I have recently been reading about the formation and early evolution of the Solar System, and how the outer Solar System very likely did not form in its current configuration and has almost certainly undergone some reshuffling and migration, and possibly ejections of ice giants that are no longer with us. However, the inner System seems to be a little more glossed over, even though it has its own set of problems, for instance, the origins and properties of Theia, the hypothesised impactor that led to the formation of the Moon, and Mars' low mass. Simulations seem to suggest a 0.5-1 Earth mass planet should have formed in Mars' position, and several explanations have been proposed to resolve this discrepancy, such as the Grand Tack, a hypothesised migration route Jupiter took in its infancy, before Saturn stabilised it and migrated out again. However, there are also problems with this explanation. This has led me to the question.

What if the inner planets, like the outer, also did not form in situ and have migrated and swapped positions? I have seen some scenarios in which Neptune and/or a third ice giant formed between Jupiter and Saturn, the sandwiching between two gas giants leading to a deficiency in material. What if in the same vein, Mars actually formed between Venus and Earth, ending up a low mass object due to the deficiency in accretion material, and was later pushed into its modern orbit by Earth. In fact, we could perhaps take this further: the faint young Sun paradox (I'm aware other solutions exist). Maybe the forming configuration was Mercury - Earth - Mars - Venus, with a wave of instability stemming from the two larger terrestrials perturbing Mars leading to Mars being kicked into it's current position, with the resulting instability of the larger terrestrials leading to them swapping positions. What implications would such a scenario have on the origins of life? If we assume said scenario removes the need for the Grand Tack, that begs the question of whether this 0.5-1 Earth mass planet did indeed form, and was later ejected during this wave of instability, perhaps by Earth or a combination of Earth and Jupiter, if we assume it was closer to 0.5 Earth masses than 1. I could go on to suggest the modern Asteroid Belt was formed by this planet and another hypothetical terrestrial that formed in the vicinity, but the catastrophic collision scenario has been largely debunked, although the removal of the GT could bring this back into question.

I haven't included Theia and the impact as that likely happened in the Earth's infancy, long before this wave of instability took place, and it has been suggested in recent times that it was in fact an outer Solar System object that had migrated into the inner Solar System. Whether the latter point is true or not, doesn't really matter to this scenario, unless it happened in the Solar System's infancy.

So, how credible is this?

  • $\begingroup$ For the inner planets you will need to explain the density gradient, e.g. link.springer.com/article/10.1007/BF00642464 (Weidenschilling, 1977) - but Mars and asteroid belts need explanation anyway with their lighter density $\endgroup$ Oct 17, 2022 at 17:36
  • $\begingroup$ Metal deficiency. Metals (in the chemical sense) are heavier than silicates, so it preferentially accreted into Venus and Earth, and perhaps the potential fifth terrestrial before Mars could begin its formation. Terrestrial Five may also explain lower density in the Asteroid Belt if it preferentially accreted from the metals of the region. If the Asteroid Belt is the remnant of formation material kept stable by Jupiter, and not the product of a catastrophic collision, it would make sense that the heavier contents of the formation material have been depleted by the planets. $\endgroup$
    – Cryoraptor
    Oct 17, 2022 at 18:20
  • $\begingroup$ It is theorized that Mercury may be the depleted/stripped and exposed core of a former Super-Earth which was disrupted, potentially due to a giant impact or tidal forces, due to its very thin crust, large core, and high iron/nickel content. $\endgroup$
    – WarpPrime
    Oct 17, 2022 at 20:07
  • $\begingroup$ Venus supplies some interesting characteristics that your question might want to integrate: Venus has (1) a retrograde rotation (so does Uranus); and (2) an axial tilt of 177.36 degrees to orbit. These suggest that something may have tipped Venus over. $\endgroup$
    – WPWPWP
    Nov 19, 2022 at 4:41
  • $\begingroup$ This is very true; I forgot about Venus' bizarre axial tilt. For an impactor to have flipped Venus over, it must have a. Been a considerable percentage of the mass of Venus, possibly a super-Mars, and b. Impacted at an extreme angle. Two separate impacts have been suggested to try and explain this. This, along with Theia, perhaps suggests that both Mars and Mercury could have their origins within a belt of Mars-mass planetesimals between Venus and Earth, and the former two are statistically unlikely survivors of a larger population. Mercury could have started as a super-Mars, $\endgroup$
    – Cryoraptor
    Nov 20, 2022 at 9:21


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