The plot of Thundarr the Barbarian got me thinking about the science behind

the passage of a runaway planet between the Earth and the Moon

and its effect on the Earth and Moon themselves, as well as their orbits.

For the following scenario:

  • planet has half the mass of Earth, and is moving at 16km/s
  • moving perpendicular to the plane of the ecliptic
  • passes directly between the Earth and the Moon with closest distance to Earth of 192,500 km

What sort of effects, if any, might this have on Earth and those of us residing there during the passage in later months and years?

  • 2
    $\begingroup$ Welcome to astronomy.SE. Wonder if you posted in the right place? Be certain to read, or try posting at worldbuilding.SE. That your physics question would be off topic at physics.SE should leave one to consider carefully the question and expected answers. Also, be certain not to post in two places without putting a link to the other question. $\endgroup$
    – Rob
    Jun 30, 2018 at 23:00
  • 1
    $\begingroup$ I think it can be argued that this question can be on-topic here. An astronomical body passing by a group of other mutually orbiting bodies certainly can disrupt their orbits and change the evolution of the system, and these kinds of effects can be important in some astrophysical situations. $\endgroup$
    – uhoh
    Jul 1, 2018 at 7:02
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    $\begingroup$ Earthquakes, tidal waves, super volcanoes, ecosystem disruption, climate change, satellite misalignment, famines, plagues, wars, change of orbit relative to sun, change in rotation, change in moon distance, change of tides, and many other effects. $\endgroup$ Jul 2, 2018 at 5:20
  • $\begingroup$ @com.prehensible Why would a Planet that passes close by the Earth cause wars ? $\endgroup$
    – Peter U
    Jul 2, 2018 at 21:24

1 Answer 1


It doesn't look good.

Tidal effects. The passing planet will certainly cause disruptive tides as it passes. The exact height of the tides would be hard to calculate as it depends on how the tides flow around the land masses, but the effects would be much greater than lunar tides. The tidal effect could also have consequences for the earthquakes.

Disruption to the orbit of the moon. The moon's orbit will be significantly altered. It could cause the moon to enter an elliptical orbit (with consequences for tides). There would not be enough disruption to cause the moon to leave Earth orbit immediately

Disruption to Earth's orbit. The orbit of Earth would also be significantly changed, it could enter a significantly elliptical orbit, this would change the amount insolation over the year and greatly change the Earth's climate

If the disruption were significant enough, it could bring the Earth's orbit uncomfortably close to that of Venus, this could result in further disruption to the moon's orbit

Fortunately space is big, and even if there are a lot of rogue planets out there, it would be a very rare event for one to pass so close to star


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