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I am envisioning a planet that has two orbiting stars: a small yellow-white F class and a larger orange K class star. The stars were once closer and created a more tumultous tectonic history but since then the planet has stabilised and in may ways is less tectonically-active than Earth. It is a planet 1.5x the size of Earth and older than Earth. Darkness (night time) is short-lived, a true night only lasts four hours out of the day, with dusks (a more orange hue due to K star) and dawns (a more golden hue due to F star) averaging an hour and a half. A day lasts 20 hrs. The K star rises first and then 80min later the F class star. The atmosphere is slightly denser than Earth's is, creating a slightly more pronounced Greenhouse Effect.

Therefore I have two closely related questions.

  1. Could I have an absence of moons (large satellites) and still have decent tides (relatively Earth size) due to the gravity from the two stars?
  2. Can my planet still be climate-stable and tectonically-stable with the two suns so long as planet is in the Goldilocks zone and possesses a stable orbit?

This is in relation to my post (Could a planet have metalloids but very few metals?)

THANK YOU to all who may read or reply to this, the advice is greatly appreciated! :)

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    $\begingroup$ Bear in mind that, since gravity is a function of mass and distances, specific values for those would help in providing a true and accurate answer. $\endgroup$
    – Amocito
    Commented Apr 12 at 18:27

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Without a rigorous calculation, based on this equation, we can see that the gravity exerted on the Earth by the moon is about 2.2 times larger than the force exerted by the sun. I'm making the assumption here that your larger F-type star has a color index (B-V) of 0.30 - 0.33, giving us a mass of 1.50 - 1.61 solar masses, and the smaller K-type star has a color index of 1.34 - 1.40 for a mass of 0.59 - 0.64 solar masses. Without specific distances for all three bodies, it's difficult to say, but my gut instinct is that it might be a little short of Earth's tides, but close.

However, for a rigorous answer, I think we need actual orbital distances.

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  • $\begingroup$ Thanks @Amocito. Apologies, I will try to be more precise. As I lack a sturdy knowledge in astrophysics I will look around for some binary systems in existence and try to guesstimate some orbital distances that seem plausible. $\endgroup$
    – Zip Zap
    Commented Apr 12 at 18:50
  • $\begingroup$ I suppose that the K-class star could be 0.1–0.4 to 0.3–1.3 AU for habitability, so let's say .9 for the K-class star. 2.9AU for the F-class star. Sound plausible? Now I am not sure of the distances between the two stars though as that is confusing due to gravity and orbits acting on each other. But if "The K star rises first and then 80min later the F class star" each day, I am guessing there is some kind of math I could deploy to get that distance, although I am not committed to 80mins if another number makes things more plausible. $\endgroup$
    – Zip Zap
    Commented Apr 12 at 18:54

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