I hope this is the right place to ask these questions. If not, please guide me in the right direction.

I am currently brainstorming up a solar system which has two habitable planets (it's a fantasy setting, so both planets have humans/humanoids, sort of "seeded" to the planet, but with slightly different plants/animals, etc).

Now, even though it's fantasy, I want to use some logical astronomy in order to create the worlds. I use the book Writer's Guide to Creating a Science Fiction Universe to get facts and figures, ideas, etc. It is a first edition print from 1993, so some of it may be out of date, but it's a good starting guide for me.

Numbers are in relation to either Sol or Earth

Anyway, this is the information I have so far:
Llayrah
The star is a blue F5 or thereabouts
Mass 1.3 solar masses
Luminosity 2.5 solar luminosity
Habitable zone- 1.15AU to 1.96AU

The two planets I am focusing on:
Llayrah 2
Distance 1.4AU
Insolation 1.27 Earth insolation
Year 1.45 Earth years
Mass 1.15 Earth mass
Radius 1.05 Earth radius
Density 0.99 earth density
Gravity 1.04 Earth gravity
Rotation (day) 1.35 Earth rotation
Axial tilt 17.5 degrees

On this planet, the years and days are longer, but the smaller axial tilt means the seaons are more mild. There would be a little stronger temperature change between night and day because the planet has more time to heat up or cool down, but it shouldn't adversely effect life. It's also the same density, so metals etc would be roughly the same as on Earth.

Llayrah 3
Distance 1.9AU
Insolation 0.69 Earth insolation
Year 2.29 Earth years
Mass 1.35 Earth mass
Radius 1.27 Earth radius
Density 0.65 earth density
Gravity 0.83 Earth gravity
Rotation (day) 0.95 Earth rotation
Axial tilt 27.9 degrees

On this planet, it receives much less energy from the sun, but I'm not entirely clear on how this would effect life. I'm sure that plants would evolve to manage just fine, but how would it effect other animals?

It's year is more than twice that of Earth, and together with it's higher axiel tilt, I am imagining that seasons would be much harsher, with summers being much hotter, and winter being much colder. Am I correct here? I want to make sure I understand this properly: the tilt effects how harsh the seasons are, and the year effects how long they last? I'm hoping it wouldn't be too much of a difference that it would make it uninhabitable, just more unpleasant. The tilt isn't much worse than here on Earth, but I could imagine 12+ month winters/summers to get mighty uncomfortable after a while.

The lower gravity means trees would be taller, people would fall for softly, etc. What other things would this effect? How much would it effect walking and running, or doing other physical activites? Or is it close enough to Earth normal that the animals would adapt and not be much bothered by it?

It's lower density is very intriguing to me. According to my book, this means there are less heavy metals, and I see this as making metal much more valuable than otherwise. The world is still sort of medieval, with a few slightly modern things- for instance, the people still fight with bows and arrows, swords, and things like that, but a few people have things like small firearms such as revolvers of shotguns (nothing remotely like high precision rifles and things like that though). How much of the people's evolution into an Earthlife modern world would be hindered due to the lower incidence of strong, heavy metals?


And a few other questions just in general.

My star is blue, and I know this would effect the world, giving it a slightly blue-tinge to it. Would this effect eyesight at all, or would people not really even notice it? For instance, if I were to stand on this world, it would probably look kind of surreal to me, and those people would probably find Earth to look odd because our sun is different. But in the grand scheme of things, would this make any noticable difference?

And for atmosphere: I would like my planets to have slightly different colored skies.

I'm sorry if this might sound a little daft, but is the color of our sky effected mostly by the gasses present in the atmosphere?

I am aware that oxygen is only about 21% of the air on the planet. On my fantasy worlds, could I realistically change the sky color without negatively effecting the oxygen content?

What if I wanted the sky on Llayrah 2 to be violet, and the sky on Llayrah 3 to be sort of butterscotch? Without having to do a breakdown of all the gasses in the atmosphere, could I just assume that these planets have that sky color (it is fantasy, afterall) without being too ridiculous about it?


To ramble just a little longer, for comparison, Mars is about 0.5AU from Earth, and my planets are also roughly 0.5AU from each other. Could I bring the farther planet in closer? What if I wanted Llayrah 3 to be only 0.25AU from Llayrah 2? Would this adversely effect the planets? Tidal forces? And how large would they look to each other at this distance? I ask about this because I realise that Llayrah 3 is quite far out, and that the much lower insolation from the sun could play havok with the photoreceptive plants and animals on the world. How close could they realistically be?

I know I haven't mentioned any moons or rings around my planets, but I want to work on these other, more important, things before I add more features to the sky.

I know this is very long, but I hope I haven't lost anyone, and I would greatly appreciate any advice on the sun and planets I've outlined, and any additional thoughts from people who know a lot more about astronomy than I do.

Thanks in advance

edit: I noticed a small error in my calculations with insolation, and I've corrected it here.

Hi Jessalynn,

good luck with your project. I think that it's great that you're trying to work out plausible physical expalantions for your planets.

Concerning sky color, there are a few things that would affect that. Our sky is blue because it has fine dust in the atmosphere which reflects blue light, but passes red light. I'm not sure how you would get a violet sky, especially on the bigger planet with the denser atmosphere. You could plausibly argue for a thinner atmosphere with some persistant very thin clouds that reflect red so they would superimpose some redness against a dark blue sky, giving an overall violet color.

For the butterscotch sky, that might work better with a cooler thicker atmosphere, such as what we saw on Titan.

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Haha, yes, I see I posted it in the wrong section. Thank you whoever moved it for me.

Antoniseb, thank you. The violet color was just something I thought up on the spot, so it's not set in stone. Would the gasses making up the atmosphere effect the color of the sky? I admit I'm not really very knowledgable about this sort of thing, so I don't really know where to start to get the colors I have in mind.

The butterscotch idea came from a thread I read about Mars, and the color of its atmosphere, and again was pretty arbitrary.

Mainly, I want a different sky color because I can't imagine that every lifebearing planet would have a blue sky like ours, plus I like the idea of playing around with different colors and imagining how it would effect the way things look. I have the program Terragen on my machine, and it's really fun changing the sun settings on it to see how it looks in the final render.

Probably not, however, it is possible. Most of the plausible gasses are transparant. You could imagine Bromine gas giving a kind of brown color, but it's not very plausible to have a global supply of bromine suspended in the upper atmosphere. You could somehow contrive to have whatever it is that gives Jupiter's red spot it's red color to be in the upper atmosphere.

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If you want the atmosphere to be breatheable, I think you're pretty much stuck with a blue sky. You could change the color by dust suspended high in the atmosphere like Mars but otherwiese, no go. Mars' atmosphere should actually be a dark blue except for its dust.

The planets themselves can actually come relatively close to each other and not effect each other too bad. True over long periods (like millions of years), planets will influence each other's orbits, but it doesn't look like you're concerned with that long of a timespan. Earth and Mars are probably going to interact with each other within the next billion years, likely ejecting one from the solar sytem (probably Mars since is has a lower mass). You could have the two planets come close (even closer than 0.25 AU) and still have them stay in stable orbits for millenia.

For heavy metals, yes, overall there may be less heavy metals on a less dense planet, but remember, a planet is a big place. There should still be enough for any industrial projects.

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Some thoughts regarding the effect of the color of the sun:

This image shows the quantity of each color that reaches the earths surface:
http://www.ncsu.edu/scivis/lessons/c...gline-alt2.gif

The spectra is a combination of the light coming in from the sun, and the light getting through (which is a function of the chemistry of the atmosphere). If the chemistry is the same, the spectra will just have a smaller red, but bigger blue/UV.

from this picture:
http://astrofili.org/members/osserva...elo/foto22.JPG

i take it to mean that theres going to be about twice the blue light than red on your planet, compared to similar amounts of blue/red on earth.

I read this to mean that the sky will be a lot more purple/violet than ours.

This:
http://www.mbari.org/staff/ryjo/cosmos/Cabs.gif
is an absorbtion spectra of chlorophyll, with less red around the red absorpbtion is less likely to evolve, and plant pigments that 'evolve' would probably be brown/red-ish....

Im guessing a lot more UV, dont know how much UV 'earth-type' atmosphere (ozone layer etc) can handle, but i assuming more will get through but who knows. More UV might mean more mutations.... and there may be some issues with skin pigmentation...



N.B. I am not an expert

Thank you, that's interesting.

If I'm understanding this correctly, then I can have a violet or purple atmosphere without too much problem. It also means the plants would be not be green (I could go with red/yellow/brown plants without too much hassle... even here on Earth we have plants that are different colors, sometimes multiple colors, so I'm thinking it would be logical to reduce the greenery and bring out other colors. Perhaps red/yellow/orange/brown and other "warm" colors.).

The UV light/skin pigmentation I *think* I could get away without worrying too much about. As on Earth, too much time in the sun can cause sunburn and eventually cancer, so perhaps making it so the people stay inside or take a rest midway through the day, or wear extra protective clothing.

It's given me more things to think about, at least

Consider the evolutionary problems with a spicies that must have protective clothing to survive. Not saying that it can't be (maybe they didn't evolve there?), but it might be worth considering.

What would be the other options?

Both planets have humans and humanoids (typical fantasy drek, really).

The inner planet will get more energy from the sun, and the outer planet will get less, compared to Earth. Actually, someone else pointed out that I can change their orbits to closer than about 0.25AU, which would let me get their insolation closer to Earthlike without sacrificing what makes these planets different than Earth.

I'm not sure where these humans/humanoids evolved (as I haven't decided on that yet), so they would have to adapt to the environment while any animals that evolved there would already be suited to it.

BlackStar,

You have Private Messaging turned off, so I have to give you this warning here in the thread. "Geek Alert" is not within the boundaries of our overall be-nice policy. Please avoid making this kind of attack in the future.

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One problem I would exclude before startin g is the lifetime of the central star: how long does a star with 1.3 solar masses live; is this long enough to have life evolve to sufficient complexity?

That could create interesting twists:

• Some scientist(s) find(s) out that the biospheres on their planets couldn't have evolved during the liftime of their star, so they have to have been seeded (Oh my, I just hope no IDer gets wind of this ). Of course, they are prosecuted, their results are suppressed etc. (just like here )
• Life on both planets can evolve to sufficient complexity, but oly just before the evolution on their central star forces the two civilisations to develop interstellar spaceflight etc.

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Are both the civilisations going to be at the same technological level?

If the evolved indepently on their own home world, how does first contact go, or does one side keep their existance a secret?

I used to play a fantasy role play grame called Traveller from Games Designer Workshop, so you may find some useful stuff there if it is still in print for designing worlds.

There used to be a complex process for designing subsectors, which I computerised reducing a long evenings work designing them to a few seconds on the computer. However that was on a ZX Spectrum

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At 0.5 AU distance, Llayrah 2 would appear 37” (arcseconds) is diameter as viewed from Llayrah 3, and Llayrah 3 would appear 45” is diameter as viewed from Llayrah 2.

At 0.25 AU distance, Llayrah 2 would appear 74” is diameter as viewed from Llayrah 3, and Llayrah 3 would appear 89” is diameter as viewed from Llayrah 2.

For comparison, the largest Venus and Mars can appear from Earth is about 60” and 25” respectively.


That’s a good question. Based on the scant information I’ve been able to find, it seems a good guess for the lifetime of a 1.3 solar mass star is about 5 billion years, or about 1/2 as long as the Sun. If intelligent life took as long to evolve in the Llayrah system as here, then the star is likely nearing the end of its lifetime (though it could still be hundreds of millions of years away).

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The issues are what goes "in the eye of the beholder" and the "eye of the beholder".

The sun radiates more blue than violet. However, a hotter star, such as your F5, could radiate just slightly more violet than blue. An F0 might be about 5% more violet than blue (based on blackbody radiation).

The very small particles in the atmosphere will scatter the short wavelength light far better than the longer wavelengths. Violet scatters a little better than blue for this reason. We don't see a violet sky because the Sun's blue light intensity is greater than violet, and our eyes do not see violet as easily. This is why our sky is blue. Dust and an atmosphere of larger particles will change things and, no doubt, plays a key roll in the brownish carmel sky of Mars.

However, you have some evolutionary advantage. It could be assumed an evolutionary track for the eyes of your citizenry, and animals, would cause them to adjust to their star and see the violet easier. [In some cases, a non-matching adjustment can be better. For instance, white-tail deer have been shown to have the ability to easily see violet, possibly some u.v., too. This, along with their larger eyes, gives them some advantage from nocturnal predators and pre-dawn deer hunter movements.]

For additional celestial color, nebula can appear green in color to the human eye given enough illumination and oxygen [OII, OIII]. The BA did an article in Astronomy about this called Alien Skies. A much better eye could see other colors in nebula such as red and blue. [Note, if one could see all three mentioned, then a RGB mix could produce a full spread of colors]

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

It looks like my 5 billion year guess was pretty close. I just found this Web page which gives a formula for estimating star lifetimes. The formula is,

Lifetime = 1 / (star mass)^(p-1) * 10^10 years

where star mass is given in solar masses and the value of p varies between 3 and 4, with p = 3 for the rare massive stars (M* > 30 M_sun), and p = 4 for the more common low-mass stars (M* < 10 M_sun ). We therefore have,

Lifetime = 1 / (1.3)^(4-1) * 10^10 = 4.55 billion years.

It looks like the lifetime of the Llayrah system is about the current age of our solar system. Interestingly, if we assume intelligent life takes the same amount of time to evolve everywhere as it did on Earth, then a 1.3 solar mass star is the largest that can support an intelligent species.

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Actually it isn't dust in the atmosphere which causes the majority of the blue colour; it is the air molecules themselves. When the air is thin the air molecules are the right distance apart to cause Rayleigh scattering. So the sky appears blue mostly because of the gases in the atmosphere rather than the dust.

But if you want a butterscotch sky you could imagine a sky with lots of brownish dust like Mars; to get a green sky you could perhaps imagine a layer of 'sky-plankton', photosynthetic aerial organisms.

Why not have your population genetically adapted to hgh UV levels? Perhaps they could have glossy violet-black skin which appeared silvery-white when viewed in ultraviolet wavelengths.

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As eburacum said the color of the sky is blue because our atmosphere is made mostly by oxigen and nitrogen. Since the oxygen is only 21% I think you could obtain a breathable athmosphere having a different color by replacing the nitrogen with another gas: this will change the Rayleigh scattering and therefore the color of the sky. Another possibility is to have a thicker athmosphere; in this way you'll obtain an effect similar to what's happening when our sun is just above the horizon: because the light has a longer path in the athmosphere than when the sun is high in the sky the Rayleigh scattering become important also for the red light. That's why the evening sky appear red around the sun.