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This is a sequel to my previous question How big could a moon be before its orbit became too unstable or tidally locked to its planet

OK, let's imagine that the thing everyone thought was a moon was actually a twin planet sharing a barycentre with BlinketyBlink.

They are currently not tidally locked, the twin is slightly smaller, with a similar basic composition and a Venus-like atmosphere.

They are far apart enough to not cause deadly disturbances on BlinketyBlink's geophysical conditions (weather, plate tectonics, volcanism, tidal waves...)

Everything is just perfect and life goes on normally on BlinketyBlink.

My question is : could the twins have an Earth-like orbit around their Sun? That's to say about 365 days, slightly elliptic and that would maintain regular weather patterns (summer to winter seasonal periods) on BlinketyBlink?

My imagination gives me this Solar system with respective Goldilocks region

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    $\begingroup$ I suggest putting in a proper title and mentioning it is related to a previous question on the body. $\endgroup$ – Bellerophon Oct 1 '17 at 12:54
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    $\begingroup$ I tried to give this a better title. Feel free to go ahead and Edit further. $\endgroup$ – a CVn Oct 1 '17 at 13:03
  • $\begingroup$ @MichaelKjörling thanks, I was trying to do it, but formulating scientific based questions is not my forte. ;) your title is much shorter and to the point. $\endgroup$ – shieldedtulip Oct 1 '17 at 13:06
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    $\begingroup$ Can I propose the sister planet be called BlanketyBlank? ;-) $\endgroup$ – Joe Bloggs Oct 1 '17 at 14:23
  • $\begingroup$ LOL Funnily enough the Brother planet already has a name - Totengot - But I can't make up my mind about BlinketyBlink's name :P $\endgroup$ – shieldedtulip Oct 1 '17 at 14:26
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Short answer is"yes", living on Blinketey blink would be very much like living on earth with two key differences.

"Moon light" would be several times brighter. Our moon is actually about the same reflective index as coal, but the atmosphere of the second member of the binary will be much more reflective, and the apparent size will be quite a lot bigger.

Tides will also be stronger.

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    $\begingroup$ Yeah you're talking land tides of 10+ metres, sod living anywhere near the coast, or the mountains for that matter. $\endgroup$ – Ash Oct 1 '17 at 13:37
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    $\begingroup$ @ash I think the tides would be manageable, but you would have to understand them and be very careful. Areas of the coast with significant slope, hills or cliffs should provide safety and allow people to wander into the inter tidal zone between tides for a short period, provided they returned to safety in time to avoid death. But large flat plains near the coast would be deadly unless you had a very strong and resilient boat to hand. $\endgroup$ – Slarty Oct 1 '17 at 14:06
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    $\begingroup$ @shieldedtulip No Ocean Tides would total as much as 215m, there's a square function in the amplitude equation for tidal range given a particular lunar mass so by more than tripling the size of the "moon" in the amplitude equation you up the amplitude by a factor of a little over 13 so I'd say forest cover starts about 130 metres above mean sea level. I wouldn't want to live in the mountains because 10 metres of land tide is the equivalent of a major earthquake every 12 hours. $\endgroup$ – Ash Oct 1 '17 at 14:29
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    $\begingroup$ Just move it to two-and-a-bit times the distance from Earth to Moon. There’s an inverse cube law for tidal strength with distance, so that will counterbalance the increased mass. $\endgroup$ – Mike Scott Oct 1 '17 at 14:45
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    $\begingroup$ @shieldedtulip Yes, the only necessary differences would be that the second planet would look much bigger and brighter, months would be much longer, and eclipses would be more common but you couldn’t see the corona during an eclipse. $\endgroup$ – Mike Scott Oct 1 '17 at 15:55
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In the situation you describe the center of mass of the system will orbit around the central star following the usual ellipse.

Both planets will "dance" around their center of mass, with the result that they will get more close or more far from the star with this dance.

Since you state they are far enough to not disturb each other too much, this means the change in distance from the star can be significant.

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    $\begingroup$ No, they only need to be about 600,000 miles apart for the tides to be the same as they are now, and +/- 300,000 miles' distance from the sun will have no noticeable effect on the weather. $\endgroup$ – Mike Scott Oct 1 '17 at 14:47
  • $\begingroup$ @MikeScott, I think I have read somewhere that the difference in distance from the sun between aphelion and perielion for Earth is noticeable (though it has no big impact on climate) $\endgroup$ – L.Dutch Oct 1 '17 at 15:36
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    $\begingroup$ That does have an effect on the climate; it’s why seasons are a bit more extreme in the southern hemisphere, because the Earth is at perihelion in the southern summer. But the difference is about 3 million miles, five times what we’re talking about here. $\endgroup$ – Mike Scott Oct 1 '17 at 15:53
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    $\begingroup$ @Mike Scott Beat me to it. Do you know if there’s a formula for calculating tide heights based on mass and orbital distance? $\endgroup$ – Slarty Oct 1 '17 at 15:59
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    $\begingroup$ @Slarty It depends on a lot more factors. Depth of the water, volume of the water, composition (density) of the water, the shape of the sea bed, the shape of the shoreline, ice, temperature of the water, the opposing gravitational forces of the sun vs the other planet (sun tides), the proximity of other planets in the system. (Earth has a tidal system for every close planet, even Saturn and Jupiter, because of their mass. When they all line up, flood time!!!) If you keep all of the other variables undefined, you can probably make the tides any height you want. $\endgroup$ – Justin Thyme Oct 1 '17 at 16:48
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It's all a matter of the size of the sun and the distance of the orbit from the sun. How big do you want your planet to be?

Our seasons are dependent on the tilt of the earth's spin in relation to the orbit around the sun. Each planet could have a different tilt. This is completely variable. The axis of spin of one planet could even be on the plane of rotation around the sun.

It is also variable as to how the planets orbit each other. Perpendicular to the sun-planet line? On the same plane as the orbit around the sun? Or somewhere in between. If on the same plane, the eclipses would be spectacular, and on every orbit of one planet around the other. If it was completely perpendicular, there would never be an eclipse.

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  • $\begingroup$ On the last point, since the two planets orbit the sun, twice per year wouldn't the perpendicular orbit would line up with the sun, making eclipses possible? $\endgroup$ – Dronz Oct 1 '17 at 18:14
  • $\begingroup$ They only orbit the sun once per year. $\endgroup$ – pojo-guy Oct 1 '17 at 18:29
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    $\begingroup$ @pojo-guy Once per THEIR year. Just to clarify. $\endgroup$ – Justin Thyme Oct 2 '17 at 1:58
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    $\begingroup$ @Dronz Think of a barbell spinning around a central rod (like the jugglers do with a plate) with the rod itself being spun around in a circle at the other end. The ends of the barbell never cross the rod. They are always spinning around the axis of the rod, as the axis of the rod spins around the sun. But I see your point, if the planets always spun on the same plane, not on the axis, then as that plane circled the sun, twice a year they would come between the sun and the other planet. $\endgroup$ – Justin Thyme Oct 2 '17 at 2:12
  • $\begingroup$ Sorry, 'the axis of the rod' should be 'the axis that is the rod'. $\endgroup$ – Justin Thyme Oct 2 '17 at 2:18
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Actually, thinking about it some more, I'm not so sure that a two-body co-orbit that's perpendicular to the solar orbit would be a stable situation. At the two points in their solar orbit where A and B are equidistant from their sun the sun would be pulling on both of them equally (or almost, depending on their size differential) but at the other two points, 90° away, one would be closest to the sun while the other would be furthest away. It seems to me that this would introduce variability into their orbits, making this arrangement unstable. The reason this doesn't happen with the Earth-Moon system is because the Moon's orbit is (pretty much) aligned with the Earth's orbit. But, then again, I'm not an expert. Would anyone out there with an astrophysics degree care to chime in?

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protected by Community Oct 2 '17 at 3:53

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