comparison more meaningful, we have scaled the results to the size of the planet. In each case we can see what fraction of the interior is occupied by the core and what fraction by the mantle.

We can also carry out theoretical modeling of planetary interiors, just as we do for stellar interiors. Just as in stars, planetary interiors must be in hydrostatic equilibrium, meaning that dP dr pGM(r 2

where M(r) is the mass interior to r. If the density can be approximated as being constant, then

so the equation of hydrostatic equilibrium takes the form dP dr

We can use equation (24.6) to estimate the central pressure of a planet (just as we did for the Sun), by taking the pressure at the surface to be zero and integrating:

to get

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