English: Hafnium-182 radioactively decays to tungsten-182 with a half-life of about 9 million years. This system can be used to date when core formation occurred for a particular planet. Hafnium is lithophilic (rock loving) and tungsten is siderophilic (iron loving). Early in a planet's history it would have been undifferentiated, meaning that it would not have been layered according to density (with the densest material being towards the interior of the planet). If differentiation (core formation) occurred relatively early in a planet's history then hafnium would not have much time to decay to tungsten. Being that hafnium is lithophilic, it would remain in the mantle (or outer layers of the planet). Then after some time, hafnium would decay to tungsten. On the other hand, if differentiation (core formation) occurred later in a planet's history, then hafnium would have decayed to tungsten. Being siderophilic, tungsten would move towards the interior of the planet along with iron. In this scenario, not much tungsten would subsequently be present in the outer layers of the planet. As such, by looking at how much tungsten is present in the outer layers of a planet, the time of differentiation (core formation) can be quantified.