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What is the effect of radius in a planet's steady state temperature?

Physics Asked by VintageLime on December 28, 2020

Let’s say there are two planets, without an atmosphere, sharing orbit and equal in every way except their size: planet 1 is 10x larger than planet 2.

Heat into the planet $Q_{in}$ from radiation is proportional to the cross sectional area, $pi r^2$. Heat out $Q_{out}$ is proportional to the surface area, $4 pi r^2$. This implies to me that the ratio $Q_{in}/Q_{out}$ should remain constant for both planets, both scaling with $r^2$, resulting in the same steady state temperature. I find this rather counterintuitive – I’d have thought having a larger surface area to mass ratio would lead to a lower steady state temperature (making planet 2 the cooler one). Am I missing something? It just feels wrong.

One Answer

If you are talking about a steady state temperature, then both heat in and heat out depend on the emitting/absorbing area. In that case, for spherical objects and assuming heat can be readily transferred inside the object, then size will not be a factor.

The size will be a factor if things cannot reach a steady state by the time of observation or there are temperature gradients within the object. This is more likely to be the case with large objects with a large heat capacity per unit area. It may take them a significant time to "warm up" and if illuminated from one side they might be more likely to exhibit inhomogeneous temperatures.

Correct answer by ProfRob on December 28, 2020

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