Where Worlds End: Tides, Heating, and the Survival Edge for Close-In Exoplanets

What sets the smallest orbital distance (periastron) at which a planet can persist near its star? We develop an analytic, physics-based planetary population boundary in periastron versus stellar luminosity and test it on a global catalog of close-in planets. Two key mechanisms govern the edge: tidal planetary disruption establishes a nearly constant floor at low luminosity, while radiative heating sets a rising boundary that asymptotically scales as the square root of stellar luminosity. Variations across star–planet systems make the transition between tidal and heating regimes smooth. This yields a closed-form two-regime model with few parameters: tidal plateau, heating-limited scaling, crossover luminosity, and transition width. Using lower-envelope extraction on main-sequence hosts, we confirm the tidal floor and the square-root scaling at high luminosity, locate the transition and critical stellar irradiation threshold. Evolved hosts were considered separately to avoid effects from the proximity to the stellar surface. This yields a unified analytic law for the inner edge across main-sequence hosts that provides predictive periastron limits for a given stellar luminosity.