Hot-Jupiter radius inflation

Hot-Jupiter radius inflation is the anomalously large radius of many strongly irradiated gas giants relative to isolated cooling models. Delayed cooling and energy deposition by tides, atmospheric circulation, or electrical currents are leading contributions.

Delayed cooling preserves a young giant planet's large radius by reducing the escape of internal energy. Enhanced atmospheric opacity, an irradiation-maintained outer radiative blanket, and composition gradients that inhibit convection can slow Kelvin-Helmholtz contraction. These mechanisms retain existing heat rather than supplying a new deep heat source.

A stabilizing composition gradient can suppress large overturning motions while permitting double-diffusive convection in small layers separated by interfaces. If heat loss is less efficient than in a single well-mixed adiabat, the planet retains more entropy and contracts more slowly. This is a proposed delayed-cooling contribution to hot-Jupiter radius inflation, with an effectiveness that depends on the gradient and layer structure.

A giant planet can also remain inflated when power is deposited sufficiently deep to alter its internal energy budget. Candidate sources include tidal heating, Joule heating by wind-driven currents, and dissipation of circulation-driven mechanical energy. The depth and efficiency of deposition matter, as discussed in models of hot-Jupiter heating.

Atmospheric winds across a magnetic field can induce electric currents. Their Ohmic heating converts mechanical energy into heat, at local rate $q=\mathbf{J}\cdot \mathbf{E}=J^2/\sigma$ in the simplest resistive description. To maintain hot-Jupiter radius inflation, current paths and deposited heat must influence the deep interior; dissipation restricted to optically thin upper layers need not maintain the planet's entropy.