There is some mysteries and signs in our solar system that leads me, in my humble opinion, to suspect that there was once another planet or planets in our solar system that have since come undone in a calamitous event in ancient geographical time that affected the other planets that are still here today.
Firstly there is the odd tilt of our own dear earth. This, it is suspected was created by a meteor hit in ancient geo-time (not the one that killed the dinosaurs, apparently, which was a relatively more recent event). Some "extreme" science buffs theorise that asteroid might have been an ancient moon. What that older meteor did was put the earth at an angle and change the path of the planet so we now have seasons. I suspect a piece of ancient exploding planet was the "asteroid".
Another suspicious piece of possible side effect is the rings of Saturn (and possibly the hidden rings of Neptune). If there was a planet that got obliterated by a large comet or some other large object or force, with great impact and effect, could the dust and rocks of Saturn be some of the shattered remains of it? Or maybe some of the moons of both Saturn and Jupiter? It may sound far fetched, but it only gets more out there then that. The famous red spot of Jupiter - a perpetual storm of differing hue - be started by a piece of gas and dust that was not native to that planet (Jupiter) being thrown down with force from the impacted planet. If a lot of heat fusion and evaporation occurred, could it have burnt Mars so the obvious signs remaining that there was once water there where there is none now could have occurred?
What about the odd position of Mercury, with one side always facing the sun and the other always facing away? Could it be an old moon from an old planet? Or just turned about face by a big rock? That is obviously not its natural original position. As the planets cooled and formed from the new sun they would have formed going with the flow, like eddies and pools in a stream. Not strewn about like lost billiard balls. Unless something(s) happened with planet rocking effect.
What about the ice moons and planets like Io or Pluto? Lets say there was a fair sized planet that was disintegrated with some force in ancient geo-time. If a lot of heat was created and its now gravitationally free planet (which may have held water) underwent the side effect of a lot of heat, the water gases would have evaporated and then cooled and drifted in space. They may have been thrown far by the impact. Hence (hypothetically) high impact water would have been thrown further then cooler relatively lower impact water. Creating, possibly, Io nearby, and Pluto far away. Like giant cosmic drops in the ocean.
Research into the possibilities could include rewinding the dance of the planets to see if there was a time when the rings of Saturn and Neptune could have lined up to form from the same event (the dust taking longer to reach Neptune of course). Natural gaps in the planets paths and distances could offer further clues.
Other clues are mythologies of ancient peoples, where "the stars were shaken from there places" (could be a humble earthquake unless the stars moved Permanently), or evidence on this planet and others that they may have had different geographies or directions in other ages. Also moons with substantially different chemical make-ups to the parent planet. And if the compounds of the planets in the solar system do not make sense in basic physics and and chemistry with lighter gases and elements cooling and forming planets further away from the sun and heavier denser ones settling as planets closer to the sun. Which I already know they don't. Anyway, its all just ideas. Will try to do more research and expand or tailor the concept.