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A couple days ago, I posted this question about what it would feel like to walk on a Rocheworld.

For some background, a Rocheworld is a double planet system where the two planets are so close together that they have actually started merging together. This is unlikely to occur in real life, but it is still cool to think about. I recommend checking out the original question and answer if you're at all confused.

The answer I got was that it wouldn't be extremely different, but over time you would notice that gravity was different in different areas. If I'm interpreting the answer correctly, gravity would be lower on the opposite ends of the Rocheworld and in the region where the two planets overlap and would be heavier along the middle of each planet.

Now that's how walking would feel. However, in the Issac Arthur video I linked he mentioned that these Rocheworlds could very well be mostly water with continents on the ends of each planet.

This leads to my two main inquiries: How would the gravity of a Rocheworld affect the ocean currents, and could you sail from one half of a Rocheworld to another?

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  • $\begingroup$ Sail, on a boat on water - no. A body of water needs gravity under it. Fly? From one planet to the other, just maybe. Zero G, which would effectively happen between the planets means water would float all around you. You couldn't sail on that. But zero G doesn't necessarily mean zero pressure. The center of the Earth is zero G but high pressure. There could theoretically be some air pressure all the way through, making flight possible but not sailing. Bit of a longshot though. $\endgroup$
    – userLTK
    Oct 15, 2017 at 6:48
  • $\begingroup$ @userLTK That's not true. If the bridge is relatively thin, gravity could get very low, but if there is a bridge at all, it will not go to zero at the surface. You'd get zero gravity at the center of mass, but that's way beneath the surface, and no more of a problem than the fact that there's zero gravity at the center of the Earth. $\endgroup$ Oct 15, 2017 at 20:14
  • $\begingroup$ @LoganR.Kearsley It would depend on the mass of the two planets. If the mass and density are the same then the centre of mass would be equidistant between them $\endgroup$
    – Slarty
    Oct 15, 2017 at 21:02
  • $\begingroup$ @Slarty That's not the point. The relative location of the center of mass is completely irrelevant. No matter the relative sizes of the two lobes or the center of mass is placed between them, if there is a solid or liquid bridge connecting them--not merely a shared atmosphere--then you cannot walk, swim, or sail to the center of mass, and you will not encounter any point of zero gravity as you travel along the surface. If the bridge is relatively thin, the gravity can get very low, but it cannot go to zero. $\endgroup$ Oct 15, 2017 at 21:25
  • $\begingroup$ @LoganR.Kearsley you certainly cannot move between them realistcly (see my answer). On reflection and choosig my words carefully, the centre of mass of the system between the two planets would not be in "zero gravity" but it would be in free fall. $\endgroup$
    – Slarty
    Oct 15, 2017 at 22:28

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If the gap is bridged by water, you could absolutely sail from one lobe to the other. And if the gap is only bridged by atmosphere, then you could still fly an airplane from one lobe to the other, as in the classic Rocheworld series by Robert Forward.

That assumes, however, that the water in the gap is liquid. Since the "interior" areas of a rocheworld get much less light than the outer poles, it might very well be frozen over!

But it doesn't have to be, so we'll assume we're looking at a world warm enough to have a liquid water bridge between lobes.

Just as when you are walking from one side of a solid rocheworld to the other, gravity will be low on the bridge. You will thus have to deal with all of the consequences of sailing in low gravity. This won't change the keel depth of your ship, but it will result in greater deviations from the mean- i.e., your ship will rise higher and sink lower as it bobs up-and-down on the waves, and it will move up-and-down more slowly. Waves themselves will also be larger and slower.

The currents would not be significantly influenced by gravity per se, but they will be influenced by the temperature, and spin. Even if it's not frozen, the bridge will be one of the colder-water areas of the world, which means, absent landmasses getting in the way and Coriolis forces pushing things sideways, warm currents will tend to flow towards it on the surface, and cold currents will flow away from it in the depths. But, of course, there will be Coriolis forces-and fairly strong ones at that! Along the equator, if the world is large enough, the seas should be fairly unaffected, but near the rotational axes you'd expect strong rotating currents. If the world is not large enough to keep the equatorial areas of the bridge isolated from the axial currents, you'd get directional currents leading from one lobe to the other on one side, and back again on the other side.

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    $\begingroup$ For a better feel of what swimming in low gravity could be like, see here. That article is about the Moon, with around 1/6 of Earth's gravity; but do note that if the planets are basically tangent to each other (as the OP's will be, if they're not waterworlds with oceans thousands of kilometers deep), the low-gravity regions will have very little gravity. So that xkcd article may well be quite relevant. $\endgroup$ Oct 15, 2017 at 5:51
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Assuming that both planets are very similar in size and mass, the area between them is not going to be a good place to sail or do anything else. It would depend on the exact distance between the worlds but assuming they are close enough, a bridge could form and fill up with water and a vast area around the bridge could fill up with low pressure air.

But lots of warm tropical water and reduced pressure is going to make for stormy conditions. I suggest that the bridge area would be immensely unstable. In such low gravity conditions any big circulating storm system that migrated onto the bridge might easily have sufficient energy to disrupt the bridge entirely. Ordinary storms can send spray high in the air even under 1g, so under very low gravity conditions I dread to think of what would happen to the seas surface, it would probably be broken into a mass of swirling foaming bubbles and drops with much of it sailing off into the low pressure area around the bridge and eventually returning to crash back into the bridge.

Mud sand and rocks are often swept up by storms on earth even under 1g. Under low gravity conditions on and around the bridge the situation is going to be even worse. Any storms are going to pick up all sorts of material from thousands of miles around including anything floating, any shallow sea mud/sand and the entirety of loose materials from any islands.

I suggest that the bridge would be an unstable spinning nightmare of water, foam, mud, rocks and debris. And given the circumstances I don’t think there would be any chance of a ”calm day”. It would be continuous. Storms are going to migrate towards the bridge in the same way that storms on earth migrate away from the equator towards the poles, because the bridge area has very little angular momentum, whereas all the other parts of both planets will have angular momentum relative to the bridge.

The electrical storms would probably also be “spectacular”.

I should also point out that the whole scenario is unstable and it would not end well for either planet. Frictional forces across the bridge and surface effects would eventually rob the system of its angular momentum causing the planets to approach closer and closer until they merged.

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You can't sail, fly or walk between the worlds. The problem isn't low gravity, but zero gravity. Let's draw a line from world A to world B. It doesn't matter where either end lies. Obviously, at one end gravity points to world A and at the other it points to world B. Gravity is a continuous function, thus there must be a point somewhere on that line where the forces balance and you're in zero gravity.

Note that the endpoints don't matter, this is true of any line you draw between the worlds. Plot those zero-gravity points--you get a surface in space between the worlds. (If the worlds are identical the plane will be flat. If they're not I'm not sure of the shape but it doesn't matter anyway.) Points on this plane are clearly not under the gravitational control of either world. What's the atmospheric pressure on a world with no gravity? Zero, obviously, as it can't hold an atmosphere. The same thing happens here--the planets can't retain atmosphere or ocean that touches this plane and there's no path between the worlds that doesn't touch it. The atmosphere and ocean will bleed away until they are no longer shared.

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  • $\begingroup$ Physics isn't my forte, but I'm a little confused. Are you saying there would be no gravity all over? How would they even stay together? I could understand no gravity in the area where they're close together before the two merge, but I don't understand why the whole system would lack gravity. $\endgroup$ Oct 15, 2017 at 8:06
  • $\begingroup$ No he’s not saying there would be no gravity. It’s simply that there are two very large gravitational attractions working in opposite directions. When you are at the centre of mass of this system planet A is pulling you one way with force x and planet B is pulling you in exactly the opposite direction with force x. So you are in free fall. In “normal” orbital free fall the attractive force of the planet is counter balanced by extreme speed that allow the curvature of the planet to become an important issue and you fall continuously. In this case instead of extreme speed we have another planet $\endgroup$
    – Slarty
    Oct 15, 2017 at 12:37
  • $\begingroup$ @Slarty And since it doesn't need speed the lack of speed won't hold the atmosphere. $\endgroup$ Oct 15, 2017 at 19:27
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    $\begingroup$ This is false. Or rather, it is true only for one point - the center of mass. Things on the plane will not be attracted towards one planet or the other, true. But they will still be attracted - just toward the center of mass instead. (Assuming the planets are the same mass. Otherwise it gets more complex) $\endgroup$
    – TLW
    Oct 15, 2017 at 19:31
  • $\begingroup$ Simple disproof from conservation of energy. Assume the planets are exactly touching. I take one kg of mass down from infinity to the side of one planet away from the other planet, doing useful work. I then move it along the surface to the point where the two planets touch. This cannot cost energy, as I am not moving away from either body's center of mass. I then move it away along said plane to infinity, and repeat the cycle. $\endgroup$
    – TLW
    Oct 15, 2017 at 19:35

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