Plausibility check for a number of worlds in my 'verse.

[SMJB]

Here's the backstory of the overall scenario: Once upon a time (read: 4,000+ years in the future) there was a human civilization of immense and vague power. Probably not including things like FTL and artificial gravity, but I intend to leave that open to interpretation. What they did have was eternal life, cybernetic implants (a necessity for any human hoping to compete in a post-Singularity universe), and a zero percent crime rate courtesy of AI Gods whose tactics might be mind control. And rumors of a secret tech-destroying superweapon that would destroy civilization if said AI Gods ever went to war with one another. I think you can see where this is going. During the golden age of civilization, people would download any specific information they needed to do their jobs directly into their implants, being faster and more flexible than several years of schooling, and so when said implants were turned to slag, they lost the information, hence explaining why no one was able to rebuild civilization in spite of there being several million survivors per world (more before cannibalism). (Their immortality also ended, though, so by their standards there were no survivors.)

Scattered around this 'verse is a number of star systems. They don't have any contact with one another, FTL not existing--think of it as a number of separate 'verses which just happen to share a backstory. As my main purpose is to explore first-contact scenarios, most of the star systems I'm focusing on have multiple habitable planets (part of the reason I'm keeping the size of the civilized galaxy vague--it's however large it needs to be to plausibly have all of these worlds, and however old it needs to be to be that size). Now, here's what I've got:

Firstly, I have an F6 star, the relevant details of which are copied from Pi3 Orionis, giving it a luminosity of 2.822 (times the sun) and thus, according to this, a habitable zone stretching from 1.6678 to 2.8367 AU (being conservative). It has three worlds in this area, all of which were home to microbial but no multicellular life, making terraforming easy.

The innermost world is right on the edge of the habitable zone, 1.668 AU (giving it a year of ~2.154 years) and is very warm. The North Pole is located at one end of an Australia-sized continent, and the arctic sea is encased between this and two other continents, and is filled with islands. The temperature in these northern lands averages sixty degrees Fahrenheit (15 C), and the way the continents are arranged mitigate turbulent seas. It is much like the Mediterranean, but the Australia-sized continent is home to a Viking-like culture that raids the others. The technology level when this world is visited by people from the next world out is basically late bronze age/ early iron age. The original survivors had the genes for all of the skin colors presently extant on Earth and then some, but the oblique angle of the solar rays in their northerly environment selected for light skin. I'm thinking they're white, but then again, the warmth means they'll be wearing less clothes, and thus catching more rays...

The next world out is at 1.945 AU (with a year of ~2.713 years), though I'm considering having it switch places with the last planet if that makes the climate more plausible (humanity went extinct on the last planet and story-wise it's just a prop to show that the survivors didn't always survive; I haven't decided on any hard data on it because it hasn't been necessary). It is very cold: seventy degrees Fahrenheit (21 C) at the equator. The majority of the world's continental mass straddles the equator, and hence its highest civilization follows suit. Once again, the genes for all possible skin colors were present, but being a the equator meant direct solar rays, meaning the environment selected for darker skin. There are other ethnic groups from cooler environs, but the Blacks are dominant, in much the same way Whites are in our world. Being expies of modern First-Worlders, they have a history of exploiting minorities and feel real guilty about it. Their technology is about a century or so ahead of us, making them the most technologically advanced faction in all these stories, but they got into space late, between their planet's high gravity and lack of a moon.

The second star system is a double-star with its relevant information stolen from Alpha Centauri with one habitable world around each star. I don't currently have the exact orbital distances, years, and day lengths on me. (I've seriously got, like their calendar years plotted out and everything. speaking of day length, though, what's the shortest probable day length for a terrestrial planet?)

The world around the larger star is about halfway between Earth and Mars in orbital distance and was in a Snowball Earth phase before being terraformed, and there are lots of glacial lakes, some the size of the Mediterranean. There was a Pangaea-like supercontinent that started breaking up about ten million years ago, creating a network of narrow seas crisscrossing the landmass. When civilization developed on this world, it naturally spread along these channels, rivers, and lakes from one end of the supercontinent to the other, meaning the culture is largely homogenous. Between a three-sided cold war/arms race and a space-crazy ruling dynasty, one of the superpowers managed to create a ship capable of putting a colony on the other planet (when the stars are at their closest to one another) in spite of the world's technology otherwise being little better than ours and sometimes worse otherwise. The lack of cultural diversity has left them with a rather naïve view about imperialism.

The world around the smaller star has the same approximate orbit as Venus. Most of it's continental mass is also in one continent, but this one is bisected by the northern ice cap and has several distinct regions. The northeast of the eastern half of the major continental mass was a sea that got cut off from the ocean, and is home to Mongols nomadic herders. The northwest is Victorian Europe a series of peninsulas and islands which is home to one of the two highest cultures on the planet; the southwest is the home of the other. The southeast is...undecided, as of yet. The natives of the western half of the continental mass have the most open, most free culture in the solar system...and they're cannibals. I had fun with this world, giving them laser semaphores (hey, at least one mad scientist invented lasers before most people'd ever heard of electricity in real life) and difference engines. The world is also one of the very rare ones in this 'verse with native life, and the exoskeleton of the local land lobsters, which they use for armor, just so happen to be made of something a lot like kevlar and so bulletproof (well, as far as man-portable black-powder weapons are concerned, anyway), so the age of knights hasn't ended yet. They also use coal, being the only world yet touched upon that even has coal deposits. So yeah, lots here to make experts from the last world be very wrong in their expectations.

The next system I haven't decided on a star for because I don't have to calculate any interplanetary journeys for the--it's a double planet, so the worlds are only 1.5 million kilometers from one another. The colonization of the lower-tech world begins when the higher-tech world develops Apollo-type technology, which they do before they invent color TV. These worlds are not (supposed to be) tidally locked. Incidentally, here's a specific thing I could use help with: how long is a "month" for this world?

This last one I haven't really worked on all that much, but I might as well include it because it's in the same verse and if anything I need more help with it than all the others, albeit it's more of the "What the hell am I doing?" than the "Do you see any glaring holes in my logic?" sort of help. This one is not a first contact story, so there is only one planet. It's a pure "space story", whereas with the rest space travel is just how we get from point A to point B. Between the draw of a whole bunch of moons and a ring system to exploit, a less strenuous 8 mps^2 gravity to fight against, and a rather heated space race, the people of this world managed to get into space and stay there--which is good, because their planet got hit by an asteroid. So basically it's a world in space. I want four moons averaging about half of our moon's mass and a ring, all in stable orbits around this smaller-than-earth planet.

[Simon_Jester]

The second star system is a double-star with its relevant information stolen from Alpha Centauri with one habitable world around each star. I don't currently have the exact orbital distances, years, and day lengths on me. (I've seriously got, like their calendar years plotted out and everything. speaking of day length, though, what's the shortest probable day length for a terrestrial planet?)

The early Earth had a day length of around six hours- the main thing that's slowed it down is the moon's gravity dragging on the bulges created by the tides. Does your planet have weaker tides, and/or a shorter history since its formation?

I had fun with this world, giving them laser semaphores (hey, at least one mad scientist invented lasers before most people'd ever heard of electricity in real life)

Er... citation needed? It's basically impossible to build a working laser without refined knowledge of atomic physics and quantum mechanics.

The next system I haven't decided on a star for because I don't have to calculate any interplanetary journeys for the--it's a double planet, so the worlds are only 1.5 million kilometers from one another. The colonization of the lower-tech world begins when the higher-tech world develops Apollo-type technology, which they do before they invent color TV. These worlds are not (supposed to be) tidally locked. Incidentally, here's a specific thing I could use help with: how long is a "month" for this world?

Without knowing the masses of the planets, there's no way to know. Also, keeping them not-tidally-locked could be rather hard, if we want to stick to fairly rigorous planetology.

This last one I haven't really worked on all that much, but I might as well include it because it's in the same verse and if anything I need more help with it than all the others, albeit it's more of the "What the hell am I doing?" than the "Do you see any glaring holes in my logic?" sort of help. This one is not a first contact story, so there is only one planet. It's a pure "space story", whereas with the rest space travel is just how we get from point A to point B. Between the draw of a whole bunch of moons and a ring system to exploit, a less strenuous 8 mps^2 gravity to fight against, and a rather heated space race, the people of this world managed to get into space and stay there--which is good, because their planet got hit by an asteroid. So basically it's a world in space. I want four moons averaging about half of our moon's mass and a ring, all in stable orbits around this smaller-than-earth planet.

I'm not entirely sure how stable that would be over many thousands of years...

[chornedsnorkack]

The second star system is a double-star with its relevant information stolen from Alpha Centauri with one habitable world around each star. I don't currently have the exact orbital distances, years, and day lengths on me. (I've seriously got, like their calendar years plotted out and everything. speaking of day length, though, what's the shortest probable day length for a terrestrial planet?)

The world around the larger star is about halfway between Earth and Mars in orbital distance and was in a Snowball Earth phase before being terraformed, and there are lots of glacial lakes, some the size of the Mediterranean. There was a Pangaea-like supercontinent that started breaking up about ten million years ago, creating a network of narrow seas crisscrossing the landmass.

At Toliman A, Earth equivalent heating is at 1,23 AU - Mars is at 1,52 AU.
At that distance, the orbit would be 1,305 Earth years, meaning 476,5 Earth days. Or 61,2 years in the 79,9 Earth year orbital period of AB... close to 60 - Year of Water Snake?
Naturally, slightly cooler climate at slightly longer distance would also mean slightly longer year/fewer years per orbit.
Also note that if the orbit of Ab is slightly inclined to the orbit of AB, there would be precession, with period of about 4000 years.

This would cause variability of axial tilt... and variability of climate, even in absence of any terraforming.

Concerning day length - interestingly the minor planets are clustered around 7...8 hours, and all major planets are slower.

The world around the smaller star has the same approximate orbit as Venus.

Meaning close to Earth equivalent heating - Earth equivalent would be 0,707 AU, Venus is at 0,723 AU.
Assuming 0,907 solar masses for B - for some reasons, there are various estimates as far as 0,934 - the distance of Venus would mean 0,6455 Earth year period, meaning 236 Earth days, and also 123,8 years per orbit.
Also note that the ratio of orbital periods between Ab and Bc (Bb is the hot super-Mercury at about 0,042 AU) is close to 2:1. It can be interesting what the exact period is... although I do not expect a real, physical resonance.

The next system I haven't decided on a star for because I don't have to calculate any interplanetary journeys for the--it's a double planet, so the worlds are only 1.5 million kilometers from one another. The colonization of the lower-tech world begins when the higher-tech world develops Apollo-type technology, which they do before they invent color TV. These worlds are not (supposed to be) tidally locked. Incidentally, here's a specific thing I could use help with: how long is a "month" for this world?

For 1 Earth mass each and a distance exactly 4 times Earth-Moon (slightly more than the quoted exactly 1,5 millions), it would be about 5,65 (sideric) months. At this kind of periods, lack of tidal lock gets plausible.