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I have a binary planet as the setting for a sci-fi story, and I would like to know if this system makes sense or if there are any major flaws. I am also open to suggestions for improvements.

Here is a description of the planets:

Both globes are nearly identical in composition and size. They are dry rocky planets with iron cores and sparse amounts of water. Each planet is approximately 85% the diameter of Earth, 62% as massive, with 85% the gravity. The atmosphere is about 50% thinner than the Earths.

The 2 bodies are tidally locked, at a distance of 4 Earth diameters (center to center) and rotate about a barycenter at a rate of 1 rotation per 8 Earth days. So they will experience huge temperature ranges each day. The slow rotation would mean less powerful storms.

The planets orbit their F0 star once every 3 years, at a distance of 2.5 AU. Axial tilt and eccentricity are near zero, so they do not experience seasons the way that Earth does.

Do these parameters make a habitable planet for alien life forms?

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    $\begingroup$ One is supposed to edit closed questions, not to delete and repost them. $\endgroup$
    – L.Dutch
    Commented Apr 6, 2023 at 3:21

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Casting aside the viablility of the solar system model you're proposing:

Do these parameters make a habitable planet for alien life forms?

F0 stars have a luminosity around 7.25 (as a multiple of Sol's luminosity) with a color index quite a bit bluer than Sol, and are about 25% hotter at the surface. While the 'habitable' zone of an F0 is thought to be between 2 and 3.7 AU, the star's radiation is significantly more hostile to primitive life than Sol.

If you're asking whether it's possible for an advanced civilization to colonize the planets, then sure. But if you're asking if native life could develop on these worlds then the answer is probably no, at least for anything we might recognize. Certainly not on the surface which is bombarded with UV light. And since UV penetrates water down to ~50m, shallow surface lakes aren't going to provide the sort of protection needed for life (as we know it) to get started.

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    $\begingroup$ Also since this star's radiation is more ionizing, the planets would likely lose all their water before life could even develop somewhere. $\endgroup$
    – Vesper
    Commented Apr 6, 2023 at 7:30
  • $\begingroup$ At what distance from the star would the planets need to be for water to remain on the surface so that life could develop naturally? $\endgroup$
    – grat
    Commented Apr 6, 2023 at 13:18
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IT seems that the binary planet system you described would have a much shorter day than you describe.

Part One:

The sizes and distances of he two planets.

Earth has a mean radius of 6371.0 kilometers and thus a mean diameter of 12,742 kilometers. Four times that is 50,968 kilometers center to center.

Each planet is approximately 85% the diameter of Earth, 62% as massive, with 85% the gravity. The atmosphere is about 50% thinner than the Earths.

So each planet would have the 0.85 diameter and radius of Earth. Each planet would have a radius of 5,415.35 kilometers, And so the distance between the surfaces of r the planets would be 50,968 minus 2X 5,415.35 kilometers, or 40,137.3 kilometers.

Part Two: Surface Gravity and Escape Velocity.

According to this online surface gravity calculator, https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc each planet would have a surface gravity 0.85 that of Earth.

I note that it would be rare for a planet to have an escape velocity and a radius with the same proportion relative to that of Earth.

But the escape velocity, and not the surface gravity, of a planet is the important factor for the planet retaining an atmosphere. No description of the properties of a fictional planet is complete without listing the escape velocity.

Here is a link to an online escape velocity calculator: https://www.calctool.org/astrophysics/escape-velocity

According to it each of your planets should have an escape velocity of 9.553 kilometers per second. That is 0.854 of Earth's escape velocity of 11.186 kilometers per second. There is a noticeable difference in the ratio of the planet's escape velocity relative to Earth and the ratio of its surface gravity relative to Earth. In many cases the difference between the ratios would be much greater.

So the surface gravity of a world and the escape velocity of that world have to be calculated separately, and they should never be described together as it's "gravity".

Part three: The Orbital period.

So what about the orbital period of the two planets? Would it be eight days? To me eight days intuitively seems too long.

According to this online orbital period calculator: https://www.omnicalculator.com/physics/orbital-period

A binary system with two planets each with 0.62 the mass of Earth and a semi-major axis of 50,968 kilometers would have an orbital period of 1 day and 4 hours.

A binary system with two planets each with 0.62 the mass of Earth and a semi-major axis of 100,000 kilometers would have an orbital period of 3 days and 6 hours.

A binary system with two planets each with 0.62 the mass of Earth and a semi-major axis of 200,000 kilometers would have an orbital period of 9 days and 6 hours.

A binary system with two planets each with 0.62 the mass of Earth and a semi-major axis of 175,000 kilometers would have an orbital period of 7 days and 13 hours.

A binary system with two planets each with 0.62 the mass of Earth and a semi-major axis of 185,000 kilometers would have an orbital period of 8 days and 5 hours.

So you should be able to calculate a system with either the separation of 50,968 kilometers or an orbital period of f8 days, but not both.

Part Four: The Distance from the Star.

The planets orbit their F0 star once every 3 years, at a distance of 2.5 AU. Axial tilt and eccentricity are near zero, so they do not experience seasons the way that Earth does.

An F0V class star should have luminosity 7.24 that of the Sun. So the Earth Equivalent Distance, or EED, where a planet would achieve the same amount of radiation from the star as Earth gets from the Sun would be the square root of 7.24 times 1 Astronomical Unit (AU). The square root of 7.24 is 2.69. So the EED of a F0V star should be about 2.69 AU.

At a distance of 2.6 AU from the star, the planets should receive about 1.037 as much radiation as Earth gets from the Sun. So the problems with ultraviolet radiation mentioned in Corey's answer would be stronger than if the planets were at the EED or father from the star than the EED.

These are my observations about the astronomical set up of the system.

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  • $\begingroup$ Thank you for the detailed response and the explanation about escape velocity. That is something I had not considered. That makes sense about the length of day, so I will need to move the planets about 14.5 EEDs apart for an 8 day orbital period. Are there any other factors that could slow the orbital period down? How do the tidal forces between the planets affect their rotation? $\endgroup$
    – grat
    Commented Apr 6, 2023 at 14:23
  • $\begingroup$ @grat No! No! An EED would depend on the luminosity of the star in question. IN the Case of FO V class stars, which are much more luminous than the Sun, their EED is more than one AU from the star. An AU is 149 million kilometers, and my answer says that the EED of a n F0V star is 2.69 AU. or 400 million kilometers, which is many, may times the maximum distance at which the two planets could remain gravitationally bound to each other. My answer showed the two planets would have to be somewhere between 175,000 and 185,000 kilometers apart to have an orbital period of 8 days. Continued. $\endgroup$ Commented Apr 6, 2023 at 17:53
  • $\begingroup$ @grat You can use the linked orbital period calculator to find the distance at which the orbital period will be exactly 9 Earth days long. The tidal interactions will slow down the rotational periods of both worlds and also push them farther apart. their original separation would have to have been greater than the Roche limit of each world. The planets should be tidally locked each other by the time of the story. A really conscientious writer would have someone calculate whether it would be possible for the two planets to be pushed to the desired separation during their lifetimes. $\endgroup$ Commented Apr 6, 2023 at 17:55
  • $\begingroup$ @grat you should lookup the sliding scale of science fiction hardness. tvtropes.org/pmwiki/pmwiki.php/SlidingScale/… And decide what scores you want your stories to have. If you want a higher hardness score you will have to work out a lot of details such as the ones mentioned here. $\endgroup$ Commented Apr 6, 2023 at 18:04
  • $\begingroup$ PS. I don't know exactly how long a F0V star can remain on the main sequence. Probably on the order of about 2 billion years. It took Earth about 4 billion years to develop an oxygen rich atmosphere breathable for humans or life forms with similar requirements. So if you want your planets to have breathable atmospheres they should probably have been terraformed by an advanced civilization sometime in the past. $\endgroup$ Commented Apr 6, 2023 at 18:08
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No examples have been found now.

This paper says it is possible and fairly likely, though physical models though.


I'm not sure how the temperature changes are supposed to be huge minus eclipses though. The distance change to the star is relatively not very large.


Also, are they tidally locked to their sun, or to each other, because it cannot be both?


If they have magnetospheres, and are tidally locked to the sun, most of them are inhospitable due to evaporated or frozen water, but a narrow band of glacial meltwater will be able to sustain all the life you want.

The physics of the system might be impossible though. Though it would just mean a slight change with no story relevance if it was that way.

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    $\begingroup$ Tidal locking normally happens with the closest mass due to tidal energy depending on distance to another gravitating mass at power of minus three. So those planets are tidally locked to each other. $\endgroup$
    – Vesper
    Commented Apr 6, 2023 at 7:32
  • $\begingroup$ Correct the planets are tidally locked to each other. They orbit around a single star, they are not tidally locked to the star. $\endgroup$
    – grat
    Commented Apr 6, 2023 at 13:07
  • $\begingroup$ In regards to the temperature and distance to the star: I assumed that because earth experiences different temperatures for summer and winter due to moving 23 degrees about its axis, then a planet moving 4 earth diameters would experience significantly more temperature difference. $\endgroup$
    – grat
    Commented Apr 6, 2023 at 13:15
  • $\begingroup$ @grat Wouldn't that only cause a tilt effect if the system was tilted? $\endgroup$
    – Aseku Vena
    Commented Apr 6, 2023 at 13:18
  • $\begingroup$ So the change in temperature is due to more surface area being exposed to sunlight and not the distance from the sun? $\endgroup$
    – grat
    Commented Apr 6, 2023 at 16:48

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