Compact Star - Planets

Planets

At low density (planets and the like) the object is held up by electromagnetic forces. These forces constrain electrons to occupy orbitals around nuclei, which give rise to chemical bonds and thus allow stiff objects such as rocks to exist. These objects are so stiff that they do not compress very much when mass is added. Adding more (cold) mass therefore makes the object larger: radius increases with mass.

Eventually a point is reached where the central pressure is so great that all matter is ionized so that the electrons are stripped from the nuclei and move freely. No chemical bonds now exist to hold up the object. This point is reached at the center of the planet Jupiter. Add more mass to Jupiter and the increase of pressure is smaller than the increase of gravity, so the radius will decrease with increasing mass. The object will shrink.

A planet such as Jupiter has about the largest volume possible for a cold mass. Add mass to Jupiter and the planet's volume, somewhat counter-intuitively, becomes smaller. The central density now is large enough that the free electrons become degenerate. This term means that the electrons have fallen into the lowest-energy states available. Since electrons are fermions, they obey the Pauli exclusion principle, and no two electrons can occupy the same state. The electrons thus occupy a wide band of low-energy states. Compressing the mass forces this band to widen, creating the quantum-mechanical force of electron degeneracy pressure which now holds the center of the planet apart. (The ions present contribute almost no force.)