At what distance would binary planets not be tidally locked?

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Discussion Overview

The discussion revolves around the conditions under which binary planets, specifically Earth-sized and mass planets orbiting a sun-like star, would remain in rotation without becoming tidally locked. Participants explore the implications of distance, system age, and mass on tidal locking and rotation dynamics.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant proposes that the distance of 702,904.853 km could allow for rotation without tidal locking, questioning if this distance is sufficient.
  • Another participant asserts that the age of the system is a critical factor in tidal locking, suggesting that it is possible for the planets to rotate while orbiting a common barycenter.
  • A different participant indicates that while the planets may rotate, they could still be synchronized if the system is very old.
  • One participant notes that the distance mentioned is nearly twice the Earth-moon distance and discusses the implications of mass on tidal effects, suggesting that tides would be similar to those experienced by Earth from the moon.
  • There is a clarification request regarding the meaning of "synchronized," whether it refers to having the same rotation period or a relationship between orbital and rotational periods.

Areas of Agreement / Disagreement

Participants express varying views on the relationship between distance, mass, and tidal locking, with no clear consensus on the exact conditions that would allow for rotation without tidal locking. Some agree that rotation is possible, while others emphasize the importance of system age and mass.

Contextual Notes

The discussion includes assumptions about the planets' masses and distances, as well as the potential for different rotational dynamics based on these factors. The implications of tidal forces and their dependence on distance and mass are acknowledged but not fully resolved.

Who May Find This Useful

Writers and creators interested in speculative fiction involving planetary systems, as well as those exploring the dynamics of binary celestial bodies.

Hainted
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Working on a story idea that involves binary worlds. To keep the math simple I'm assuming the planets to be Earth sized, and mass, orbiting a star identical to the sun. I managed to find someone to help with the first equations and discovered that based on the time it takes them to orbit each other (48 days) they would be 702,904.853km apart, and based on their year (432 days) they are 167,323,260km or 1.1 AU from the sun. I also discovered that a full "moon" would be 8 times brighter than our moon.

My question is Would 702,904.853km be far enough apart for them to rotate, or would they be tidally locked? Is it even possible for them to rotate and still be orbiting a common barycenter, and if so what would the distance be?

FYI I don't need the equations( I'm 2 decades out of school) but I don't need it explained like a 5yo either. Just looking at plausibility of this.
 
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Hainted said:
Is it even possible for them to rotate and still be orbiting a common barycenter, and if so what would the distance be?

Yes, it is possible. What matters is not the distance but the age of the system; the tidal lock develops over time. So for story purposes, you can have it however you want :smile:.
 
Thanks, I'm not doing Hard SF, but I want to avoid making a complete fool of myself scientifically.
 
Yes I would say it would be probable that the worlds would rotate, but still may be synchronized if the system is very old.
Your distance is nearly twice the Earth moon distance of 384,400 km, but an Earth mass is roughly a little more than 8 times a moon mass, and since tides are raised to the cube of the distance, the tides on each world would be nearly the same (slightly more) as the moon raises on the earth.

You could still have the worlds rotate at different rates, or even make one world a little smaller and say it is synchronous and the larger one is not. About the only thing to watch out for is that smaller worlds are more likely to be synchronized by larger worlds.
 
Synchronized as in Same rotation period (I.E. 24 hour day) or synchronized as in one orbit(48 days) equals one day/night period?
 

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