Making a world with a year-long day

[Landis963]

Hello all.  

This is, I guess, the second leg of my quest to create a Cosmere world in D&D.  Now, for reasons of my own understanding (namely, "because I think its cool") I want my primary world to have a year longer than its day.  Not tidally locked a la Taldain, but definitely long enough for the change from day to night, or from night to day, to be a momentous occasion, possibly comprising several days.  However, and here's the rub, I also want this world to be livable by humans.  

I've got a couple questions with regard to how such a system would be laid out, and if there would be other planets involved. Note that all of these questions are coming from a layman's perspective.   

1. What color is the star?  Obviously making it too bright means that the habitable zone is farther out, and that means a longer distance that the planet needs to travel before it completes a full rotation.  Conversely, making it too dim pushes the habitable zone close enough to the sun that solar weather starts to become an issue, and I'd rather avoid that particular headache.  
2. How large/dense is the planet?  Once we've determined the size and color of the star, we can then start looking into how large a planet needs to be to settle into the resultant habitable zone.  Too small and it flies out of the habitable zone in one direction, too large and it flies out in the other direction.  
3. Is there anything about the planet that would regulate temperatures?  Venus' scorching temperatures are mostly due to its thick cloud cover, so instituting such a dense atmosphere could be useful in making it play nicely with life.  These clouds could be Invested or not, as this is so early in the creation process that basically everything is in flux.  
4. How many moons?  At least one moon will be necessary for several reasons (ease of timekeeping for the players chief among them, although I currently have it playing a role in the behind-the-scenes of the story).  
5. Are there any other planets in the system?  As this is a D&D world, I'm currently assuming that the outer planes are locations within the Cognitive Realm.  Which means at least two other planets (for the Hells and Heavens respectively) and between 4 (the elemental planes) and 6 (the elemental planes +plus Feywild and Shadowfell) others exist somewhere within hiking distance of the Prime Material's subastral.  These other planets (assuming we pack them all into one solar system, which seems a stretch) might have effects in the Physical as well, especially with regard to the stats of the star we agree upon.  
6. Is there some math formulae we can use for all of this?  It would help immensely if we could just define a variable or two, plug them in, and go.  

[physicskid]

1) if you want a short year you need to be very close to the star. This implies a cool star so you don't burn up, probably a red dwarf. This star would look pink or red to a human standing of the surface but local life would have eyes that work further toward the infrared.

2) This is basically determined by the composition of the planet, ie whatever you want it to be. I think a water world would be interesting from a story perspective (no metals framing would be interesting) and the excess water might help with temperature stability.

3) long nights means very cold nights and long days means very hot days. Temperature is stabilized by large bodies of water and thick atmospheres but it's still a problem and an atmosphere thick enough to help would ruin your sunrise. one interesting idea would be for the oceans to freeze and life to hide underwater and underground for the week long night. sunrise would be a very welcome sight indeed.

4)moons give tides and if tides are big enough they can cause geothermal heating. other than the consequences of tides/ rotational stability I can't think of any reason it matters. if you want multiple moons consider giving them resonant orbits like the Galilean moons.

5) no reason why not. bear in mind that orbits can't get too close to each other and gas giants are going to really gravitationally sculpt the system if you really want to get into mapping it though. also look up the Trappist system. red dwarf star bunch of habitable (maybe) planets.

https://en.wikipedia.org/wiki/TRAPPIST-1

6)check out wolfram alpha. and if you're interested in hard Science fiction try the Atomic Rockets site.

http://www.projectrho.com/public_html/rocket/

 

great to see someone looking at plausibility in their scenarios. have an upvote

Edited May 24, 2017 by physicskid
added postscript

[Eagle of the Forest Path]

On question 2, bear in mind that there would be little to no centrifugal force on this planet, so if you want gravity around 1G your planet has to be lighter than earth.

For 3, it depends on the type of cloud cover. Greenhouse gasses (sorry, I can't be more specific) allow heat in and keep it in (like on Venus, as you say); volcanic clouds keep heat out (like on Scadrial). By using either of these you can get more play on the type of star and your planet's orbit radius.

You can probably find more on https://worldbuilding.stackexchange.com/

[Jofwu]

Most of this is totally up to you, honestly.

1. Color/size of star just depends on how long you want the day/year to be.

Here's a nice little chart for comparison. Size increases from bottom left to top right. Consider the extreme case of the recent TRAPPIST-1 announcement. It's a star all the way down near DX Cancri on that chart. It has seven planets crammed around it, three of the m the habitable zone. The whole system is well within the orbit of Mercury. The planet orbits are each just a few days long and the habitable planets are fairly close in size to Earth. Note that, in order to cram this many sizeable planets into a small place the orbits have to be somewhat resonant.

So that's the extreme case of a red dwarf with Earth-sized planets. You probably want something bigger than that. For a sunset on Earth, the Sun moves roughly 20 degrees across the sky to go from day to night. Assuming that's about the same for your star/planet, and you want the sunset/sunrise to take 168 hours (7 Earth-days) then you need a 3024 hour solar day (126 Earth-days). Decide how long you want the year to be and then you can calculate the length of the sidereal day. (i.e. how fast the planet needs to spin)

Of course you can achieve a long day no matter how long your year is. Year length and rotation speed both play into the length of a solar day (and thus how long a sunset/sunrise takes). As long as the rotational speed is reasonable (say 1 rotation per 6 hours max speed) you can take a year length and reach the solar day you want.

2. Planet size is pretty flexible. Like I said, there's evidence of Earth-sized planets around very small stars. A planet doesn't fly away or fall into the star because of it's size. That doesn't really come into play unless the planet is comparable in size to the star, which it won't be. Density is pretty flexible.

3. A thick atmosphere would certainly help with keeping temperatures uniform, but you don't want to go the direction of Venus because then you get ridiculously high air pressures at the surface. A bit thicker than Earth should be fine. Go as high as you can on greenhouse gasses without getting unrealistic for life.

4. Moons is up to you, but more moons means stability is complicated. You don't have lots of moons on terrestrial plans. The Moon makes the possibility of other moons around Earth difficult. A second moon would have to be very small and very close to Earth or small and much further out. Mars has two moons that are both very small. Roshar has three small moons in close elliptical orbits, but they're also technically unstable.

5. As you can see with TRAPPIST-1, other planets are totally possible. They might need to be in resonant orbits though.

6. A formula for what exactly? There's no catch-all formula for planetary science.

[Landis963]

I'm seeing a lot of "just pick one," so I think I'll just start picking ones.  

1.  My current thought is a K star, dimmer than Sol but not so much that it becomes a red giant or similar.  Using the star graph that @jofwu provided, I'm thinking it would be along the main sequence, possibly in the neighborhood of Eridani 9.  

2.  While a water world would be interesting, and I am currently toying with the idea of having the starting continent be an archipelago, I don't think I'd like the entire world to be ocean.  Besides, having year-long nights and human-habitable environments means that water will freeze into glaciers when pointed away from the star.  With all that said, I suspect my world will be smaller than Earth, somewhat.  Say .95 Earth mass.  

3.  Thick cloud cover (if uniform) would indeed make sunrise and sunset difficult to discern, in addition to blocking out the view of any moon or stars.  However, if they are Invested, then anything else that is Invested with the same power might shine through.  The question then becomes "what proportion does cloud cover have to greenhouse gas presence?"  

4.  My current draft of the system has one moon orbiting the planet that is Invested by a Shard - or rather the equivalent of such in this galaxy.  As such, I have no problem with its light penetrating through any cloud cover.  

5.  In terms of the Heavens and Hells, all I need are 2 other planets with subastrals.  Which means two other planets with enough Cognitive presence to create a subastral, which means they're probably in the habitable zone around the star.   Resonant orbits are probably a good idea, both thematically and in general, so that's what I'll assume.  

[Jofwu]

11 minutes ago, Landis963 said:

3.  Thick cloud cover (if uniform) would indeed make sunrise and sunset difficult to discern, in addition to blocking out the view of any moon or stars.  However, if they are Invested, then anything else that is Invested with the same power might shine through.  The question then becomes "what proportion does cloud cover have to greenhouse gas presence?"  

Venus isn't warm because of cloud cover. That's not how the greenhouse effect works. What you have is sunlight shining through the atmosphere and warming up the surface normally. Warm things then radiate heat as infrared light. On a planet with no atmosphere, this light shoots back off into space. But some gasses found in an atmosphere (greenhouse gasses) absorb infrared light themselves. This means the heat has a hard time escaping the atmosphere.

Venus is particularly warm because (besides being closer to the sun) it has an atmosphere 100 times more massive than Earth's (and 90 times the pressure at the surface), and it's loaded with a higher percentage of greenhouse gasses as well. The clouds are just a byproduct of all this. So you don't need lots of clouds to keep your planet warm at night. Just say the atmosphere is a bit thicker than Earth, with a bit more greenhouse gasses.

An extensive ocean would also help a lot with temperatures, I believe. Ocean temperatures are very constant. Throw in some ocean currents that can bring warm water to landmasses on the other side of the planet. Consider how Europe is a temperate climate thanks to the Gulf Stream, despite being at the same latitude as Canada.

 

[Landis963]

@physicskid also mentioned the possibility of using tides to bring warm water from the day side to the night side, so that's a possibility as well.  

EDIT:Sticking to the topic of atmosphere for a moment: what natural conditions give rise to a thick atmosphere such as Venus'? How would life react?  I already know that a dimmer sun leads to darker plant life, but how would they grow under the influence of extra greenhouse gasses?  

Edited May 25, 2017 by Landis963

[physicskid]

Well for slight increases in atmospheric CO2 you get carbon greening. plants don't have to open their stoma as wide to get enough CO2 so they can use water more efficiently and photosynthesize faster. But there's really no data on plants in very thick Venus like atmospheres because we haven't found any yet. 

One idea for maintaining a more stable temperature is to get non-solar heating on the night side. maybe instead of a planet try a moon of a relatively close gas giant. tidal locking will give the moon a day equivalent to its orbital period around the gas giant but if the orbit is eccentric tidal flexion from the gas giant will help heat the oceans. you will also get TWO types of sunrise. one is an ordinary sunrise and one is when the sun emerges from being eclipsed by the gas giant. add other moons magnetic phenomena etc until you think the visuals are spectacular enough. (the tidal flexion heat would probably manifest as volcanism so maybe work that into your magic system?)

[Glamdring804]

On 5/23/2017 at 11:57 PM, Eagle of the Forest Path said:

On question 2, bear in mind that there would be little to no centrifugal force on this planet, so if you want gravity around 1G your planet has to be lighter than earth.

Actually, this effect would be too small to matter, on the order of about 0.3% weaker gravity. 

https://en.m.wikipedia.org/wiki/Gravity_of_Earth#Latitude

[Landis963]

Another instance of "Just Pick One":  Day length is about half the year.  I'm currently thinking the entire orbital period is 276 Earth days, which means 137 Earth days to go from dawn to dawn.  Judging from the formulae on that webpage you cited, I need to solve for the sidereal day thusly:  T[sidereal day]=(276*137)/(276+137)=37812/413=91.55 Earth days.  I might change that up later, I'm not certain, really.  And the orbital period itself changed multiple times (during the writing of the post) before I even got to the formula, so there we are.  I'm still thinking only one moon though.  Phases are basically useless because not only is the sun in the same position over multiple lunar orbits, but the moon itself is Invested enough to glow under its own power (Basically this is to allow plants a method of cheating their way through a 3-month night).  I don't believe I'll need a second moon - it seems like it would be distracting, although I might resurrect the possibility of the moon having a moon.    

Edited June 1, 2017 by Landis963
Got the math wrong.