Young giant planets retain high formation entropy and radiate gravitational and thermal energy as they contract. Their infrared self-luminosity, especially at wide angular separation from a young nearby star, made the first directly imaged exoplanets much easier to detect than mature reflected-light planets. The inferred brightness depends on whether formation followed a high-entropy hot start or a low-entropy cold start.
By years, deuterium burning and most rapid Kelvin-Helmholtz contraction have ended. The intrinsic luminosity is governed mainly by the remaining interior entropy and ionic heat capacity, slow contraction supported by partially degenerate electrons, and the atmospheric opacity that controls escape of heat. Composition-dependent processes such as helium rain can add energy. For an irradiated planet, absorbed and reradiated starlight may dominate the observed luminosity, but it does not equal the planet's intrinsic cooling luminosity.
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