Maybe the inhabitants of that planet left and blew up on the way out since they thought they were going to be victims of some kind of section eight colonization.
The hypothesis correctly anticipated the orbits of Ceres (in the asteroid belt) and Uranus, but failed as a predictor of Neptune's orbit and has since been discredited further.
Then again, I was tongue in cheek regarding Richard "Face on Mars" Hoagland and his, shall we say 'creative' theories about Mars and his hyperdimensional physics.
And we won't even get started about Hoagland's theory that Earth's Moon has a shattered glass dome on it that the Apollo Astronauts photographed.
But Richard was always entertaining to listen to on Art Bell's late night radio program.
And one where the asteroid belt is located.
Don't read further if you are fond of sleeping:
What really sinks to the middle is the heavy metals, most notably uranium.
Yes, the core of our planet is a nuclear reactor.
And more uranium is sinking towards the core over time.
When enough arrives the core goes hypercritical and explodes.
Just like what happened to the planet between Mars and Jupiter.
Sweet dreams...