Astronomy
Thermal Evolution of Carbon-Rich Exoplanets and Diamond Layers
Quick fact
Some carbon-rich exoplanets, like 55 Cancri e, could have mantles up to one-third diamond by volume, and the high thermal conductivity of diamond means heat escapes much faster than through Earth's rocky mantle, dramatically altering the planet's cooling history.
Why this is interesting
Imagine an entire planet where the mantle is made of diamonds—not rock. How would that change the way the planet cools and evolves?
Read the full explanation
Understanding Thermal Evolution of Carbon-Rich Exoplanets and Diamond Layers
Most rocky planets, like Earth, are made largely of silicates and metals. But some exoplanets form in environments with far more carbon than oxygen. On these planets, the carbon can combine with other elements to form minerals we don't see on Earth—graphite at lower pressures and diamond at higher pressures. During the planet's formation, heavier materials sink to form a metallic core, while lighter silicates form the mantle. But if the mantle is rich in carbon, deep below the surface where pressures are extreme, the carbon can crystallize into diamond, creating a thick layer. This diamond layer would dramatically change how the planet conducts heat. Diamond is an excellent conductor of heat, far better than typical rock. So heat from the core would escape more easily through the diamond layer, possibly making the mantle convect differently and cooling the planet faster than a similar rocky planet.
A deeper explanation
The thermal evolution of a planet is governed by its heat budget: initial heat from accretion and differentiation, plus long-term radiogenic heating from isotopes like uranium, thorium, and potassium. In a carbon-rich exoplanet with a diamond mantle layer, the high thermal conductivity of diamond (about five times that of Earth's mantle rocks) provides a fast path for heat to escape. This efficient heat loss would cause the mantle to cool more quickly, potentially shutting off mantle convection sooner. Without vigorous convection, plate tectonics—if they ever existed—would cease, and volcanic activity would decline. However, the abundance of radioactive elements in the carbon-rich mantle can still generate significant heat, delaying cooling. The balance between rapid heat loss through diamond and internal radiogenic production determines the planet's thermal evolution. This evolution influences crust formation, magnetic field generation (via core cooling), and the composition of gases released into the atmosphere. Thus, the diamond layer is not just a curiosity; it is a key control on the fate of the planet.