I have two tidally locked planets of roughly equal size (about the size of Mars) orbiting each other around a barycenter equidistant between them. They complete a day-night cycle every 72 Earth hours. Together, they revolve around a single star similar to our sun. One revolution is approximately one Earth year. Both planets have life on them. Is this arrangement possible?

Both have large liquid iron cores and magnetic fields, but the poles are oriented toward the east-west rather than north-south. That is, the "north" pole of one planet's magnetosphere always faces the south pole of the other, and at midday on either planet, one of the poles faces the sun. Is this possible, and would it expose an area of the planet to intense solar radiation?

If one is standing on planet A at midnight, looking up at planet B, will the eclipsed planet B appear red like a lunar eclipse on Earth?

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    $\begingroup$ Welcome ! there are a few things that would help clarify any answers to your question(s). - What are the diameters of the locked planets, how far apart are they and at what distance to the parent star? $\endgroup$
    – Joe
    Jul 26, 2018 at 21:18
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    $\begingroup$ Welcome to Worldbuilding,Wolf E. B.! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Gryphon
    Jul 26, 2018 at 21:19
  • $\begingroup$ I'd like to ask a few more questions also: are the planets' axes tilted? Earth has a 22 degree axial tilt. This will help resolve your last question. $\endgroup$ Jul 26, 2018 at 21:27
  • $\begingroup$ Thanks for the responses. Each planet is about 4200 miles in diameter. Their distance from the sun is similar to Earth's, but I haven't determined how far apart from each other they are; I don't know how to calulate that to get the rotation speed I want. $\endgroup$
    – Wolf E. B.
    Jul 27, 2018 at 4:45
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    $\begingroup$ The magnetic poles part i think is the least plausible of your scenario. While i dont know much about why Uranus has such a weird magnetic phenomenon, it probably has more to do with it being an Ice Giant. I dont think metallic planets like earth would be possible to have strong poles in such a way. Most of the field is produced due the the rotation of the inner core. It's not likely that the core would rotate perpendicular to the rotation of the surface. Just about everything in a star system rotates around the same axis that it orbits on, unless its a captured body. $\endgroup$
    – Stephan
    Aug 2, 2018 at 20:09

1 Answer 1


I'm inclined to believe that life would not exist on either planet. If I understand the arrangement the liquid cores are going to try to rotate in the direction of the planet's spin but also in the direction of the tidally locked pair of planets which is at 90 degrees. This will cause the liquid core to not spin and not generate a magnetic field thus the atmosphere gets ripped away by the solar wind and exposes the populations to extreme radiation.

Imagine two basketballs filled with water and tied to each other with a piece of string. You have the basketballs rotating along the axis of the string and rotating in the plane of the string. What will the water do? Because there are no hollows inside the balls the water can't "bunch up" on one end. They can be tidally locked but not magnetically locked since there is no magnetic field.

The other idea to consider is that they are each in geosynchronous orbits of each other. Therefore they can't be more than half the distance to the sun apart otherwise they would hit/graze the sun. This will dictate the speed they are rotating and the distance apart, along with their masses.

You might consider Trojan Points to resolve these issues.

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    – JBH
    Aug 4, 2018 at 2:45

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