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Okay, I have a desert planet that orbits an M0 star and an G0 star at an circumbinary orbital radius of .9 AU from the shared center of gravity of its twin suns. It’s tilted on its axis by 23.5 degrees, has days that are 24 hours long, and has a year that’s about 67.5% as long as Earth’s. Its mean surface temperature is 308 K, and the range of temperatures that naturally occur on its surface varies from 279 K to 337 K. Its Kármán line is 120 km from its surface, and the atmospheric pressure at surface level is 2.81 atm.

Humans can breathe the atmosphere at the surface unaided, due to its chemical composition. The planet’s radius is 97% of Earth’s, its mass is 91.2% of Earth’s, and its surface gravity is 9.51 N/kg. There is water on its surface, allowing it to support complex life. Is the planet I described realistic? Why or why not?

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    $\begingroup$ I... don't see why not. $\endgroup$ Apr 23, 2021 at 0:54
  • $\begingroup$ You say "There is water on its surface, allowing it to support complex life." Does the planet have a biosphere, or are you wanting a biologically dead planet that humans can survive on when they arrive? $\endgroup$ Apr 23, 2021 at 9:18
  • $\begingroup$ @KerrAvon2055 The planet has a native ecosystem, and is more populous around the poles, where lower temperatures allow for permanent bodies of water, which regulate the temperature and support life. $\endgroup$ Apr 23, 2021 at 15:52
  • $\begingroup$ @TysonDennis A note on wording. The star system is binary, not circumbinary. The orbit of the planet around the two stars is circumbinary. $\endgroup$ Apr 23, 2021 at 18:19
  • $\begingroup$ Note that unless there is very little water (no oceans, at most inland seas) you are very likely to end up with something close to Eocene Climate. Rainforest around the equator, deserts and scrubland (or savannas if grass analogues exist) in the middle latitudes and temperate forests near the poles. $\endgroup$ Apr 24, 2021 at 16:16

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As for a reality check I have to start with your choice of stars. Our sun is G2-V star, which obviously has a temperature that puts the goldilocks zone at 1au. Your much cooler G0 star is giving heat to a planet at 0.9au, and I don't believe it will be inside the goldilocks zone for your stars. The M0 is almost negligible in its irradiance contribution.

I want to move on to the very low density of your planet. It is unlikely this has an iron core due to its low mass to volume ratio. It is nearly the size of earth, yet has 9% less mass. What is the core made of? This means it is unlikely to have a magnetic field, and that means it doesn't have a magnetosphere. Life will problematic, even with a cooler stellar irradiance. Your planet will be suffering cosmic radiation bombardment.

Now for the problem of tidal forces and the Love numbers on your light-weight planet. Having a binary star makes the tides very erratic as the tidal forces won't have a normal harmonic. There will be much shifting of mass in this planet; expect a great deal of volcanism and tectonic activity even without a moon. The constant and erratic squeezing and stretching of your spheroid planet will be making a tremendous amount of heat and pressure inside the mantle from friction, and it wants to get out! We know how that happens.

Your surface temperatures, breathable atmosphere, and liquid water I feel put this planet outside the realm of a reality-check. Your story needs some high tech or magic to work, I am afraid.

The question you framed doesn't give us enough parameters for more than a 0th order answer I am afraid, but maybe some participants will make some guesses for you.

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  • $\begingroup$ The planet has exactly the same density as Earth. (Because $0.97^3 = 0.912$, exactly as given in the question.) $\endgroup$
    – AlexP
    Jun 18 at 13:39

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