Help with a fictional Star System

[Bill Phil]

If the larger planet is less dense, and bigger, the gravity could be 1g. Size and mass both factor into the equations.

Magnetic fields are also not that important. The atmosphere would block a lot of the radiation.

[SargeRho]

That's the kind of reply I was hoping for

The Gas Dwarf is "inside" the habitable zone, like Tenorra, not "inside" it like Roasted and Toasted. Oh the wonderful flexibility of some words...

For Tenorra's largest moon, I wrote 'relatively thick' on purpose, instead of thick. It's still very tenuous, less than Mars' atmosphere in fact, basically just on the border of requiring a heat shield to land. They do indeed use space tugs and freighters propelled by solar-electric VASIMR engines. Or did, before they got Lazor'd by the invading fleet.

On the tenuous hydrogen atmosphere: Similar to Europa's oxygen atmosphere, perhaps slightly denser, basically it's just barely there.

By barely habitable I mean it's tidally locked, one side is mostly frozen over, the "hot pole" is, well, very hot. Its atmosphere is slightly less dense than Earth's, the surface pressure is somewhere around what we have on Earth at 4km Altitude, but less oxygen in the atmosphere than Earth. I chose the M8V dwarf exactly because it's on the border between a red dwarf and a high mass brown dwarf: I wanted to add a very unlikely, but technically possible planet in the universe - as I intend to make more games within the same universe if this one is successful, or at least write some short stories in it.

[RuBisCO]

Well, apparently we read different books A series by Anne McCaffrey i've read described a habitable planet, Earth-like and with two (or three, i do not remember now) hefty moons orbiting it. Everytime there was a conjunction of two (or stars forbid three) moons, surface of the planet became a hell hammered by hurricanes non-stop until conjunction ended. It was so bad everyone were forced to evacuate to underground bases or orbital stations - and any stragglers caught on the surface were considered as good as dead.

Again I'm talking about a binary planetary system of tidally locked worlds, your example does not cover that. The scenario has been used in sci-fi to portray two waring groups, people of either world fighting each other, specifically if one world is less habitable then the other it makes for drama between the "haves" world and "haves not" world

[Armchair Rocket Scientist]

The Gas Dwarf is "inside" the habitable zone, like Tenorra, not "inside" it like Roasted and Toasted. Oh the wonderful flexibility of some words...

Ahh, you mean the gas dwarf is in the inner portion of the HZ, while Tenorra is further out?

If the planets are that close together, it sounds likely that the gas dwarf and Tennora would have to be in an orbital resonance for the system to be stable. 3:4 or 2:3 are possibilities, since those are stable for Neptune, but they may not be stable when body 3 in the 3-body problem "Tennora" is over 10% the gas dwarf's mass.

For Tenorra's largest moon, I wrote 'relatively thick' on purpose, instead of thick. It's still very tenuous, less than Mars' atmosphere in fact, basically just on the border of requiring a heat shield to land. They do indeed use space tugs and freighters propelled by solar-electric VASIMR engines. Or did, before they got Lazor'd by the invading fleet.

Okay. Even so, your atmosphere is likely going to have lots of CO2 and maybe nitrogen, and Tennora will have some argon of its own. The only good reason to harvest argon from the moon would be if you don't want to deal with launching it from Tennora's surface. What technology do the Tennorans have for launching stuff into orbit, and how thick is Tennora's atmosphere?

On the tenuous hydrogen atmosphere: Similar to Europa's oxygen atmosphere, perhaps slightly denser, basically it's just barely there.

Ah. Depending on how how the planet is, you're likely to get additional gas species like sodium or calcium vapor (like on Mercury).

By barely habitable I mean it's tidally locked, one side is mostly frozen over, the "hot pole" is, well, very hot. Its atmosphere is slightly less dense than Earth's, the surface pressure is somewhere around what we have on Earth at 4km Altitude, but less oxygen in the atmosphere than Earth. I chose the M8V dwarf exactly because it's on the border between a red dwarf and a high mass brown dwarf: I wanted to add a very unlikely, but technically possible planet in the universe - as I intend to make more games within the same universe if this one is successful, or at least write some short stories in it.

Okey dokey. Even the thin atmosphere is going to distribute heat to the night side. I'm guessing it will have glaciers and a "polar" cap, but any ocean won't be frozen all the way down. This planet's orbital period may only be a few days long, so you could have a significant Coriolis effect as well. The "hot pole" may be habitable by thermophilic organisms. One note: your M8V star will radiate most of its energy in the infrared. It's possible that greenhouse gases like CO2 will reflect as much radiation as they keep in under these conditions.

[SargeRho]

Yep, the Gas Dwarf is on the inner edge, while Tenorra is near the outer edge of the habitable zone - and as a result has larger poles than Earth does, and they never ran into the global warming problem in the first place.

Laurentia - the most advanced country within the Tenorran Defense Union - has a fleet of SSTOs, which are mostly used for ferrying people and smaller cargoes, as well as a number of fully reusable BFRs. The surface pressure of Tenorra is almost identical to Earth's. The CPSN has a number of fully reusable 2-stage launch vehicles. While Tenorra does indeed have some argon herself, the overabundance on Rex combined with the lower atmospheric density and gravity makes it cheaper to produce it on Rex and send it up to the fuel depots than sending it up from Tenorra.

Edited June 9, 2015 by SargeRho

[YNM]

Actually, getting a gas giant that way (between two terrestrial planet) is really hard. You might want to put a larger star than Sun inside (maybe G0V or F9V), placing Tenorra on the inner edge of the HZ, a "dwarf" gas giant (or I'll use the term ice giant here as they're mostly lighter) some considerable distance from Tenorra (maybe within the outer edge of the HZ), then a large gas giant even further. Kind of Mars. The ice giant can have a fair amount of moons, and the Tenorrans civilized them. After those two giants comes more rocky "planets" (I'd suggest something kind of ceres and vesta instead, but make them larger), then the rest of the system. The brown dwarf can be at <200 AU, if you still want it to be the largest brown dwarf ever with a few planets coming with it.

If you want your original system to work, the HZ needs to be quite wide I guess, therefore a larger central star. K2V weights around 0.8 sun's mass and around half sun's luminosity. An F9V star mass around 1.2 Sun's mass and release ~5 times sun's luminosity, probably allowing a HZ 1.5 times wider than Sun's (and 1.5 times farther as well, get more inner planets).

Something which you might found subtle, but affect what worlds you can have, is the age of the system. If it's young, it's unstable (and your original scenario might works, but it'll change in really long terms); if it's old, it's stable, but then you need to calculate some things.

Edited June 9, 2015 by YNM

[SargeRho]

How about F5V class, like Procyon A? The Habitable Zone is I think fairly large on that one.

[YNM]

F5V might be too hot... 10 times Sun's luminosity yet only a HZ more wider by 1.7 times. And a surface temperature that gives mostly blue light - if you say they're humans then it'd cause some bad glares. A heavier planet would justify this down by getting a thicker atmosphere.

[SargeRho]

F9V it is then.