What would the theoretical temperature be on Earth, if space were not a vacuum?

Question

.

Answer 1

About 6000°C, the temperature of the surface of the sun.

If space were not a vacuum, the earth would have a lot of drag in its orbit, causing it to eventually slow down and fall into the sun.

Answer 2

Assuming radiative equilibrium, this could be calculated by assuming the luminosity of the sun at the surface of an object with the radius of 1 A.U. (distance from the sun to the earth.)  You could use the Stefan-Boltzmann law (luminosity proportional to temperature to the 4th power) to calculate this. Works out to around 280K (7C) , but you'll have to show your work to get credit on the assignment.