Tidally-locked planets flipping around their axes

[Strato Incendus]

Recently, I’ve heard from Anton Petrov on YouTube that some tidally-locked planets around red-dwarf stars (such as TRAPPIST-1) have been suspected to flip around their own axis every once in a while — so that the former day side becomes the night side, and vice versa.

This is presumed to happen for the same reason as why a T handle set in a rotating motion on a space station starts flipping back and forth. The latter is an issue we discussed for spaceship designs, when it came to how multiple rings have to be arranged in counter-rotating pairs, in order to prevent the ship from flipping around its axis during travel.
I’m wondering what this would do to the presumably habitable twilight zone of a tidally-locked planet. If the day and night side swap places, but the twilight zone remains in place, the twilight zone could remain habitable, as far as I can tell. The question is in which direction the planet flips:

1) If the planet flips around its axis along the plane of motion, the habitable vertical belt that is the twilight zone would essentially be “rolling” on the circumference of the orbit. Meaning, none of the twilight zone would move into being on the day side or night side temporarily.

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.
In my story, the planet in question is Teegarden b. The question is now whether I have to postulate that it doesn’t do these flips, or whether I can acknowledge the flips, but claim it doesn’t matter as long as you only settle in the (right parts of) the twilight zone. The day side and night side are uninhabitable anyway.

Anton Petrov demonstrated the potentially catastrophic consequences of such a flip with Earth in Universe Sandbox 2: In scenario 1, Africa was the day side; then, he flipped the planet around, and suddenly South America was the day side, and Africa was the night side. I don’t recall if there were any places on Earth in that test which would have remained habitable in either scenario, AND in the transitionary period in between.

 

[Hornbein]

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.

This is what happens. A tidally locked planet rotates once a year. The axis of rotation is what flips, and that's perpendicular to the plane of motion.

 

[Hornbein]

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.

This is what happens. A tidally locked planet rotates once a year. The axis of rotation is what flips, and that's perpendicular to the plane of motion.

Look at this a bit more carefully, only the two segments of the twilight zone that are on the equator remain in the twilight zone during a flip. That's where any permanent settlements would have to be.

I wonder how fast these planetary flips. Year? Century? Other planets are involved, so I guess the flips wouldn't be all that predictable.

 

[Algr]

So the planet rotates 90°? The "sun pole" becomes a north or south pole? The video I've seen looks more like like a 180° flip.

Life on such planets would have evolved to deal with such flips. I'd expect huge wind storms and rushes of water for seeds and organisms to catch a ride upon. The nightside glaciers would melt and flow back to the new nightside. This would reduce the hotside temperatures for a long time. I wonder if frozen lifeforms could revive thousands of years later when the next flip happens?

 

[Algr]

Can you link to the Anton Petrov video? I don't see it on his YouTube page.

 

[Strato Incendus]

Sure, here it is:

 

[Algr]

Anton's video has the twilight zone passing through the poles in both the normal and inverted Earth. (180°) But the ice cap covers more than half the Earth, so both remain frozen in both orientations. It looks like the poles remain poles in both orientations, rather than Antarctica becoming a sun or night pole.

To avoid this in your story, you could just say that the planet lacks close neighbors, or is stabilized by some nature of its core that you never clearly define.

 

[Strato Incendus]

Nice idea, but that would still require me to change the destination planet. Because the one where I’m currently sending my ship, Teegarden b, has already been confirmed to have a close neighbour, Teegarden c.

So should I pick another planet in a distance of about 12.5 light years? Teegarden b is still the one with the highest Earth-Similarity Index (ESI) in Earth’s vicinity. There is only one planet with an even higher ESI, but it’s so far away that it’s not a realistic target for below-light-speed travel.

 

[Algr]

Stick with Teegarden b and go with the undefined "nature of it's core". Its neighbors boost it's magnetic field instead of flipping it. Don't dwell on the science. If you have an interesting story that readers want to believe in, and characters they like, readers won't sweat the details. Trek and Star Wars aren't enduring for their scientific accuracy, after all.

 

[Strato Incendus]

Alright, thanks a lot for your answers!

 

[Nik_2213]

Is this akin to how Mars' axal tilt may vary enough over 'deep time' for precession to potentially thaw its polar caps ?

Where-as Earth's is constrained by Moon's tidal dissipation ??

 

[Hornbein]

Stick with Teegarden b and go with the undefined "nature of it's core". Its neighbors boost it's magnetic field instead of flipping it. Don't dwell on the science. If you have an interesting story that readers want to believe in, and characters they like, readers won't sweat the details. Trek and Star Wars aren't enduring for their scientific accuracy, after all.

While this is a lot to like, I'm going to go the other way. Sometimes the scientific details lead the author to things they would never have imagined. That happened to me. On the other hand, I'm not a popular writer nor am I trying to be.

 

[Algr]

It turns out this is Trappist 1c, not Teegarden b: But if you use a real planet this could happen to you someday soon:



So I'm right the first time and ignore the second. Use an imaginary planet so that some future telescope doesn't pluto your book.

 

[EventHorizon]

Is this akin to how Mars' axal tilt may vary enough over 'deep time' for precession to potentially thaw its polar caps ?

Where-as Earth's is constrained by Moon's tidal dissipation ??

Mars's obliquity changes substantially on timescales of hundreds of thousands of millions of years. So if a planet like Mars has life, I think the life would have enough time to adapt to the changes

 

[Strato Incendus]

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.

This is what happens. A tidally locked planet rotates once a year. The axis of rotation is what flips, and that's perpendicular to the plane of motion.

Coming back to this: If the planet really flips as frequently as once per year, wouldn’t that render it more habitable? Meaning, the long day and night of being on the sun or space side would be more like seasons.

Also, keep in mind a “year” on Teegarden b is just 4.91 Earth days.

The question is of course how hot the temporary day or night side would get, during this period of uninterrupted days and nights. Then again, anyone planning to settle on HAVOC in the clouds of Venus is going to face the same problem. And HAVOC exists in my setting.

 

[Strato Incendus]

Just a heads up: I gave the art software Leonardo.AI the description of how I currently imagine a tidally-locked planet around a calm red dwarf star like Teegarden's Star. After many attempts, this is one depiction it came up with (as per the Terms of Service of Leonardo, the user who created the image retains the rights to it.):

Spoiler

Assuming that such a planet can have an atmosphere that includes oxygen, does this make sense to you?
I guess the trees might be darker shades of brown or even black, in order to absorb more of the dim red light from the star?

 

[Strato Incendus]

Since people on this forum have understandably warned me against using a real-world planet in my story, just in case James Webb looks at its atmosphere in the future and debunks my story by proving that planet is uninhabitable:

I recently asked the YouTuber Kyplanet (a new “contrarian” setting out debunk a lot of circulating misinformation about space discoveries) whether he could do an episode on Teegarden b. He said we wouldn’t know enough about it yet, doesn’t place too much importance on the high Earth Similarity Index — and importantly, told me that James Webb Telescope can’t look at its atmosphere, because Teegarden b does not pass in front of its star, relative to our position.

If that is true, while disappointing for researchers, it actually seems ideal for my story, right? Because I can count on James Webb not being able to look at Teegarden b’s atmosphere anytime soon, in order to prove or disprove its habitability. It would thus be left up to the audience’s imagination for years, if not decades to come.

Teegarden b being postulated as habitable within my setting would then become a “god of the gaps” kind of thing. And by “god”, I mean “unfalsifiable”. Not scientifically testable — and therefore, open to the realm of myths, which is what stories are, at the end of the day.

Or am I missing something here?

 

[Strato Incendus]

Coming back to this: If the planet really flips as frequently as once per year, wouldn’t that render it more habitable? Meaning, the long day and night of being on the sun or space side would be more like seasons.

Also, keep in mind a “year” on Teegarden b is just 4.91 Earth days.

The question is of course how hot the temporary day or night side would get, during this period of uninterrupted days and nights. Then again, anyone planning to settle on HAVOC in the clouds of Venus is going to face the same problem. And HAVOC exists in my setting.

Any takes on this, since it just came up again on another forum?

Perhaps a general thought experiment: How hot would the day side of a planet in the habitable zone of a red-dwarf star get over the course of 5 days facing the star, uninterrupted? Because after that, given its short orbit / year, the planet will flip, and the day side will start moving into the night side, allowing it to cool off again.

In other words, with a tidally-locked planet that has such a short year, and flips once per year, there wouldn't really be a day side and night side. Rather, night and day would each last five Earth days. And they would result from vertical flips, rather than from horizontal rotation.

 

[DaveC426913]

It seems you are assuming this polar flip would mix up day and night side. I'm not sure I follow.

If the planet flipped end-over-end, how can you assume it will mix up its day and night faces? Or is that just a contrivance for your story?

I shudder to think of the quakes and volcanos and crevasses occurring on a planet that flips every few days.

 

[Strato Incendus]

Thanks for your fast response, @DaveC426913 !

Of course, not ALL of the day- and night side will flip: The "West and East Pole" should always remain in the twilight zone.

However, the North and South pole (both of which are necessarily in the twilight zone while the planet is "upright") should pass either through the day or night side during each flip. Because either, they'll move along in front of the star, or on the rear side of the planet.

And the question is: How hot / cold would these areas get, over the course of 5 days, if that's the maximum duration they spend in the heat of the day side, or the cold of the night side?

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.

This is what happens. A tidally locked planet rotates once a year. The axis of rotation is what flips, and that's perpendicular to the plane of motion.

I shudder to think of the quakes and volcanos and crevasses occurring on a planet that flips every few days.

And hence, the second question becomes:

Assuming there are still habitable areas on such a planet at all, would it be smarter to place a colony on the West or East pole, which always remain in the twilight zone (probably more stable)?

Or could you place one on the North or South pole, so that the colonies would pass through the day and night side with each flip, and thereby experience seasons? Because having somewhat of a day and night rhythm could actually be preferrable for sentient beings, even if that rhythm is prolonged, compared to Earth. As long as the side effects (quakes, volcanos, wind speeds etc.) aren't too life-threatening, of course.

 

[DaveC426913]

However, the North and South pole (both of which are necessarily in the twilight zone while the planet is "upright") should pass either through the day or night side during each flip. Because either, they'll move along in front of the star, or on the rear side of the planet.

Why? It kind of depends on the axis of the flip. What if the axis runs through the star? Then the flip will have zero impact on daylight. In fact, in the extreme, such a flip wouldn't even be visible from the planet's surface, unless one could observe the stars.

 

[Strato Incendus]

Ah, you mean if the planet "lies" on its axis, kind of like Uranus does?
So if I understand correctly, this scenario would indeed be a case of the planet "rolling" on its orbit whenever it flips. Would that be less threatening to life on the planet overall?

I just figured this scenario wasn't possible, based on @Hornbein 's post:

2) If however the planet flips around its axis in a 90-degree angle to the plane of motion, the poles of the planet, normally in the twilight zone, would temporarily point towards the star or away from it. Meaning, one of the poles would become the day side, the other pole the night side for a while. The rest of the twilight zone — the part always pointing away from the star at a 90-degree angle — would remain in the twilight area.

This is what happens. A tidally locked planet rotates once a year. The axis of rotation is what flips, and that's perpendicular to the plane of motion.

 

[DaveC426913]

Ah, you mean if the planet "lies" on its axis, kind of like Uranus does?

No. This tidally locked planet has its axis pointing north/south, rotating once per revolution (i.e. about the z-axis, below). Otherwise it wouldn't be tidally locked.

So if I understand correctly, this scenario would indeed be a case of the planet "rolling" on its orbit whenever it flips.

I think that's key: to nail down what kind of flipping could occur.

Yes, Hornbein may have the right idea. I just don't know if he meant it as the only kind of valid flipping.

Gonna post a diagram, so's'n we can all talk about the same things.

Horbein is talking about a fip around the x-axis. The poles would pass through zenith and antizenith.

I was imagining a flip around the y-axis (so, as you say, rolling in its orbit) which would evoke virtually no visible change day/night from the surface, except the stars. What I do not know is if this kind of flip is possible.

Do we know, for example, that these planets are experiencing the Dzhanibekov Effect, as it is known? Is this the only mechanism that can result in a flip?

We would need to understand - or invent - the appropriate asymmetries and densities that trigger the effect.

 

[Hornbein]

Do we know, for example, that these planets are experiencing the Dzhanibekov Effect, as it is known? Is this the only mechanism that can result in a flip?

We don't know that, but I surely can't imagine any other mechanism. Not that that proves anything.

If a planet is tidally locked then its axis of rotation must be perpendicular to the orbital plane. Otherwise it wouldn't be tidally locked.

It seems to me that this flipping can happen only if the planet hasn't enough mass for gravity to compel it into an ellipsoidal shape. The equator is an oval or something instead of a circle. Then wouldn't it be a huge asteroid instead of a planet? I'm no expert though. Maybe there's some other way. If so then Mercury might flip over.

Here's a very dramatic Dzhanibekov flip. The driver Peter Dumbreck was very fortunate to survive more or less uninjured. The vehicle so happened to be in the part of the flip when it impacted so that the car landed upright in soft earth. The automobile was impaled by a tree that missed the driver.



As you can see the flipping doesn't take a simple path, it's not a simple rotation. It's chaotic. There is no point on the planet's surface that is always in twilight.

 

[DaveC426913]

If a planet is tidally locked then its axis of rotation must be perpendicular to the orbital plane. Otherwise it wouldn't be tidally locked.

I said that! I want credit!

Here's a very dramatic Dzhanibekov flip. The driver Peter Dumbreck was very fortunate to survive more or less uninjured. The vehicle so happened to be in the part of the flip when it impacted so that the car landed upright in soft earth. The automobile was impaled by a tree that missed the driver.


As you can see the flipping doesn't take a simple path, it's not a simple rotation. It's chaotic. There is no point on the planet's surface that is always in twilight.

Cool as that might appear, I kind of have to reject it as an ideal example of the Effect. Way too many mitigating factors: the shape of the car and the force of the wind are surely going to swamp the effect. It's practically an airplane.

This is a much more germane example.