 Giant gas planets like Neptune experience strong gravitational forces pushing the mass inward. The deeper into the planet we go, the greater the pressure, the greater the temperature, and the greater the mass density. The extremes all exist at the center, so we'll focus on planetary cores. The high temperatures in the interior create significant kinetic energy within the gas that creates pressure forces pushing outward. The planet takes on a volume that equalizes these two forces at each and every distance from the center. We say that the planet is in hydrostatic equilibrium. This provides information about the pressures felt in the gas giant's core as a function of its mass. Now if we add mass to the planet, the particle density and the interior temperature will increase according to the laws of thermodynamics. At 10,000 degrees Kelvin, the electrons are stripped from their protons and form a free electron gas. The ideal gas formula still holds and shows that the rise in temperature increases the outward pressure. As a result, the planet's volume grows. Jupiter is 18 times more massive than Neptune and 23 times larger.