Sounds like a stupidly easy question to find out with a quick internet search, but it’s not.
I don’t want to know the average surface temperature, or the average ocean surface water temperature, or read another article about climate change.
But that’s all I found in the past hour.
I’d like to know the average temperature of all molecules that comprise earth, or a best guess scientific estimate.
Why is it hard? At least to get an approximation since you can’t measure everywhere.
We know temperatures of the mantle and both cores. We know their size. We can ignore the crust as a rounding error. This approximation will improve as our measurements get better.