The total mass of the asteroid belt is approximately 4% of our Moon. Enough to make some believe that the asteroid belt is the remnants of a planet named Phaeton. Supposedly destroyed in a cataclysmic event that took place recent enough to have been recorded in myth by our ancestors.

Phaeton and its moon is thought to have orbited the Sun in the area where we now find the asteroid belt and the dwarf planet Ceres. Phaeton was then destroyed, leaving us with the current constellation. The asteroids are remnants of Phaeton, and Ceres is what used to be its moon.

Valles Marineris and the asteroid belt

Others have suggested that the asteroid belt comes from Mars. The thinking is that the scar on the surface of Mars, known as Valles Marineris, was created through a cataclysmic event that sent rocks from Mars flying into space.

However, the amount of matter excavated from Valles Marineris doesn’t add up to more than the total amount of material found in the asteroid belt. All of it, without loss, must therefore have found its way out to the asteroid belt. But that’s not how things work.

The Sun and Jupiter would’ve gobbled up much of the excavated material, and a lot of it would’ve been lost to space. Or ended up as meteorite showers on other planets, including Earth. There’s also the issue of Ceres, which definitely didn’t come from Mars.

We can therefore dismiss this theory as highly unlikely.

Calculating the size of Phaeton

If the asteroid belt is the result of a cataclysmic event, the original object must’ve been located in its same orbit. It must’ve also been a great deal larger than the total material found there. Because a major part of the resulting debris would’ve been lost.

How much larger it must’ve been depends on the intensity of the event. Which can be inferred from the debris.

Asteroids are a great deal smaller than planets, so if the original object was a planet, it must’ve been pulverized in the event. There must’ve been a massive explosion, and a lot of material must’ve been lost as a result. Maybe as much as 96%, which would make Phaeton the size of our Moon.

However, if planets are hollow, which we have reasons to believe, the diameter of Phaeton may have been more like that of Mercury. But that too is hardly a big planet.

For Phaeton to have been anything bigger than Mercury, more than 96% of its original material must’ve been lost. Even if we go to a 99% loss, we only grow the planet’s diameter by 50%. That’s not even the size of Mars, so we must conclude that Phaeton must’ve been a small planet.

Rogue planet blowing up a smaller planet