Artemis - the fifth planet
Sean Raymond presented an interesting talk on Artemis, a fifth terrestrial planet (about Mars mass) that he conjectures existed in the Solar System for about 700 million years after formation.
The basic scenario is that among the "planetary embryos" that formed in the inner system, an additional Mars massed object formed and persisted at about 2 AU (outside the orbit of Mars). It loitered there in a near circular orbit for about 700 million years before being promptly ejected by Jupiter.
The Jupiter ejection of Artemis is conjectured to have come about from another recent dynamical model.
In this scenario the outer planets were in a more compact configuration, with Jupiter and Saturn in, or inside, a 2:1 orbital resonance, and Uranus and Neptune close in and reversed in position.
Interaction with an outer debris disk of planetesimals moved the outer planets out, and Jupiter inwards.
The outer planetesimals were ejected, Neptune flipped position with Uranus, and both moved outward along with Saturn.
When Jupiter and Saturn swept through the 2:1 resonance, it triggered strong perturbations of the inner terrestrial planets, and poor Artemis was ejected after interacting weakly with the inner planets.
This happened about 3.8 billion years ago, about 700 million years after the formation of the Solar System, and was also the event that triggered the Late Heavy Bombardment.
So Artemis is now drifting somewhere in interstellar space, all by itself. Be fun if it had a large moon, eh?
It would have gone into an epicyclic orbit about the Solar orbit, and could still be relatively close by, probably within few tens of parsecs of the Sun, unless a bad luck encounter with another star over the last 18 or so Galactic orbit sent it off in a very different orbit.
This has been postulated before, but the interesting thing is that the scenario is a little bit beyond a "just so" story, there are some kinematic anomalies that fit this scenario and further modeling, and observations, may refute the model, or confirm it indirectly.
We'll probably never know for sure.
This is NOT the same as this scenario...