Earth is a giant capacitor. This is true regardless of its internal makeup, but it's…
This has led some to conclude that the matter making up the asteroid belt must have come from something else than an exploded planet. An alternative source often suggested, is Valles Marineris, the scar on the surface of Mars.
However, the total mass excavated from Valles Marineris is far too little to make the alternative hypothesis believable. There is also the dwarf planet Ceres, which definitely did not come from any excavation.
The matter excavated from Valles Marineris may in total have been about 4% of our Moon, but there is no way all of this found its way out to the asteroid belt. Much of it rained down on Earth in meteorite showers. The Sun and Jupiter must have gobbled up much of it too, and a lot was no doubt lost to space.
Seen in this perspective, 4% of our Moon, or 22% of Pluto sounds about right for an exploded planet the size of Mercury.
Rogue planet blowing up a smaller planet
Mercury has a volume about three times that of our Moon, which means that the asteroid belt has a total volume corresponding to about 1.5% of Mercury.
However, this assumes that planets are solid to the core. If planets are hollow, which we have good reasons to believe, the above mentioned numbers may be off by upwards of 100%. The asteroid belt may be a full 4% of Mercury. This would be especially true if the exploded planet had a particularly thin crust, making it all the more likely to have exploded.
In the case of an explosion, only the bits ejected in the direction of the planet’s orbit would remain in orbit. Bits ejected in any other direction would be sent hurling out into space to be lost for ever, or gobbled up by other planets and the Sun. That would have left very little of the original planet in orbit.
The relatively small mass of the asteroid belt is therefore no reason to abandon the theory that it is the remnants of the planet Phaeton.