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If two planets were tidally locked and shared an atmosphere, as described in this question, what would the weather patterns of the planets be like? What weather patterns would form, and could these weather patterns travel from one planet to the other?

For the sake of simplicity, assume both planets are Earth-like in composition and mass, with high amounts of water on the surface, liquidy interiors, metal cores, humid oxygen/nitrogen-filled atmospheres etc. One can assume that the planetry pair are about the same distance from there star (which is of the same type as the Sun) as Earth is from the Sun.

Loduwijk
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tox123
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  • humid oxygen? What should it be? – L.Dutch Mar 20 '19 at 03:30
  • I think tox123 meant an athmosphere made up by oxygen and nitrogen, which contains noticable amounts of water vapour. – DarthDonut Mar 20 '19 at 07:43
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    @L.Dutch I think that entire section between the preceding comma and the period is supposed to be a description of the atmosphere all together. "humid oxygen and nitrogen filled atmospheres" So it would be an atmosphere with lots of oxygen and nitrogen and it would be humid. I don't think humid is an adjective for oxygen in that sentence. – Loduwijk Mar 20 '19 at 20:31
  • @L.Dutch Is that edit better? tox123: If I misunderstood, feel free to roll back the change. – Loduwijk Mar 20 '19 at 20:33

1 Answers1

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Planets sharing an atmosphere would produce a highly unstable situation. At the closest point between the two planets gravitation would be negligible and would result in huge amounts of water, foam, rock and debris being drawn up into the atmosphere.

It is likely that this region would form a rotating storm between the two planets. Given the high energy content of storm systems and the very low net gravitational forces in the area it would result in a storm of biblical proportions and would absorb any storms approaching it from either planet. No Storm would be able to cross this boundary.

Weather would be dominated by the ring storm between the two planets. The weather systems would be powered by the energy from the sun coupled with the difference in ground speed between the point of closest approach and the far side of each planet (similarly on Earth there is a difference in ground speed between the poles and the equator that drives our climate).

This would cause all of the weather systems on each planet to spiral from the far side towards the point of closest approach where they would be consumed by the world storm system circulating in that area. Similarly on Earth storm systems spiral away for the fast moving equator to the stationary poles.

Slarty
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  • I'm sorry, this is just wrong. Yes there would be a zero g point between the planets but that would not extend down to the two surfaces. – Tim B Mar 20 '19 at 11:40
  • No it is correct. I said negligible not zero. With atmospheres touching the gravitational forces of each planet would almost cancel out so it would take very little energy to move huge amounts of material up in the air and a storm system contains huge amounts of energy. – Slarty Mar 20 '19 at 11:48
  • It would be cancelling out at the point where the atmospheres touch. When you descend from that point towards the surface gravity would increase until you had real gravity by the point you reached the surface. The gradient would be different as you rose but surface gravity would be present. – Tim B Mar 20 '19 at 12:01
  • I'm not saying there would be no affects at all - but you certainly wouldn't see rock/water being "drawn up" unless there was a powerful tornado or similar doing it. You would be able to stand on the surface and be pulled down to the ground below you just fine, no risk of floating up. – Tim B Mar 20 '19 at 12:04
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    The net gravitational acceleration difference between a point on the surface of the Earth and a point 50km up is around 0.15m/s/s. This would be the net gravitational force on the surface of both planets at the point of closest approach – about one tenth of the gravitational acceleration on the surface of the Moon. Imagine a storm under those conditions. – Slarty Mar 20 '19 at 12:15
  • Don't forget the centripetal force from the orbit though, that would be significant at that point. – Tim B Mar 20 '19 at 14:36
  • Would it? Assuming the nearest point on the surface is 50km below the centre of mass of the 2 planet system and there is one revolution every 24 hours the nearest point is moving at 3-4 m/s relative to the centre of mass. So centripetal force will be very small indeed and since centripetal force acts towards the centre of the circle in which the object is moving that force (such as it is) would be directly up. – Slarty Mar 20 '19 at 15:46
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    In the classic rocheworld configuration there is an 80km separation and 6 hour rotation period - so yeah you are right, there is a centripetal effect but it's only 0.00034g. Interesting. I've just tried doing some research on the effective g force across the rocheworld surface and not found any good sources. – Tim B Mar 20 '19 at 17:36
  • I am not sure your approximation is correct but I'm also not sure it's wrong. Approximately gravity is zero at the core, increases until you reach the surface, then descends on a square root curve as you move away from the surface. So you have 1G pulling down at the surface, being offset by 1G pulling upwards from 80km away. So the question is just how much the 1G from the other body has reduced by the time it reaches the surface, which as you say is not a lot. Interesting. – Tim B Mar 20 '19 at 17:38
  • The Rocheworld books by R L Forward are for the most part scientifically accurate and generally did not portray the gravity as reducing by quite that much but they could always have got something wrong and having said that there is an incident in them where water is pulled up from Eau to Roche - although that was triggered by an interaction with a 3rd body. – Tim B Mar 20 '19 at 17:42
  • This does sound cool, and maybe there is truth to it, but even if there is I think you exaggerate some of it. You write that no storm could cross the center area, and you compare it to Earth storms moving toward the pole... I'm not an expert, so I don't know if they move toward the pole in general, but I do know that on Earth they do (also) come down from the poles toward the equator. Perhaps that is the exception, I don't know, but it does happen. Still is a cool setting to imagine. – Loduwijk Mar 20 '19 at 20:43
  • @TimB (Slarty automatically notified). I think the figure "configurations of a binary star system..." in this wp article give an idea of the shape of the system's athmosphere. Specifically this image is what the athmosphere would look like. The 8-shaped line is the equipotential, which I think means (very) roughly constant average athmospheric pressure. – Rafael Mar 20 '19 at 21:17
  • I'd rather postulate that the situation would be so unstable that the two planets would almost certainly not last long without crashing into each other. Binary planets certainly are possible, but these two would effectively be circling each other within the shared atmosphere, draining momentum causing their orbits to decay. – jwenting Mar 21 '19 at 09:42
  • @jwenting that would only be true if the atmosphere was not also rotating with the planets... which of course it must be because it is gravitationally linked to them. There's going to be some drag, probably expressed as heating of the atmosphere, but it would take deep time to cause the pair to decay and collide. – Corey Sep 17 '20 at 00:04
  • The tidal forces would be huge and catastrophic. Earth's Moon is responsible for the largest part of tides on Earth (the remainder being from the Sun) of many metres in places. imagine if it was a thousand times closer (inverse square relationship with gravity) and weighed almost one hundred times as much. Trillions of tons of water shifting up and down by many km every day would soon sap the rotational energy. – Slarty Sep 17 '20 at 11:08