# 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 Jan 2 '16 at 1:12
• 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. Sep 18 '16 at 11:20

The mass of the asteroid main belt is estimated at 4% the mass of our moon according to Wikipedia so any object formed from the aggregation of that mass would not be a planet.

It would be the size of a very small moon.

Even if all the asteroids in the solar system were combined, the total mass would be below a third of the moon's mass.

• Yes, but that low number might have been caused precisely by the influence of Jupiter. Without Jupiter, it might've been very different. Oct 8 '14 at 15:35
• As long as it orbits the Sun, isn't it a planet regardless of mass? Oct 8 '14 at 19:51
• @Scottie Pluto is crying bitter tears Oct 8 '14 at 20:32
• I thought Pluto was deemed not a planet because it's orbit is not on the same plane as the rest of the planets and because it's path crosses in front of other planets, not because of it's size. Oct 8 '14 at 20:51
• @Scottie the IAU definition has it orbiting the sun, sufficient mass to establish hydrostatic equilibrium... and has 'cleared the neighborhood around its orbit'. That last point is where Pluto lacks the requirements to be a planet. Assuming that the astroid belt was all one object, and it cleared its orbit it would be a planet rather than a dwarf planet. The neither then inclination nor the eccentricity of the orbit is a factor there.
– user595
Oct 9 '14 at 1:17

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.

Ceres constitutes about 1/3 of the mass of the asteroid belt. Assuming the same density, the whole asteroid belt combined into one planet would have 3 times greater volume than Ceres and $\sqrt[3]{3}=1.44...$ times greater radius.

Wikipedia states that 99.9% of the mass originally in the asteroid belt was lost in the first 100 million years of the solar system - I am assuming after the protosun initiated fusion. If so, that would put the potential mass at 4000X the moon, or almost twice Neptune's mass. That would be one big planet!

The current mass of the asteroid belt is just a few percent that of the Moon.

However, if Jupiter did not exist, it might be that there would have been a full-sized terrestrial planet where the asteroid belt is now.

I say "might" because I am basing my statement on the Grand Tack Hypothesis. This has it that Jupiter formed at around the snowline (around 3 au) and then migrated inwards to an orbit at around 1.5 au. This had the effect of scattering out most of the planetesimals that existed between about 1 and 3 au and may even have destroyed an initial generation of protoplanets.

The Grand Tack Hypothesis was developed partly to explain the low-mass of Mars and the very low-mass of the asteroid belt.

Without Jupiter it is likely there would have been considerably more mass between 1 and 5 au in the Solar System available to form planets.

Just for clarification Ceres is not considered an asteroid, but a dwarf planet like Pluto. I think people confuse Ceres and call it an asteroid due to its orbit running in or bordering the asteroid belt.