Despite the Solar System looking quite stable, clockwork-like on human timescales, to such a degree the movement of its members is used to track time and make calendars since antiquity, it is pretty chaotic on longer timescales, of several million years. For example, recently there was news about a study(1), where researchers determined that close-passing stars alter our entire planetary system's orbital evolution via their gravitational perturbations on the giant planets, making retrodiction of past orbital evolution very uncertain beyond ~50 Myrs ago. Another example, for the future, was a study from 2009(2) that raised the possibility of Mercury eventually running amok, causing destabilization of all the terrestrial planets about 3 Gyr from now, with possible collisions of Mercury, Mars or Venus with the Earth (although not likely. the probability of a large increase in the eccentricity of Mercury required to kickstart it was estimated about 1%, and the simulated orbit that lead to collision with Earth appears to be a single event out of a sample of 2,501 solutions).

Given these possibilities, I think that, beyond scenarios involving a direct collision with Earth, one can imagine a near miss situation, where our planet narrowly avoids a direct hit, but the Moon is lost, say either shattered, ejected from the solar system, or plunging into the sun. So I ask, assuming this actually happened at some point between Solar System formation and now, and somehow humanity still managed to arise on this planet, would we be able to determine Earth once had a long lost companion?


(1). Kaib, Nathan A., and Sean N. Raymond. Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution. arXiv:2402.08734, arXiv, 13 Feb. 2024. arXiv.org, https://doi.org/10.48550/arXiv.2402.08734.

(2). Laskar, J., and M. Gastineau. “Existence of Collisional Trajectories of Mercury, Mars and Venus with the Earth.” Nature, vol. 459, no. 7248, June 2009, pp. 817–19. www.nature.com, https://doi.org/10.1038/nature08096.

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    $\begingroup$ We would be puzzled by the changes in day length observed in ancient sediments. $\endgroup$
    – Jon Custer
    Feb 26 at 21:49
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    $\begingroup$ @JonCuster how can that be measured? $\endgroup$
    – vsz
    Feb 27 at 7:05
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    $\begingroup$ @vsz - see, for example, pnas.org/doi/full/10.1073/pnas.1717689115 - "Quasiperiodic variations in insolation, known as Milankovitch cycles, serve as a primary control on climate change over timescales of 10$^4$–10$^6$ y (1). Their expression in the stratigraphic record provides a powerful tool for reconstructing geologic timescales, or astrochronologies, and evaluating Earth history." $\endgroup$
    – Jon Custer
    Feb 27 at 15:26

1 Answer 1


The collision between the Proto-Earth and "Theia", the hypothetical planet responsible for the Moon's formation, is thought to have given us the essential elements for life to form, since it brought the appropriate materials which Earth might have otherwise been deficient in from a different region of the protoplanetary disk. Without this collision, life on Earth as we know it might not exist.

The Moon also significantly influences Earth's tides, which in-turn stabilized Earth's climate by redistributing heat, allowing the planet to be globally habitable.

Now, to answer your main question: if humans formed without a moon, whether we could tell if we initially had one or not depends on a few factors, such as how long it lasted. As aforementioned, the Moon plays a huge role in our ocean's current. With that in mind, if it was around for long enough, it should have had enough influence to create certain anomalies in fossils formed by tides that couldn't otherwise be explained, just to list one example. So yes, there would very likely be some clues.

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    $\begingroup$ I wonder whether any event that could cause the Earth to lose the Moon wouldn't also kill of all life on Earth. Meaning that any living things on a moonless Earth would have developed after the Earth lost the Moon so there would be no fossil remains of life when the Moon was still there. $\endgroup$
    – quarague
    Feb 27 at 13:32
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    $\begingroup$ @quarague life doesn't have to still be alive to fossilise. Quite the opposite, usually. $\endgroup$
    – OrangeDog
    Feb 27 at 13:54
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    $\begingroup$ @quarague If something really managed to kill all life, it might be more surprising if life re-emerged spontaneously. But we still don't know how life originally started, so it's hard to know the probability. $\endgroup$
    – Barmar
    Feb 27 at 17:04
  • $\begingroup$ @quarague, I guess it depends on the mechanism by which we lost the Moon, but it certainly would have had major implications for us. It's possible that life could have survived given the proper conditions, though as mentioned, the Moon is thought to have had a huge role in forming life, so one minor change, let alone such a major one, could have easily ended it all. $\endgroup$
    – 4NT4R3S
    Feb 27 at 17:13
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    $\begingroup$ This reminds me of the novel "Origin" by Stephen Baxter. $\endgroup$
    – Jim421616
    Feb 27 at 17:24

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