Question about a fictional planet - star system

[RainDreamer]

In the past, I read a sci-fi sorta story, the name of which escaped my mind at the moment, and the content is mostly irrelevant to the question.

In that story, there exists a planet-star system in which the planet somehow have one side constantly facing the star. The result is that the planet is constantly baked on one side from the light, and the opposite side is left to freeze. Right in the middle of the two zones, in the twilight, is a habitable area, like a ring around where a race of people live.

My question is, is it possible for such a system to exist in reality? Where a planet constantly face the star on one side only? And possibly inside the habitable zone of the star at the same time?

[KBoy420]

I am nowhere near qualified to answer this. But everything in space is in an orbit of something, right? So I don't see how it could be possible because a planet in orbit of a star, regardless of if it rotates on its own axis or not, would still be exposed to sunlight on all sides at some point throughout its orbit, no?

[ZetaX]

My question is, is it possible for such a system to exist in reality? Where a planet constantly face the star on one side only? And possibly inside the habitable zone of the star at the same time?

A rather unlikely encounter, but yes, it's possible. It's the usual tidal locking/bound rotation we see with the moon.

But everything in space is in an orbit of something, right?

No. But that is not relevant.

[KBoy420]

Learn something new every day! Need to go do some more research now.

[Frozen_Heart]

A tidally locked planet is certainly possible.

The moon is tidally locked to Earth.

[Mr Shifty]

Given enough time, all satellites will tidally lock to their primaries. The time to lock is strongly dependent on the distance between the two objects. (It's approximately proportional to the 6th power of the semi-major axis of the satellite.) This is why no planets in our solar system are yet locked to the sun (though Mercury is in a 3:2 resonance.) But solar systems with larger suns or large planets close to a cooler sun might be expected to be locked in a smaller timescale.

Is the novel you're remembering Ark by Stephen Baxter by any chance?

[Sirrobert]

Yup, it's called tidal locking. Our moon has the same thing, it's always the same side that faces the Earth.

Basicly what happends is that while the planet rotates around it's parent (star), it's own rotation (around itself) takes exactly the same time. So 1 day is the same lenght as 1 year

[bdito]

Absolutely. Given the billions upon billions of galaxies out there the odds are in favour of something like that happening somewhere.

[Drunken Hobo]

It is indeed it's possible. You could even argue that Earth already has a "ring around where a race of people live", only it's based on latitude, not longitude:

With a tidally-locked planet, the image would be rotated 90 degrees, and habitability would be based on longitude. You would have to have a thick enough atmosphere to distribute heat to the dark side though, otherwise the atmosphere would all just condense & freeze on the dark side.

[Ikaneko]

Definitely, but unlikely.

It would be a biologist's dream though. Think of the variations in the life forms!

That said, the atmosphere's pressure would be messed up, with the cold side almost frozen and the hot side full of pressure.

What would happen??

[Bill Phil]

Tidally locked bodies are pretty common...

If there is life there, it's most likely prokaryotic, since prokaryotes were dominant for billions of years on Earth.

[NFUN]

There was a NOVA episode about this. It would be possible and potentially likely in systems with red dwarf stars because habitable planets would need to be close to their star in order to get enough heat. The show speculated that creatures living in the twilight could filter the air passing by them from the wind mentioned earlier.

[brdavis]

Definitely, but unlikely.

Actually, the most common worlds in the habitable zone would be expected to be tidally locked to their stars… far from uncommon. Rather the opposite. By far the most common stellar type are M Dwarfs… and their habitable zones are going to be very close.

It would be a biologist's dream though.

There have actually been a number of SF stories in such settings.

That said, the atmosphere's pressure would be messed up, with the cold side almost frozen and the hot side full of pressure.

If there was a strong pressure difference, you get winds. So what you'd expect is warming near the sub solar point with that hot air rising and flowing towards the terminator, cooling and sinking over the anti-solar point and then flowing back towards the sub-solar hemisphere. Ground winds would be blowing from the dark to the light side all across the terminator.

It turns out that simulations (yes, there's peer-reviewed literature on this) shows that the slow rotation (tidally-locked does -not- mean non-rotating… very much to the contrary) changes this: general airflow is from the hot to the cold side other the terminator at the "east" and "west" (leading and trailing) poles, cooling and flowing back to the day side over the north & south poles (where the winds would be strongest). It turns out that for a reasonable atmosphere you would not suffer 'atmospheric collapse' (the dark side doesn't get cold enough to 'freeze out' the atmosphere), but you would certainly have an interesting problem with the water cycle. The dark side would accumulate massive ice sheets, which would flow (as glaciers… wind-carved glaciers) back across the terminator onto the day side, melting as they went, with the resulting melt water evaporating and moving back to the dark side to fall out as snow.

Yeah, it would be a heck of a place

Biologically, besides the "frozen hell" on the dark side, and the "scorching baked dry hell" of the day side, there would be a temperate band between fire and ice, with an active water cycle. Trees (or other plants) there would have an interesting advantage: the star would never move in the sky. Forget optimizing for for an energy source that never moves. Fixed leaves almost perpendicular to the ground (the star would be low on the horizon), nothing much growing in the shadows because anything shadowed is *always* shadowed, etc. Anything living on the night side would either be based on trace nutrients blowing in from the day side (atmospheric filter-feeders?), or hydrothermal vents (which may or may not exist… plate tectonics here would be questionable without a nice uniformly hydrated crust.

Yeah, I teach university level planetary science… I love this stuff

--

Brian Davis

[GoSlash27]

As brdavis said above, most "earth-like" planets discovered so far are tidally locked. The concept of a habitable planet with one side facing it's star is not only feasible, but established as common.

Whether an intelligent complex organism could evolve there or not... I don't know. The conventional wisdom says you need tidal marshes and a magnetic shield. Both would be hard to come by without rotation.

Best,

-Slashy

[magnemoe]

Given enough time, all satellites will tidally lock to their primaries. The time to lock is strongly dependent on the distance between the two objects. (It's approximately proportional to the 6th power of the semi-major axis of the satellite.) This is why no planets in our solar system are yet locked to the sun (though Mercury is in a 3:2 resonance.) But solar systems with larger suns or large planets close to a cooler sun might be expected to be locked in a smaller timescale.

Is the novel you're remembering Ark by Stephen Baxter by any chance?

As I understand the 3:2 resonance is a local energy minimum, to get to tidal locking you first has to spend energy to slow it down more so Mercury will not become tidal locked unless some external force changes its rotation speed.

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There was a NOVA episode about this. It would be possible and potentially likely in systems with red dwarf stars because habitable planets would need to be close to their star in order to get enough heat. The show speculated that creatures living in the twilight could filter the air passing by them from the wind mentioned earlier.

Yes, the main problem with an tidal locked planet is that all the water will end up as ice on the cold side, also co2 might freeze there.

This depend a lot on water, you would want an wet planet so water can reach the ice, in that case it would keep temperature on backside up. So no eurasia sized continents.

If the planet has an wobble like not a circular orbit it would help, however the orbit would become more circular over time because of tides.

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As brdavis said above, most "earth-like" planets discovered so far are tidally locked. The concept of a habitable planet with one side facing it's star is not only feasible, but established as common.

Whether an intelligent complex organism could evolve there or not... I don't know. The conventional wisdom says you need tidal marshes and a magnetic shield. Both would be hard to come by without rotation.

Best,

-Slashy

This is an artifact of the detection method, its magnitudes easier to detect planets close to small stars.

However if you want to stay in the goldilock zone around an red dwarf you have a few options, tidal locked, perhaps 3:2 resonance but that would be worse for advanced life.

Double planet like Pluto or moon to a gas giant.

[RainDreamer]

Well, this sure is fascinating. The story I read was "The Wall of Darkness" by Athur C.Clarke, which also deal with the mind boggling nature of mobius space at the same time. I was interested in the configuration of the planet-star system though. I thought it would be rare, but turns out it is more common than I thought. Lots of interesting things to think about too, with the interaction between dark/light side. It is also quite poetic at the same time.

[KerbMav]

.

Regarding rotation: Would the planets rotational axis be vertical to its orbital plane?

North/south pole being cooked and frozen every half year ... even more interesting.

[KerikBalm]

It turns out that simulations (yes, there's peer-reviewed literature on this) shows that the slow rotation (tidally-locked does -not- mean non-rotating… very much to the contrary) changes this: general airflow is from the hot to the cold side other the terminator at the "east" and "west" (leading and trailing) poles, cooling and flowing back to the day side over the north & south poles (where the winds would be strongest). It turns out that for a reasonable atmosphere you would not suffer 'atmospheric collapse' (the dark side doesn't get cold enough to 'freeze out' the atmosphere), but you would certainly have an interesting problem with the water cycle.

Define "reasonable atmosphere"?

Water being a potent greenhouse gas, would mostly be locked up in ice sheets. Water content plumets, so then it would have to be closer and not rely on a greenhouse effect... but then if it was closer... before the tidal locking and H2O accumulation on the darkside... isn't there a significant chance of runaway greenhouse, and then you end up with a Venus like planet?

I don't know... the idea of tidally locked, habitable planets seems very very sketchy to me. When I hear all these people talking about the habitable one of a red dwarf... I sort of doubt that a red dwarf would have a habitable zone at all. Sure, it has a zone where the black body temperature of an object would be within the range needed for liquid water... but habitability is much more complicated than that.

The dark side would accumulate massive ice sheets, which would flow (as glaciers… wind-carved glaciers) back across the terminator onto the day side, melting as they went, with the resulting melt water evaporating and moving back to the dark side to fall out as snow.

Yeah, it would be a heck of a place

Biologically, besides the "frozen hell" on the dark side, and the "scorching baked dry hell" of the day side, there would be a temperate band between fire and ice, with an active water cycle.

Well, thats a very small environment... and then we've also got to wonder if any abiogenesis model would work on such a planet

I'm deeply skeptical of the idea of planets with a habitable surface around red dwarfs....

I am nowhere near qualified to answer this. But everything in space is in an orbit of something, right? So I don't see how it could be possible because a planet in orbit of a star, regardless of if it rotates on its own axis or not, would still be exposed to sunlight on all sides at some point throughout its orbit, no?

The rotation period would be the same as the orbital period... that's how.

Tidal locking is very common in KSP, go look at Kerbin from the mun, Duna from Ike, Jool from Laythe, etc.... it should help you understand

Edited January 28, 2015 by KerikBalm

[Sirrobert]

As brdavis said above, most "earth-like" planets discovered so far are tidally locked. The concept of a habitable planet with one side facing it's star is not only feasible, but established as common.

Whether an intelligent complex organism could evolve there or not... I don't know. The conventional wisdom says you need tidal marshes and a magnetic shield. Both would be hard to come by without rotation.

Best,

-Slashy

He just said that a tidal locked planet still rotates. Just not very fast

Regarding rotation: Would the planets rotational axis be vertical to its orbital plane?

North/south pole being cooked and frozen every half year ... even more interesting.

That's not the same thing as tidal locked. But it would be interesting

[KerbMav]

That's not the same thing as tidal locked.

Bending my head a bit more ... you are obviously right!

[michaelsteele3]

Yes. It is possible.

By the way, you should check out this documentary on youtube "What if the Earth stopped spinning?"

Pretty depressing, but still fictional.

[ZetaX]

More nitpicking:

Regarding rotation: Would the planets rotational axis be vertical to its orbital plane?

North/south pole being cooked and frozen every half year ... even more interesting.

The scenario in the last line is the case of the axis being in the orbital plane, not vertical to it.

[AngelLestat]

You would have to have a thick enough atmosphere to distribute heat to the dark side though, otherwise the atmosphere would all just condense & freeze on the dark side.

That is right in a rocky planet, and if you have a thick atmosphere then you also need more distance to the star to avoid higher temperatures on the surface.

Being far from the star also reduce a little how strong the super rotation winds will be. To avoid higher pressure then your gravity needs to be lower with a magnetic field.

But I have a better idea.

A tidal locked world with a Huge Sea. Water is very effective to absorb heat and carry that to different places.

Take a look at england, they should have very cold weather, however they live in a constant warm myst due the main sea current.

The heat capacity of the oceans are 1000 times greather than our atmosphere. And this taken into account that you use just the first 30 meters deep of that capacity.

Density and thermal capacity of water is a lot higher than gasses.

So with this scenary, you can control how much water this world has, to make the story you want.

The water would control how much difference in temperature you might fine, it would also control the winds at surface level, they would not be so strong, but at higher altitudes you would have constant faster winds that can be used by airships to fly from one point of the planet to another.

By the way, you should check out this documentary on youtube "What if the Earth stopped spinning?"

Yeah that documentary is pretty inaccurate.

Edited January 28, 2015 by AngelLestat

[brdavis]

Define "reasonable atmosphere"?

Well, the paper in question used a global climate model, 3-D. For an Earth-sized planet with Earth-normal insolation, it turns out the atmosphere (a CO2 based atmosphere) is stable against collapse down to 30 mb… so about 3x the Martian surface pressure. An Earth-sized planet with 1 Atm of CO2 can prevent atmospheric collapse with an insolation level of just 30% Earth's. and a nitrogen-dominated atmosphere doesn't start to freeze out unit the dark-side temperature reaches about 80 K… which is very easy to maintain.

...isn't there a significant chance of runaway greenhouse, and then you end up with a Venus like planet?

Not really. Sure, you can come up with a "planet too close" scenario… but a dense CO2-rich atmosphere is in fact what you want if you are near the outer edge of the habitable zone.

I don't know... the idea of tidally locked, habitable planets seems very very sketchy to me.

Agreed, it seems more than a bit outrageous… which is one of the reasons people have done peer-reviewed research on the subject. It turns out, our intuition (perhaps coming from evolving on a non-tidelocked planet) appears to be wrong.

I sort of doubt that a red dwarf would have a habitable zone at all. Sure, it has a zone where the black body temperature of an object would be within the range needed for liquid water... but habitability is much more complicated than that.

Agreed; there's a lot involved. for example, type M dwarfs tend to have both huge "sunspots", as well as often are flare stars. The first could drop temperatures to where atmospheric collapse does happen, while the 2nd could drive temperatures too high over too much of the surface. That was actually another thing the paper worked out - would such a planet be stable (maintain an atmosphere) under these conditions? It turns out… the models show it would. Surprise again .

Well, thats a very small environment... and then we've also got to wonder if any abiogenesis model would work on such a planet…

Current thinking is that life on Earth was likely started around oceanic thermal vents. I could certainly see that happening in a tide-locked world setting.

There are plenty of other habitability issues (could you have a carbonate-silicate cycle without global oceans? Or would you stall plate tectonics? Would flares from the star start stripping the atmosphere?). But that's true for almost any planet. There are even some possible advantages: around a flare star, a one-faced world always has a protected side… not true for a rapid rotator like Earth.

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Being far from the star also reduce a little how strong the super rotation winds will be.

Hmm. Not sure I follow - the super rotation of the upper atmosphere is a product of the heating, but also the slow rotation of the planet (like Venus). Being far from the star isn't going to help much (unless you are so far it's too cold or not tide-locked).

A tidal locked world with a Huge Sea. Water is very effective to absorb heat and carry that to different places.

True, but I don't have a good reference for the oceanic circulation of a global sea on a one-faced world . Yes, it would help.

...the winds at surface level, they would not be so strong, but at higher altitudes you would have constant faster winds that can be used by airships to fly from one point of the planet to another.

It would if nothing else have some interesting airflow patterns (fun story settings).

Yeah that documentary is pretty inaccurate.

[KerikBalm]

Well, the paper in question used a global climate model, 3-D. For an Earth-sized planet with Earth-normal insolation, it turns out the atmosphere (a CO2 based atmosphere) is stable against collapse down to 30 mb… so about 3x the Martian surface pressure. An Earth-sized planet with 1 Atm of CO2 can prevent atmospheric collapse with an insolation level of just 30% Earth's. and a nitrogen-dominated atmosphere doesn't start to freeze out unit the dark-side temperature reaches about 80 K… which is very easy to maintain.

...

Sure, you can come up with a "planet too close" scenario… but a dense CO2-rich atmosphere is in fact what you want if you are near the outer edge of the habitable zone.

If the point is to find a planet where liquid water exists, you need to assume that there is water vapor in the atmosphere which (as climate change denailists love to point out) is also a potent greenhouse gas.

So... the atmosphere needs to have H2O in it. When the planet is formed, that H2O will not be immediately sequestered in ice sheets on the dark side (if there is even a dark side early in formation). That H2O is going to affect the temperature. Models for the evolution of Venus have Water vapor being the most potent greenhouse gas early in its history (until its oceans boiled off completely, and all H20 was lost to photolysis).

How is the planet going to handle this early on?

Its climate may be stable later with massive ice sheets on the dark side sequestering all that H2O... but would those massive ice sheets form in the first place with that H2O free?

Feedback loops often lead to stable "on" or "off" conditions. If something is "off" it takes a large perturbance to switch it "on", and vice versa.

Just because you can show a stable condition could exist, doesn't mean it will occur. The planet doesn't instantly form tidally locked with ice sheets on the dark side. So I'm asking if there is a viable model of planetary evolution that reaches that condition.

Additionally, it takes some time for CO2 atmospheres to form, no? from volcanic outgassing?

Might it be that the outgassing rate was never sufficiently high, and it just accumulated on the dark side as it outgassed (again, this depends on how quickly the planet becomes tidally locked)?

http://www.ncbi.nlm.nih.gov/pubmed/11538226

" For fully saturated, cloud-free conditions, the critical solar flux at which a runaway greenhouse occurs, that is, the oceans evaporate entirely, is found to be 1.4 times the present flux at Earth's orbit (S0). This value is close to the flux expected at Venus' orbit early in solar system history. Is is nearly independent of the amount of CO2 present in the atmosphere, but is sensitive to the H2O absorption coefficient in the 8- to 12-micrometers window region.

...

The surface temperature of a runaway greenhouse atmosphere containing a full ocean's worth of water would have been in excess of 1500 degrees K--above the solidus for silicate rocks. The presence of such a steam atmosphere during accretion may have significantly influenced the early thermal evolution of both Earth and Venus."

I think a model that doesn't account for an early steam atmosphere is incomplete.

It turns out, our intuition (perhaps coming from evolving on a non-tidelocked planet) appears to be wrong.

I'm not ready to say that based on one paper. As a molecular biologist... I'm used to a whole lot more controls, and looking for other variables that could affect the outcome. You haven't linked the paper, but so far it sounds to me like the atmosphere model is too simple (considering only a CO2 atmosphere.

Current thinking is that life on Earth was likely started around oceanic thermal vents. I could certainly see that happening in a tide-locked world setting.

Not really, thats sort of the pop-culture interpretation, which is a little out of date. As per the RNA world hypothesis, when looking for the first life, we look for the first self replicating RNAs. We've made a number of RNA directed RNA polymerase Ribozymes (that is, RNA that copies other RNAs).

It turns out, cryogenic conditions are actually much better for this to occur, and as water begins to free, inclusions in the ice start to concentrate various compounds. By some models, the first cells were more like ice bubbles.

One should also consider where the nutrients form, vs where the life may start. The best conditions for the synthesis of the precursors (such as nucleotides of the RNA bases from formamide) often need heat, but then the best conditions for those bases to form a self replicating RNA seems to be a lot colder (so maybe they form in one set of conditions, and diffuse to another location with other conditions).

Then there's the observation that nucleic acids absorb strongly in the UV, possibly implicating exposure to light in abiogensis (ruling out deep sea vents, though life later could have moved there, and they'd be a good spot for chemical precursors). Then we have to consider the first self replicating nucleic acids would lack a helicase enzyme -> but in the lab we replicate nucleic acid sequences without helicases by thermal cycling. Some studies suggest a Day-Night cycle would have provided that thermal cycling to allow separation of strands for the next round of replication -> perhaps some current near an undersea heat source could provide that, but the losses due to diffusion may be too high.

If the day/night cycle is needed for abiogenesis, then the tidal locking would have to occur slow enough for life to arrise first.

There are plenty of other habitability issues (could you have a carbonate-silicate cycle without global oceans?

One also has to consider that when photosynthesis happened on Earth. In the Devonian, as Plants colonized the land, and evolved woody proteins that couldn't yet be effectively degraded, CO2 concentrations collapsed.

http://en.wikipedia.org/wiki/Late_Devonian_extinction#Plant_evolution

Something similar may have again happened in the carboniferous, as plants evolved new adaptations and spread even more inland, and increase in biomass (sequestering a lot more carbon)

Earth's atmosphere (while undeniably affected by CO2) has a lot of other gases contributing to the greenhouse affect, and was never in danger of freezing out.

How would a tidally locked planet handle such an event?

What of buffer gasses? You need some Nitrogen in the atmosphere, its hard to imagine life without nitrogen (its not as important as carbon, but still very important).

As I said... I'm still deeply skeptical.