Why does Uranus emit far less internal heat than Neptune?
In plain words
Neptune radiates more than twice the energy it receives from the Sun, while its near twin Uranus radiates only slightly more than it absorbs. Either something inside Uranus traps heat or Uranus lost its heat early.
Precise statement
Neptune's internal heat flux is about $4e2\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ (Voyager-era estimate), whereas a 2025 full-orbit energy-budget analysis gives $78\pm 18\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ for Uranus (Wang and coauthors, GRL 2025, https://doi.org/10.1029/2025GL115660). Determine the cause: a stably stratified interior with composition gradients that suppresses convection, early heat loss after a giant impact, or a different bulk composition. An answer is an interior and evolution model reproducing both planets' present fluxes, radii and gravity harmonics.
What would settle it
A Uranus orbiter measuring gravity harmonics, tidal response and thermal emission, with evolution models fitting both ice giants simultaneously.
Status in the literature
Unverified note
The 2025 analysis found that Uranus does emit internal heat, at $78 \pm 18\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$, though far less than Neptune.