Could this [fantasy] planet be scientifically possible?

[Oliva]

Hi fellas.
First of all, sorry for the bad english, as it's not my main language :)

Short story to explain my needs.
A few years ago, a night tale to my daughter was about a little girl who lived in a peculiar planet.
It was pretty hot on one side and then hottest on the pole, there was a not so large strip of temperate climate and then the other side was pretty damn cold, again with the pole the coldest point. I told her the zone frontiers were so clearly defined that if you saw the planet from the space, it would be a red/white ball with a green strip in the middle.

Then it went for a few days the same tale explaining how the girl felt among the different civilizations on both places, because the technology were much different on the three zones, mainly due to the survivability.

I always try to give her a real scientific education, by encouraging her to ask and research until she has no doubts about what she wants to know... And these days it happened that she came and asked me if this planet were scientifically possible. And asked me to explain to her how could the cold region be so cold if there was sun.

I first told her that there would be no sun there. That this planet would orbit the sun just like mercury orbits our sun or the moon orbits our planet. The cold region would be a hidden face, but during the night tale I also told her that there was no night in this planet.
She didn't seem to be much satisfied, but didn't raise any other questions... so now I have these questions:

1) would be possible to this fictional planet to exist between two stars? A 'sun class' star, who would be the system center and a not-so-bright star, who could maybe provide light enough, but not heat enough?

2) these 'clearly defined zones'? possible? like north hemisphere is all Greenland and south hemisphere is all Sahara? and the equator line is not a line, but a ~300km zone of dense forest...

 

[Chatterton]

With some mild tweaking you could have a tidally locked planet, as many suspect habitable planets around red dwarf stars would be. Like our nearest neighbour, Proxima Centauri. The planet would have one side constantly facing the sun, similarly to how the moon orbits Earth. But since it's orbiting a sun you get one side light, one side dark.

The most livable region would be a strip around the outside of the light side that could conceivably be circumnavigated if your character wanted to go on a journey. The interior of the light side would likely be hot, perhaps quite stormy. The dark side would likely be cold and miserable, although warm winds drawn in by the cold could make the edges of the dark side not too unpleasant

 

[Oliva]

The planet would have one side constantly facing the sun, similarly to how the moon orbits Earth. But since it's orbiting a sun you get one side light, one side dark.

Yeah, but the point is that this planet should have day anywhere on surface, everytime.

This point of having day everywhere everytime is something that confused me a bit to explain

So I imagined something like this:

sun ---------- planet ---------------star 2
-O----------------o-----------------------O

The orbital movement per se would be the star 2 rounding sun. The planet would be only locked on that gravity and would round the sun at the same speed of star 2.

What I thought is the possibility of that star 2 to have a low, but consistent brightness, but really low heat, so that side could be permanent frozen

 

[Khashishi]

The orbital speed goes down as the objects are farther apart. So, star 2 can't orbit star 1 as the same speed as planet orbits star 1, unless it's at the same distance. If it's at the same distance, it wouldn't be stable.

 

[Chatterton]

I think also to have a binary star system with a stable, livable planet orbiting one star, the second star would have to be quite a distance out so that it would appear pretty pale in comparison. And Khashishi's right about orbital periods. So you could have a tidally locked planet with consistent lighting on one side and variable lighting on the other side.

Also, depending on the luminosity of the second star, it could affect the 'light side' from time to time, creating brighter than normal days with double shadows when both suns are visible on that side (something George Lucas didn't seem to take into account).

 

[Algr]

A second star at a pluto-distance would create light resembling dusk on Earth. The sky would be blue, and stars invisible when it was out. However on a tidally locked planet, it would rise and set once per that planet's year. (Which might be a few days or several months.) With a third star in the system, you could get a nightless period lasting several decades, but (as mentioned above) you'd also get similarly long periods where one or both stars were on the same side of the sky.

 

[|Glitch|]

Consider the real-world example of Proxima Centauri b:

Proxima Centauri b orbits Proxima Centauri every 11.186 days because its semi-major axis is only 0.0485 AU from its parent star. This also makes the planet tidally locked to its parent star. The “optimistic” liquid water Habitable Zone for Proxima Centauri is between 0.032 AU and 0.084 AU. The “conservative” liquid water Habitable Zone for Proxima Centauri is between 0.04 AU and 0.08 AU. That puts Proxima Centauri b within the liquid water Habitable Zone, but on the warm side. Kopparapu et al. (2014) further assumes a minimum atmospheric pressure of 1,000 Pascals (0.01 bar), with nitrogen (N2) as the background atmospheric gas.

The “conservative” liquid water habitable zone is depicted in green, with Proxima Centauri b's orbit depicted in yellow.

The mass of Proxima Centauri b is 1.27 M_⨁, with a radius of 1.10 R_⨁, and a density of 5.26 g/cm³ (0.95 ρ_⨁). This gives the planet a gravity of 1.05 g (10.297 m/s²). The closest Earth-like exoplanet we have found to date.

With an atmospheric pressure somewhere between 1,000 Pascals and 100,000 Pascals, nitrogen as the background atmospheric gas, and a mean surface temperature of ~16°C - according to the assumptions made by Kopparapu et al. (2014) - the Albedo of Proxima Centauri b should be between 25% and 35% with radiative forcing at approximately 200 W/m². Compared to the 1,366 W/m² that we receive from the Sun on Earth, Proxima Centauri feels 6.83 times less hot on Proxima Centauri b than our Sun does on Earth. However, this does not take into consideration that much of Proxima Centauri's light will be in the infrared range of the electromagnetic spectrum, which we cannot see with our eyes but we would still feel as heat. So it might appear exceedingly dim, but still feel warm.

Proxima Centauri is part of a ternary system, with the other two stars being Alpha Centauri A and B, even though Proxima Centauri is separated from them by approximately 0.21 light years (~13,280 AU). Alpha Centauri A and B are only separated by 11.2 AU from each other. As a result, from someone's perspective standing on Proxima Centauri b, both Alpha Centauri A and B would appear as one star to the unaided viewer and be brighter than all the other stars in the sky, but too distant to provide any sort of useful light.

The one issue I see with the OP description of the planet are the poles. When a planet is tidally locked to its star its geographical poles (as opposed to the magnetic poles) will always be perpendicular to the planet's orbit. Therefore, both geographical poles will be in the "twilight" zone of the planet. The side of the planet always facing the star would naturally have the highest mean surface temperature on the planet, and even though the planet is not rotating, the difference in temperature between the day and night sides of the planet would be the mechanism that drives the winds. Assuming there is an atmosphere.

If there is any liquid water on Proxima Centauri b it would be found in that narrow "twilight" band that encompasses both poles. The day side of the planet would be too hot to support liquid water on the surface, and the night side would be too cold and all water would be in the form of ice. Only in the middle, where the day and night side transition, would liquid water be possible.Sources:
First radius measurements of very low mass stars with the VLTI - Astronomy & Astrophysics, Volume 397, January 2003 (DOI: 10.1051/0004-6361:20021714)
The diameters of α Centauri A and B: A comparison of the asteroseismic and VINCI/VLTI views – Astronomy & Astrophysics, Volume 404, June 2003 (DOI: 10.1051/0004-6361:20030570)
Search for Associations Containing Young stars (SACY): I. Sample & Searching Method – Astronomy & Astrophysics, Volume 460, December 2006 (DOI: 10.1051/0004-6361:20065602)
Mass-radius relation of low and very low-mass stars revisited with the VLTI - Astronomy & Astrophysics, Volume 505, October 2009 (DOI: 10.1051/0004-6361:200911976)
A Physically-Motivated Photometric Calibration of M Dwarf Metallicity – Astronomy & Astrophysics, Volume 519, September 2010 (DOI: 10.1051/0004-6361:201015016)
Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass – The Astrophysical Journal Letters, Volume 787, Number 2, May 2014 (DOI: 10.1088/2041-8205/787/2/L29)
A Terrestrial Planet Candidate in a Temperate Orbit Around Proxima Centauri – Nature, Volume 536, Issue 7617, August 2016 (DOI: 10.1038/nature19106)
Calculations of Periodicity from Hα Profiles of Proxima Centauri – arXiv 1608.07834v1, August 2016

 

[SlowThinker]

How about
1. A Sun-like star
2. A brown dwarf
3. The planet in Lagrange point between the two
That should fit the bill, and AFAIK is not completely impossible.
Of course the poles have to be in the green band.

 

[|Glitch|]

Yeah, but the point is that this planet should have day anywhere on surface, everytime.

This point of having day everywhere everytime is something that confused me a bit to explain

So I imagined something like this:

sun ---------- planet ---------------star 2
-O----------------o-----------------------O

The orbital movement per se would be the star 2 rounding sun. The planet would be only locked on that gravity and would round the sun at the same speed of star 2.

What I thought is the possibility of that star 2 to have a low, but consistent brightness, but really low heat, so that side could be permanent frozen

As Khashishi correctly pointed out, objects orbiting further away from the star will be traveling slower than objects orbiting closer to the star. Placing a planet within the L1 Lagrange Point, as SlowThinker suggested is your only viable option, but even that is tricky. The only area of true stability for long periods within Lagrangian Point L1 is between one third and one half of the Hill Sphere radius. I am not entirely sure that an object would be tidally locked to its gravitational parents (which would be equally shared between both the primary star and the brown dwarf that SlowThinker described) at that distance. However, it is not such a big stretch to claim that the planet is tidally locked to the star and the brown dwarf simultaneously. Highly unlikely perhaps, because that would mean the brown dwarf would have had to migrate very close to the main sequence star (it could not have formed that close to the star) and the planet would have had to have been captured in the L1 Lagrange Point (it also could not have formed where the gravity between the star and the brown dwarf were equal). Nevertheless, enough doubt exists that makes SlowThinker's idea the one that fits the OPs description the best. As he said, it "is not completely impossible" just extremely improbable.

 

[rootone]

That planet between the two binary stars is never likely to form in the first place.
If we magically drop a planet in there it will likely get ejected from the system at a high velocity.

 

[|Glitch|]

That planet between the two binary stars is never likely to form in the first place.
If we magically drop a planet in there it will likely get ejected from the system at a high velocity.

That is certainly true using the barycenter between two stars as location for the planet, it would be quickly ejected from the solar system. However, not if you place that planet in the L1 Lagrange Point of a brown dwarf that was orbiting a star. It would have to be placed (or captured) because a planet could not have formed there, since that L1 Lagrange Point would be continuously changing as the brown dwarf migrated closer and closer to its parent star. Assuming something eventually stabilized the brown dwarf's orbit and stopped its migration, an L1 Lagrange Point would be at a constant distance from both the star and the brown dwarf. It would not orbit either the brown dwarf or the star, but rather be captured in the gravity well. Similar to the Greek and Trojan asteroids in Jupiter's L4 and L5 Lagrange Points.

 

[AidenFlamel]

That should fit the bill, and AFAIK is not completely impossible.
Of course the poles have to be in the green band.

What if this planet had a rotation like the one Uranus has while being tidally locked? This would fix the problem mentioned above about not being able to have the poles on the opposing sides.

 

[SlowThinker]

What if this planet had a rotation like the one Uranus has while being tidally locked? This would fix the problem mentioned above about not being able to have the poles on the opposing sides.

Yes but then you would not have a bright side and dark side.
In the beginning the poles point toward the star, all is fine.
A quarter a year later, however, the planet rotates sideways. That would be similar to normal Earth rotation, with day+night cycle at most of the planet, except for the poles.
Another quarter of a year, the former bright side becomes the dark side and vice versa.

P.S.: Tidally locked means no rotation. You can't have a planet rotating like Uranus and have the axis follow the orbit.

 

[stefan r]

H
1) would be possible to this fictional planet to exist between two stars? A 'sun class' star, who would be the system center and a not-so-bright star, who could maybe provide light enough, but not heat enough?

2) these 'clearly defined zones'? possible? like north hemisphere is all Greenland and south hemisphere is all Sahara? and the equator line is not a line, but a ~300km zone of dense forest...

You could use a large star. Perhaps something like Betelgeuse or Rigel. Put a small red dwarf in a distant orbit (~400 AU) around not-Rigel. Then place the tidally locked planet in close orbit around the red dwarf. A lot of heat reaches the planet from hot not-Rigel star but not enough to thaw all the ice. The tidal locked side gets heated by both the red dwarf and by not-Rigel so It get toasty by the equator. If they are all in a plane the planet will have cold poles like Earth or mars.

The official description of things orbiting the real Rigel is much more complicated.