2]

I tried searching but I must've done something wrong, so...

How do we determine what material is at the core of a planet? Do certain metals emit something we can detect?

I really don't know how to phrase this in an intelligent way, so please bear with me.

 

[ranger]

Determining properties of a planet's deep interior are difficult because we cannot observe this region directly as we can the surface of a planet. On Earth, seismology has provided important information on the structure and composition of the deep interior including the core. In addition, properties of the Earth's outer core motions have been determined mostly from observations of the geomagnetic field secular variation. Unfortunately we are not at present able to conduct detailed seismic studies and observations of magnetic field secular variation for other planets to provide us with the same quality of information on their cores. We are however, capable of studying a planet's magnetic field morphology quite extensively with an orbiting spacecraft at relatively low altitude. Here we use numerical dynamo models to determine what magnetic field structures can tell us about a planet's core. We concentrate on properties such as inner core size and outer core convective vigor. The results are used to determine what key observations of magnetic field structures could be made by future spacecraft missions to provide information on the dynamo source regions inside planets. This type of study can aid in determining the internal structure and evolution of Mercury, Jupiter, Saturn and Ganymede for which little is known about these properties.

This is an abstract of Determining Core Properties From a Planet's Magnetic Field Morphology

 

[MadScientist 1000]

So that's how they do it! I never thought of that. Nice abstract though, but I can't get to the full version.