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.
For the record, you should probably use a lower-case ‘k’ (as in kilo, the SI prefix) rather than an upper case K (the unit of temperature, Kelvin), both in general and especially when referring to temperatures!