Weather on a tidally-locked planet

What would the weather in a tidally-locked planet's terminator be like? I've heard plenty for each side, but none for the habitable zone. Looking mostly for temperature and weather ideas.

You can logic it out. "Weather" is caused by the movement of air in responses to changes in temperature (and density, and moisture content, but those are mainly driven by temperature). Fundamental to this is temperature gradients. If it's a lot hotter over here than over there, energy will try to flow between those two places in the form of the movement of air.

So, what will you get at the boundary between a part of a planet that is always super hot and a part that's always super cold? The most ferocious storms you can possibly imagine. Thermodynamics wants to move energy from the hot part to the cold part, and convection is the most efficient way of doing so.

Now, I'm not an expert, but the way I believe it would work (in a general sense, at least) is that the side facing the sun would superheat air, which would rise rapidly and flow towards the colder side. As it does so, it draws in cooler air from its surroundings, and it also gets cooler as it moves towards the cold side, eventually falling back down and driving denser air towards the hot side.

You could have upper level winds hundreds of kph in velocity and lower level winds of similar strength but moving in the opposite direction, like a hurricane that is active all the time. Between them would be an unpredictable shear layer that would constantly be in friction, generating electrostatic forces that would power lightning storms like you've never seen. Due to the energies involved, I'd expect thousands of smaller storms to form, particularly near the terminator.

This is assuming the planet in question has an atmosphere: to be tidally locked, it's usually going to be relatively close to its parent star, and most such planets lose their atmospheres to solar wind unless they manage to undergo a runaway greenhouse effect like Venus, or are extremely massive (e.g. Super Earths).

Edited by Fighteer on Feb 21st 2020 at 8:08:19 AM

Thanks for the help! I'll incorporate that :)

There are some apps online that allow you to simulate an alien planet's atmosphere. Havent tried one yet, but they look interesting.

Are you able to name some? I can only find build your own earth & that doesn't have a tidal-locking option

On most stars from G-class (Sun-like) to K-class (Epsilon Eridani) to red dwarfs, the distance needed for gravity to tidally lock a planet in place is so close the atmosphere would be blown away every time unless the planet has one hell of a strong magnetic field and a large mass to boot. Even Super-Earths would likely be airless rocks. Anything F-class (Canopus) or brighter is certainly going to strip away tidally locked planets to airless rocks.

The only possible exception to this rule may exist around red dwarf stars owing to the low luminosity and radiation output. (Assuming said red dwarf is not a violent flare star.) There is one such possibility we know of right now in the form of Proxima b around Proxima Centauri. Proxima b is the nearest planet outside our Solar System and it may or may not have an atmosphere and it may or may not be tidally locked. (If it isn't tidally locked, it's probably in 3:2 resonance like Mercury.) Proxima Centauri puts out a low enough amount of radiation and solar wind that unless Proxima b lacks a magnetic field entirely, the planet should retain an atmosphere even if tidally locked. And it's in the Goldilocks zone of Proxima Centauri. The only drawback to Proxima b is Proxima Centauri is known to be a bit of a flare star so there's uncertainty about a great many things about it.

Would life as we know it even be possible on a tidally-locked planet? Doesn't rotation help maintain the atmosphere or is that something else I'm thinking of?

A tidally locked planet would probably have no magnetic field to speak of, since its innards wouldn't be spinning (at least not fast enough to matter). This would remove its most powerful mechanism to prevent the solar wind from stripping away the atmosphere. Even if it could retain air, it would be too hot for water to be liquid on most of one half and too cold on most of the other.

There might be a narrow temperate band along the terminator, but as I said previously, it would be beset by storms the likes of which we Earthlings can only imagine, and those storms would never stop. Add in extreme levels of radiation and it is difficult to imagine any sort of life surviving for long on the surface. Underneath the ground (or water), however, there would be a lot of free energy due to the violent thermodynamics above. It might be enough for life to evolve, though probably not with any degree of complexity.

If the planet does not retain its atmosphere, then no, it's pretty much hopeless. There's a miniscule chance that there would be an ice cap or ice sheet on the side facing away from the sun, beneath which liquid water could remain, but it would be miles thick.

Edited by Fighteer on Feb 21st 2020 at 1:33:45 PM

While I appreciate all the talk about how a tidally-locked planet would work, I'm really just looking for weather ideas/a way to simulate what this atmosphere might be like.

Not sure of any cheap way to simulate an atmosphere. Such calculations are usually heavy and done on supercomputers.

(NB: Sunward is the direction of the sun, or the half of the planet in permanent sunshine. Antisunward is the direction away from the sun, or the half of the planet in permanent dark)

But simply speaking, the main weather pattern would be obvious: A permanent high-altitude hot wind going from sunward to antisunward, and a permanent cold wind going antisunward to sunward. The part closest to the sun would have air heated and rise up, and the part far from the sun would have hot air cool down and drop.) Barring extreme geography, this would create a climate distribution akin to earth's hemisphere, where the sunward side is very hot and the antisunward side is frozen. How big each region is would depend heavily on the specifics.

Proxima/Ultima by Stephen Baxter takes place on a tidally locked planet around Alpha Centauri. The atmosphere is much thicker with a strong greenhouse effect, which evens out the heat much more and allows (from the equator) a large temperate region with a tropical region under the stellar point. Though the star flares, life can sense an incoming flare and takes shelter, or has evolved thick outer layers to survive (E.g. plants fold up)

Edited by devak on Feb 22nd 2020 at 8:55:40 PM

Thanks! That helps a lot :)

Something like that would only work near a red dwarf, of course, where the energy output in that sort of orbit isn't so great that the planet would be baked to bare rock like Mercury.

Another thing I thought of with respect to this sort of planet is that it would lose its atmosphere really quickly. On the sunward face, as air heats up to extreme temperatures, it would rise really high above the surface, high enough that even a strong magnetic field couldn't keep it shielded from the solar wind. Molecules like water vapor would dissociate into hydrogen and oxygen, and the lighter gases like hydrogen would be stripped off in the wake of the star's output long before they could cool down and fall back to the surface.

Aside from a permanent ice cap at the anti-sunward pole whose size would depend on the size of the planet and some other factors, such a world would have all its surface water destroyed in short order, along with most other atmospheric gases lighter than carbon dioxide. Some of it depends on the strength of the solar wind, as well: I don't know how much weaker it would be near a red dwarf.

You'd need a very massive planet to prevent this, quite a bit heavier than Earth. As noted, a very thick atmosphere might also work, but it would have had to develop said atmosphere before becoming tidally locked... possibly it could have migrated inward due to perturbations from other planets in the system. Imagine, for example, if some disturbance made Mercury and Venus swap places.

There are also some ways that a world like that might keep its magnetic field. One way I could think of would be for its rotational axis to be lined up with the axis pointed at the sun, like a top spinning on its side. I have no idea how such a situation could occur; perhaps it had a crazy impact at some point in its past that dramatically altered its spin, so that tidal locking only cancelled out a part of its rotation. In that case it might have a strong enough field to keep its atmosphere from being stripped.

The Coriolis effect on such a world would have some fantastical effects on its weather patterns, and I would love to see a simulation of that.

Edited by Fighteer on Feb 24th 2020 at 10:51:47 AM

Actually, they are animations, not an app, but potentially useful nevertheless. Find here

Those animations are interesting. In particular the precipitation one: i did not expect that it would rain so heavily at the substellar point.

Planets do migrate sometimes, and a somewhat heavier planet with a somewhat thicker atmosphere is not improbable. The tidally locked world need not be common (I think, based on the OP), just plausible.

Seeing those animations though, i wonder if you could tweak the geography for a more optimal habitable world. Big landmasses seem bad (High temperatures), but ocean is useless for habitability. Maybe a planet with an ocean below the Substellar but surrounded by australia-sized continents would be more optimal? Or actually given the high precipitation, i guess it would be more akin to a lake.

Edited by devak on Feb 24th 2020 at 7:28:10 PM

The thing with that Earth simulation is that we're far enough away from our Sun that the temperature gradient wouldn't be nearly as massive as if we were, say, in Mercury's orbit. We're also too far away to become tidally locked (the Moon helps with that slightly). I find it interesting that all the evaporation occurs on the cold side and the precipitation on the warm side. That doesn't feel quite right but I'll believe the sims over my layman's knowledge.

^^^ Thank you for those! That's mainly what I've been looking for.

And yeah, this type of world doesn't at all need to be common, because the story doesn't involve space travel or the like.

From the maps, it seems much of the evaporation happens at the hot half of the planet, but near the colder side. All the vapor is transported to the substellar, where it precipitates.

My presumption is that evaporation more or less corresponds to precipitation, so what we see on that map is where the storms are.