Gliese 581g - A Tidally Locked DnD World

[Aloïsius]

I was wondering about a binary system... One far away star that would bring little warm and light, but still enough for some photosynthesis activity. And one close red dwarf, with which the planet would be tidely locked.

When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.

 

[Umbran]

When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.

A planet in the "Goldilocks zone" of a red dwarf is very close in, which means the year is short. As an example, Gliese 581g (if it exists) has an orbital period of 36.5 days or so. Each season would be about 9 days.

I think having a secondary star, rather than a moon, might be a good way to inject some time dependencies that would be interesting, but it wouldn't be a season like we have on Earth.

 

[Nyeshet]

I was wondering about a binary system... One far away star that would bring little warm and light, but still enough for some photosynthesis activity. And one close red dwarf, with which the planet would be tidely locked.

When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.

This would be difficult to set up realistically. If the stars are closer than about 3 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc.

If the suns were further apart, there is a risk that the gravity of one would upset the orbits of the planets that tried to form about of the other (either preventing formation or tossing them out of the system during formation) unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky). It would still give off as much light as a full moon, more or less, but it would not obviously be a second 'sun' - let alone provide enough heat for a hotter summer, etc.



Edit: I found the link I was looking for: planet forming discs of dust do not seem to form in binary star systems that are between ~3 and ~50 AUs apart. The stars either have to be nearer than 3 AUs (and so act as a single star for orbital purposes) or farther apart than 50 AUs (in which case the second sun will appear star-like).


http://solstation.com/habitable.htm
http://solstation.com/images/bi2sep.jpg

On the other hand, I have some little experience in working on the equations for luminosity of stars in such a system, as I once worked out what a red dwarf star (0.34 sol masses) at 108 AU away would look like from a planet at a mars like orbit (1.69 AUs) from the binary star (1.29 sol masses). The red dwarf would have a luminosity of about -11.7, compared to the full moon's luminosity of about -12.7 and the earth's sun's of -26.8. So it would be no larger than any given star in the night sky, but its brightness would be that of a gibbous moon. You will get some faint shadows from the reddish-star due to its brightness, but little else.


If we presume a sun-like binary star companion at ~50 AUs, its brightness would likely be several times brighter than the full moon - but it still would be too faint to give any real extra heat to the world.



Okay, I calculated by extrapolation the apparent luminosity of a sun-like G2 star at 50 AUs using the link below. It works out to -18.37. This is roughly equivalent to the brightness of the sun as seen from Pluto (-18.2) or Neptune (-19.3). Your red dwarf tidally-locked planet not get much heat from its second sun, but low light vision should see much farther than on an earth-like world, at least when the second 'sun' is in the night sky. Mythically, locals might view this second sun as instead the king of the stars, the local myth equivalent of Jupiter due to its brightness.

http://www.johnbray.org.uk/planetdesigner/
http://en.wikipedia.org/wiki/Apparent_luminosity

 

[Nifft]

This would be difficult to set up realistically. If the stars are closer than about 2 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc.

If the suns were further apart, there is a risk that the gravity of one would upset the orbit of the other unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky). It would still give off as much light as a full moon, more or less, but it would not obviously be a second 'sun' - let alone provide enough heat for a hotter summer, etc.

Yeah. IMHO it's way easier to posit a phase-locked moon orbiting a Jovian-scale gas giant. The gas giant would also be tasked with maintaining a magnetosphere, since they seem good at that.

 

[Umbran]

This would be difficult to set up realistically. If the stars are closer than about 2 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc.

However, in this situation, it is unlikely the planet would become tidally locked, as the tides are not going to be consistent over time

If the suns were further apart, there is a risk that the gravity of one would upset the orbit of the other unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky).

I am not sure this is correct.

Take a larger, bright star. Put the Red dwarf in orbit around it. Put the planet in close orbit around the Red dwarf. A situation like our Sun, a Jovian planet, and one of it's moons, just make everything bigger. I think you can still get significant illumination from the bright star without significantly perturbing the planet's orbit. I'd need to spend some time with the math to see if it works out well for sure.

Mind you, the bright star will have a much shorter lifespan than the red dwarf, and when it dies, it may well wreck things on the poor little world so nearby.

 

[Nyeshet]

A planet in the "Goldilocks zone" of a red dwarf is very close in, which means the year is short. As an example, Gliese 581g (if it exists) has an orbital period of 36.5 days or so. Each season would be about 9 days.

I think having a secondary star, rather than a moon, might be a good way to inject some time dependencies that would be interesting, but it wouldn't be a season like we have on Earth.

A tidally locked world should have little or no tilt - and so no seasons to speak of, much like its one face has no day and its other face no night to speak of.

I'll admit I did once play with the idea of a more 'normal' world around a red dwarf star, but to make it normal I had to place a tiny half-earth mass 'moon' around a Jovian world just barely in the habitable zone of a red dwarf star. The result was a 'world' with a day 0.7x the length of an earth day (as it orbitted the Jovian at a Callisto distance, so as to keep radiation to an acceptable level) and a year of about 32 days. There were still two main problems, however.

First, the tides were absolutely incredible. Between the Jovian and the Red Dwarf, the worldlet experienced tides of about 100 meters (72.63 ave from Jovian, 25.59 ave from star). By comparison, earth's ave mid-ocean tide height from the sun and moon together is about 0.54 meters). If earth's maximum tide heights are any example from which to extrapolate, then this worldlet has a maximum tide height around 1.2 km. Its 'Bays of Fundy' would be incredible to see.

Second, when the Jovian was at its maximum distance from the red dwarf, the worldlet, during its few hours of night, would actually be just outside the habitable zone for a few hours. So once a month, for a few days, the nights on the night side would be colder than most arctic nights, or so I would presume.

As a minor point, the night side would have twilight nights when facing the Jovian, as the reflected light from its surface bathed the world in light, but solar eclipses would happen every noon on the day side during those times of the month when the Jovian was between the worldlet and the red dwarf. No corona would be visible, however, due to how vast the Jovian would appear from the surface even when compared to the red sun. On the other hand, during these eclipses, the night side would also be experiencing something unusual: Stars would only be visible when the night side faced away from the Jovian for a few days each month - also, as already mentioned, the coldest days of the year. Thus the world would have something akin to seasons, even as it had little to no tilt.

 

[Nyeshet]

I am not sure this is correct.

Take a larger, bright star. Put the Red dwarf in orbit around it. Put the planet in close orbit around the Red dwarf. A situation like our Sun, a Jovian planet, and one of it's moons, just make everything bigger. I think you can still get significant illumination from the bright star without significantly perturbing the planet's orbit. I'd need to spend some time with the math to see if it works out well for sure.

Mind you, the bright star will have a much shorter lifespan than the red dwarf, and when it dies, it may well wreck things on the poor little world so nearby.

That is a problem. Also a problem is explaining how a planet formed between two stars so near each other in the first place. A red dwarf separated from a sun-like star - let alone a much more gravitationally powerful F, A, or even O type star - by only 5 AUs (Jupiter distance) would be near enough to have any dusty disc surrounding it torn away by the gravity of the companion star - even if the potential planet was as near as the red dwarf's habitable zone. Actually, with O type stars the situation is worse. I recall an article that spoke of dusty planetary discs around young stars being literally blown away (gradually) by the solar wind of O type stars a mere 5 to 10 light years distant. Having an O type star 5 to 10 AUs distant would likely prevent any planet from forming even around a sun-like star, let alone a red dwarf.


However, red dwarves live a *very* long time. The eldest ones still have more than 10x the current age of the universe before there is any chance of them going out. So why not a captured one that formed its planets late and then joined another star (perhaps gathering a disc when passing through a nebula - long after its own flaring youthful couple billion years, then a few billion years later being captured by another star into a binary orbit - losing its outter planets in the process but keeping those one or two nearest its surface). Now we have a nice stable red dwarf with a planet within its habitable zone fully developed, and a binary companion that might be as close as 5 to 10 AUs. Such would have to be an extremely rare event, as no such binary system so close together and yet with a planet around one of the two has yet been found.

Note, however, that even an F type star would not give a notable amount of heat at Jovian distances. You would need at least an A - maybe an O, and yes, when the younger larger star goes supernova, the tidally locked world is toast. (Actually, for an O type star, the world would be toast during the red giant phase, when the red dwarf - at 5 AU - would likely end up skimming almost over the surface of the red giant's outter envelope of atmosphere).

 

[Umbran]

A tidally locked world should have little or no tilt - and so no seasons to speak of, much like its one face has no day and its other face no night to speak of.

Yes. We touched on that earlier in the thread. Lacking day and night, or seasons, some folks wanted something else to mark time in the system that the biology and cultures could key on - we've discussed moons, and now perhaps another star nearby, to help add something along those lines.

However, red dwarves live a *very* long time. The eldest ones still have more than 10x the current age of the universe before there is any chance of them going out. So why not a captured one that formed its planets late and then joined another star

Capture would be the obvious way to end up in this arrangement, yes.

Note, however, that even an F type star would not give a notable amount of heat at Jovian distances.

I don't think we are so much worried about heat as illumination - something animals could see by, or that might run a slow plant ecology - as much visible light as given by a full Moon would probably be enough for our purposes.

And, honestly, while we are using a real physical setup for inspiration, we're considering it for a *fantasy* game setting. I don't think we are worried about exact stellar spectral types, or exact distances. I brought up the Jovian example merely as something folks could easily visualize.

 

[Nyeshet]

I don't think we are so much worried about heat as illumination - something animals could see by, or that might run a slow plant ecology - as much visible light as given by a full Moon would probably be enough for our purposes.

And, honestly, while we are using a real physical setup for inspiration, we're considering it for a *fantasy* game setting. I don't think we are worried about exact stellar spectral types, or exact distances. I brought up the Jovian example merely as something folks could easily visualize.

Okay, but to be technically just one more time ...

All below presume a world 5 AU distant from a star of the stated solar mass.

Mass. Type. Lum . . Notes

2.0 . A . . -26.8 . 1.25 Ba out of 1.44 Ba lifespan, sun as seen from earth

1.5 . A/F . -25.5 . 2.52 Ba out of 3.21 Ba lifespan, sun as seen from Mars (-25.6, aphelion)

1.25. F5. . -25.0 . 4.51 Ba out of 5.01 Ba lifespan, sun as seen from (almost) Mars

1.0 . G . . -23.45 .4.64 Ba out of 10.0 Ba lifespan, sun as seen from ~Jupiter


Considering the pictures returned from Mars, it looks like you can get as much illumination as you need with just an F type type (F0 to F5), but a Sun-like star will only be enough if you think photosynthesis would work on a warmer Europa. The full moon may give just enough light for dark adjusted eyes to notice shadows, but plants don't grow much under its light (and the sun as seen from Pluto is quite a bit brighter - and just the brightest star in the night sky from Pluto's point of view).

 

[Umbran]

Considering the pictures returned from Mars, it looks like you can get as much illumination as you need with just an F type type (F0 to F5), but a Sun-like star will only be enough if you think photosynthesis would work on a warmer Europa.

Fantasy game, remember? Fire breathing dragons, and such? The biology already doesn't fit Earth norms.

We don't need scientific accuracy, we need it plausible enough to not get in the way of drama and developing story.

The full moon may give just enough light for dark adjusted eyes to notice shadows, but plants don't grow much under its light (and the sun as seen from Pluto is quite a bit brighter - and just the brightest star in the night sky from Pluto's point of view).

I don't know about where you live, but in Boston, the full moon on a clear night gives more than enough light to notice shadows; it gives enough light to read by - I was just noting that here two nights ago, even.

I think that sounds like plenty for a magically endowed world. Almost an unseelie fey feel, even, under that dim pale light in an otherwise eternal night...