Neptune Radiates 2.61 Times Absorbed Solar Energy, Exposing a 0.43-Watt Internal Heat Mystery
Updated
Updated · spacedaily.com · Aug 21
Neptune Radiates 2.61 Times Absorbed Solar Energy, Exposing a 0.43-Watt Internal Heat Mystery
1 articles · Updated · spacedaily.com · Aug 21
Summary
Voyager 2 measurements show Neptune emits 2.61 times the solar power it absorbs, implying an internal excess of about 1.6 times absorbed sunlight rather than an extra 2.5 times on top.
At 30 astronomical units from the Sun, Neptune gets only about one-900th of Earth’s sunlight; its effective temperature of 59.3 kelvin versus a 46.6-kelvin solar-equilibrium temperature creates the large energy ratio.
The inferred intrinsic heat flux is modest in absolute terms—about 0.43 watts per square metre—but still dominates Neptune’s local energy balance because incoming sunlight is so weak.
Scientists think the heat mainly reflects leftover formation energy, slow contraction and possible material separation, but no single mechanism has been confirmed and no orbiter has remeasured Neptune’s full energy budget since 1989.
The puzzle is sharpened by Uranus: a 2025 analysis found it emits only about 12.5% more than absorbed sunlight, leaving Neptune’s much stronger heat loss as a clue to divergent ice-giant interiors and evolution.