# Is my map of prevailing winds on a binary planet realistic?

My binary planet consists of a Venus sized earth-like planet, tidally locked to a mars sized “super mercury”, with their centers separated by about 7 Venus radii. I know the coriolis effect would behave very differently here, so I decided to figure it out for myself, but I want to know if it is plausible, because it is certainly interesting.

The coriolis effect, as far as I know, arises from the equator moving faster than the poles because it is a larger distance traveling over the same time. A good analogy would be a record on a player. As air moves from the poles south, it approaches land moving faster than it, and appears to fall behind. Air moving north approaches slower moving land, and appears to slip ahead. But In my case, my planet is like a circle sticker stuck on the record, the near side of the planet hardly moving at all, and the far side buzzing around as the planets orbit one another.

As you can see, the further you get from the barycenter the faster the planet is moving, which forms a series of concentric circles from the sublunar point to the sublunar antipode. Air on the near side would appear to flow similar to on earth, moving closer to the sublunar point, toward the barycenter axis, toward slower moving land, therefore being deflected in the prograde direction. But once you reach the meridian “equator”, you find traveling north and south does not change how fast that part of the planet is moving. Here, the wind would not be deflected anywhere, but move north or south. Beyond this meridian, on the far side, moving toward the equator brings you to faster moving land, and the wind reverses direction. Considering the near side far side line as the “prime meridians” you would find in a single convection cell, the wind blows away from one point on this circle, and towards it on the opposite side of the planet. Even more interestingly, the next convection cell would blow in the opposite direction, away from this point, and converging back where you started

In conclusion, I feel this would make for a very interesting world with civilizations having to navigate by these strange winds, sometimes with the guide, of the other planet, and sometimes alone without it. Is there anything I’m missing? And to whoever edits this for formatting feel free to rewrite some stuff cause my syntax is terrible when I dump information.

• Hi Sean. I think I need to help you by setting an expectation. Humanity has no knowledge of a habitable binary planet. In fact, with the possible exception of Mars, humanity has no knowledge of any other habitable planet other than Earth. We have one and only one data point to work with: Earth. You're asking us to judge as "realistic" something humanity is incompetent to judge. The only way I can think of this is: if the planets are far enough away, the climate won't differ significantly from Earth. If they're close, they'll have higher tides and nasty storms.
– JBH
Commented Jun 13, 2022 at 21:04
• @JBH what do you mean by "habitable planet" in "In fact, with the possible exception of Mars, humanity has no knowledge of any other habitable planet other than Earth."? Do you mean habitable for humans in particular, in which case Mars probably could never have been habitable, or do you mean habitable for liquid water using life in general, in which case it is theoretically possible that there is life in the subsurface oceans of several worlds in our solar system and thus Mars is not the only possible other habitable world known.. Commented Jun 14, 2022 at 15:27
• @M.A.Golding You're straining at a gnat, proven by the phrase, "...it is theoretically possible...." We know of no world (at all) with the possible exception of Mars that can be considered habitable, and therefore have but one data point we can work with to answer this question. My goal is to help Sean understand our limits. We can help achieve a goal - but it's worthless to ask us to approve a result. (a) He already has an answer and (b) we have no data to work with to approve it.
– JBH
Commented Jun 15, 2022 at 21:28
• @JBH You discount the entire process of extrapolation. We can model a hypothetical planet such as this. Yes such things are complex, but that does not mean, by any measure that they are impossible. Commented Jun 17, 2022 at 5:41
• @ThistleThornThornThorn I don't believe I've discounted anything. The OP asked if the wind patterns are realistic. An extrapolation based on hypothesis may be a useful guess, but doesn't rise to the expectations of the OP. Please keep in mind that the purpose of my initial comment was to help the OP understand how to ask future questions. There are practical limits to what we can do (even with extrapolation).
– JBH
Commented Jun 17, 2022 at 6:03

Background

So, they key consideration here is force diagrams in each reference frame. The major considerations are:

• Gravity of planet you are on
• Gravity of neighbouring planet (tidal forces)
• Accelerating reference frame (ultimately the coriolis force derives from getting nearer or further from the axis of rotation)

Ultimately I'd think about solving a static solution (where does the atmosphere generally end up -- for the Earth you get a convenient even-ish shell) and then consider what happens to a parcel of air moving within this background distribution (for Earth, Coriolis leads to Hadley cells & trade winds)

In this specific question I think OP has sketched out the static solution without considering tidal forces (but they align with the Barycentric rotation so don't super change the solution). Assuming Barycentric rotation dominates gravity, the dominant wind current would be as sketched, leading air to build up on the far side of the planet until pressure gradients matched the rotational force (if tidal force dominates, you get more air on the near side).

To estimate the wind pattern, when I consider the forces on a moving element of air, I believe you end up with something akin to the classic Earth hadley cells, but rotated 90deg so the "north pole" points at the Barycentre.

Unknowns:

• it is possible that the daily cycle of solar heating adds complexities to this that I can't fully map in my head.
• Tidal forces are actually very complicated (see: Earth's tides) at there might be standing waves in the atmosphere due to this. Add topography to the planet and Rossby waves derived from that would become an important part of the overall atmospheric dynamics

PS. I am only considering atmospheres here. For full hard sci-fi you may need to think about the stresses in the solid bodies of the planets in this setup, and whether the orbits are stable

• Tidal forces would be extreme, but since the planets are tide-locked, they wouldn't actually be called "weather". You totally need to figure out what the rotational period would be between the two planets, and that requires specifying masses, which would allow you to also sketch out the gravity wells. The atmosphere would be thicker on the barycenter side, so the atmospheric pressure would be much higher on the surface there. It would be a fun exercise. Commented Jun 17, 2022 at 16:37
• The distance between the planets and the center of gravity they orbit, ultimately the speed they orbit each other; will greatly affect the coriolis forces experienced in the atmosphere. Complex is an understatement when all other factors are added in as well. Commented Jun 17, 2022 at 17:59

I suspect the 2 planets as described would not have stabilised their orbits in the fashion you present.

What you are describing is effectively the Earth and the Moon - with their habitable atmospheres but the moon being insanely close to the Earth. I don't know the right way to calculate those orbital mechanics, but it feels intuitively wrong to have the smaller planet orbiting the larger at a distance of seven radii

It is most likely going to need to be at a similar distance of the Moon from Earth - Or even further...

The key point here, is that based on Orbital mechanics, I think your 2 atmospheres will not be able to interact at all