Thermoelastic Contraction as a Suppressor of Atmospheric Escape in Close-in Exoplanets

Abstract

The long-term retention of substantial atmospheres in close-in exoplanets presents a major challenge to classical hydrodynamic escape theory, which predicts rapid mass loss under intense stellar irradiation. In this work, we propose a fully classical, interior-driven suppression mechanism based on thermoelastic contraction of the planetary mantle. By incorporating pressure- and temperature-dependent elastic deformation into the structural evolution of the planet, we demonstrate that radial contraction can lead to measurable increases in surface escape velocity. We analytically derive a modified escape condition and introduce a dimensionless suppression index Xi that quantifies the extent to which internal mechanical response inhibits atmospheric loss. Numerical simulations across a wide parameter space show that volumetric strain values in the range 0.005 to 0.01 can enhance escape velocities by up to 10 percent, leading to a reduction in energy-limited escape rates by over 50 percent. When applied to warm mini-Neptunes such as GJ 1214b, K2-18b, and TOI-270c, the model successfully accounts for their persistent atmospheres without invoking exotic stellar conditions or chemical outliers. Our results indicate that planetary elasticity, often neglected in escape models, plays a first-order role in shaping the atmospheric evolution of close-in worlds. The theory yields specific observational predictions, including suppressed outflow signatures and radius anomalies, which may be testable with JWST, ARIEL, and future spectroscopic missions.