I have an earth-like planet with a roughly earth-like orbital period, that is in a stable co-orbital scenario with another largish body. Similar to Janus and Epimetheus but planets rather than moons.

At every approach/exchange the planet experiences geologic upheaval and climatic shift. Geologic due to tidal forces from the close approach, causing severe earthquakes. Climatic because of the exchange of orbital position.

This is for a young adult scifi. The scenario will only be described indirectly and will not have to stand up to hard scifi level scrutiny, but I would like to keep it realistic and plausible.

Is the geologic upheaval part plausible?

  • 2
    $\begingroup$ Please limit yourself to one question per post. $\endgroup$
    – sphennings
    Mar 10, 2022 at 2:57
  • $\begingroup$ This might be useful to you: github.com/gliese1337/Solia $\endgroup$ Mar 10, 2022 at 3:21
  • 1
    $\begingroup$ Reposted three times with different subject to keep @sphennings happy and hopefully get opened for more answers. $\endgroup$
    – Felix
    Mar 10, 2022 at 23:29
  • $\begingroup$ Just found this very relevant question. worldbuilding.stackexchange.com/questions/53803/… Interestingly it asks multiple questions but did not get closed for not being "focused" enough. Not sure how this criterion is being applied or indeed how it should be applied. $\endgroup$
    – Felix
    Mar 10, 2022 at 23:32
  • $\begingroup$ Looking at that question above and the comments it seems like it might be a bit too hard to answer these questions with any accuracy due to chaotic behaviour or the equations. Where there is room for doubt there is room for science fiction! $\endgroup$
    – Felix
    Mar 10, 2022 at 23:42

1 Answer 1


Earth has radius = 6371 km and earth orbits the Sun at an average distance of 149.60 million km. Perihelion, the smallest distance from the Sun is 147,093,602 km. Aphelion, the greatest distance from the Sun is 152,097,053 km. The difference between the two is 5,003,451 km, (3.3 percent).

Your two planets are earth like, and suppose the distance between the two orbits is approximately 5000 km. (Epimetheus and Janus have radii of about 60 and 90 kilometers. Their orbits around Saturn differ in size by only 50 km. They never get closer than about 15,000 km from each other i.e. ~167 time the radius of Janus. Janus and Epimetheus share the same orbit of 151,472 kilometers from Saturn's center).

  1. This does not cause any geologic upheaval.
  2. There may be slight climate shift.
  3. The two planets never get closer than about 1 million km from each other (~167 time the radius of one planet). Distance between earth and moon is 384,400 km. So one planet will see the other planet much smaller than one sees earth from moon.
  4. Janus and Epimetheus have Orbital period 0.6945 days and swap orbits after 4 years. Your planets (Orbital period 365 days) are expected to swap orbits after 2,000 years.
  • $\begingroup$ You have formulated a hypothesis. Can you please show your work that the hypothesis actually works? The obits are stable, the exchange of momentum works as predicted, and so on. In what conditions are the orbits stable? (And in the conditions described in the answer, at minimum distance one of the planets would appear larger as seen from the other than how the full moon appears as seen from Earth; I think that this is a better comparison, because most people have seen the full moon, but very few people have been to the Moon to see how large Earth appears in the sky.) $\endgroup$
    – AlexP
    Mar 10, 2022 at 16:01
  • $\begingroup$ @AlexP 1- As I told above, for earth, difference between Perihelion and Aphelion is 5,003,451 km, (3.3 percent) and it does not cause any geologic upheaval on earth. From this evidence, we can safely conclude the same for the planets in question. 2- Compare climates at same latitude in Northern and southern hemisphere (in the same season). Summer, winter etc are slightly milder in Northern hemisphere. 3- A lot of pictures are available for earth as seen from moon (Planets in question are earth-like). 4- 0.6945 days : 4 years ~ 365 days : 2,000 years $\endgroup$
    – imtaar
    Mar 13, 2022 at 8:29

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