How big would the asteroid belt planet be?
As I understand it, the asteroid belt exists because the gravitational force of Jupiter prevents the asteroids from accreting (is that a word?) into a planet.
If, however, Jupiter didn't exist and they did create a planet, how big would that planet be?
I see some of you are confusing mass and volume yes C is approximately 1/3 the mass in the asteroid belt but mass and volume are not equal example a gallon of water vs a gallon of iron both have the same volume but iron has much higher weight
Anyone interested might wanna have a look at the *"Linking Exoplanet and Disk Compositions"* Workshop of the Space Telescope Science Institute September 12-14, 2016. I don't have time and mind for all of this, but there seems to be rivaling theories about how the asteroid belt formed. It might not be pristine, but maybe has accumulated later on, after planet migration. When Jupiter moves, worlds are crushed and formed.
The largest main belt asteroid is 1 Ceres, which alone contains almost a third of the total mass of the whole main asteroid belt.
Ceres is large enough to be in hydrostatic equilibrium, i.e. its own gravity is strong enough to pull it into a roughly spherical shape. Since the mass of a spherical planet scales as the cube of the diameter (assuming constant density), piling all the other main belt asteroids together onto Ceres would only increase its diameter by a bit under 50%. It would still be a roughly similar type of body — a small sphere of partially differentiated rock and ice, with no atmosphere to speak of (as it'd be way too small to hold onto one).
Thus, to answer your question, a hypothetical planet containing all the matter currently making up the main asteroid belt would look pretty much like Ceres already does, just a bit bigger.