An unexplained energy budget
In the cold, distant reaches of the solar system, Neptune presents a significant thermal puzzle. The ice giant radiates approximately 2.61 times more energy than it absorbs from the distant Sun. This substantial internal heat source is what drives the planet's dynamic weather, including the fastest planetary winds in the solar system, which can reach speeds of 2,100 kilometers per hour. While other gas giants like Jupiter and Saturn also emit more heat than they receive, Neptune's energy imbalance is particularly pronounced, especially when compared to its nearest neighbor, Uranus.
Uranus, though closer to the Sun, is strangely inert. It radiates only about 1.1 times the energy it receives, making it one of the coldest places in the solar system. Neptune, despite being over 50% farther from the Sun and receiving only 40% of the sunlight that Uranus does, has roughly the same atmospheric temperature. The core temperature of Neptune is estimated to be around 5,400 to 7,000 K, comparable to the surface of the Sun, while its cloud tops approach a frigid 55 K (−218 °C). This vast temperature difference helps power its turbulent atmosphere. The mechanism responsible for this extra heat is a subject of scientific study, as simple models of planetary formation cannot fully account for it.
Raining diamonds
The leading theory for Neptune's excess heat involves a truly exotic form of precipitation: diamond rain. Deep within the planet, under pressures millions of times greater than Earth's atmosphere and at temperatures of several thousand degrees, methane (CH4) is thought to break down. The immense pressure squeezes the carbon atoms from the methane molecules into the crystalline structure of diamonds.
These newly formed solid diamonds, being denser than their surroundings, would then slowly sink through the planet's hot and fluid mantle. As they descend toward the core, they generate heat through friction with the surrounding material. This process effectively converts gravitational potential energy into thermal energy, which then convects toward the surface and radiates into space. This "diamond rain" could provide the additional energy needed to explain Neptune's thermal output. Laboratory experiments using lasers to create shockwaves in polystyrene have successfully replicated these conditions, observing the formation of nanodiamonds and supporting the viability of this hypothesis. The Kelvin-Helmholtz mechanism, heat generated by slow gravitational contraction, contributes to Neptune's energy, as it does for Jupiter and Saturn, it is considered insufficient on its own to explain the planet's large thermal surplus.