Jupiter can hold more than 1,300 Earths by volume, yet its core remains dark to nuclear fire. The difference between planet and star is not size on a ruler but mass in the ledger of gravity and pressure.
Jupiter is dominated by hydrogen and helium, the same raw material as stars, but it carries only a fraction of the mass needed to reach the thermonuclear ignition point. Stellar models show that hydrogen fusion in the proton–proton chain demands a minimum central pressure and temperature that scale with total mass. A true star must cross a threshold where gravitational compression drives the core past tens of millions of degrees, pushing quantum tunneling and reaction cross‑sections high enough for sustained fusion and a positive energy balance over its own radiative losses.
In that sense, Jupiter sits in a narrow band of cosmic design: heavy enough that its internal pressure creates metallic hydrogen and a powerful magnetic field, yet far below the critical mass of a red dwarf or even a brown dwarf. It illustrates an odd kind of marginal effect in planetary formation, where a small change in accreted mass would have rewritten the architecture of the entire system.
