In 1772, Johann Bode noticed a pattern in the distances of the planets from the Sun. He noted that planets followed a pattern of d = 4 + n, where n was 3, 6, 12, 24, etc. If you let the Earth’s distance be 10, then Bode’s law matched the distances of the (then) known planets to within a couple percent, with one exception. Bode’s law predicted a planet between Mars and Jupiter that didn’t exist.
In 1781 Uranus was discovered, and agreed with Bode’s law to with a couple percent, so there began a search for the missing “fifth planet” between Mars and Jupiter. In 1801 the minor planet Ceres was discovered, and was at exactly the distance Bode predicted.
When Ceres was discovered there was a lot of interest in trying to find an underlying mechanism for Bode’s law. But over time it was found that the relation is just an interesting coincidence. Neptune doesn’t agree with the law, and exoplanetary systems don’t follow the relation either. Sometimes an interesting pattern is no more than that.
Still, with the discovery of more and more objects between Mars and Jupiter (what we now call asteroids), there came an idea that perhaps the asteroids are the remnant of a destroyed world. The idea was first proposed by Heinrich Olbers (of Olber’s paradox fame) and eventually became known as Phaeton, or Maldek or Tiamat. The initial argument for the existence of such a planet came from the prediction of a planet between Mars and Jupiter by Bode’s law, but in modern times the idea lingers among non-scientists as the planet of an alien civilization, a planet destroyed by Nibiru, and lots of other wild ideas. None of these ideas have any basis in reality.
Despite all this, how do we know that the asteroid isn’t the result of some long destroyed planet? One reason is the mass of the asteroid belt. If we took all the asteroids and combined them into a single mass, it would only be about 4% of the mass of our Moon. That would make it smaller than Pluto’s moon Charon.
But perhaps the explosion of the planet sent matter out of our solar system or into the Sun. The problem with that idea is that any collision that could destroy a planet sized object wouldn’t cast most of the material out of the solar system or into the Sun. There would still be remnants that we don’t see.
But there is another point of evidence that is very conclusive, and that is the chemical variation we observe within asteroids. If the asteroids all came from a single progenitor planet, their chemical makeup would have certain similarities. The chemical composition of Earth is distinct from Mars, or Venus or any other planet. They formed in different regions of the solar system, and we can see that in their rocks. Similarly, asteroids can be classified into different families that have a similar origin, such as the Flora or Massalia families.
Chemically, the asteroids do not have a single progenitor. There was no original planet from which the asteroids came. Instead they failed to form into a single body, driven in part be the gravitational interactions with Jupiter, which tends to keep them spread apart. The chemical evidence on this is conclusive.
But the facts don’t prevent some people from continuing to push the idea of a lost world.