Possibility: killing FTL via slowboat colonists

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Re: Possibility: killing FTL via slowboat colonists

Post by Destructionator »

Somes J wrote:The dwarf is supposed to be similar to Wolf 359, right? Looking at Solstation, Wolf 359's habitable zone is .005 AU, or only 750,000 km! A quick calculation tells me that if you were 50 million km (1/3 of an AU) from the red dwarf and 40 AU from the Amillian sun you'd be getting more heat from the Amillian sun!
Wow. Yeah, Wolf 359 is what I've been running with - it is small and dim, meaning its effect on everything else is fairly small, while still being there.

But eeek, the radius of Saturn + radius of Wolf 359 ~= 200,000 km. So that's not very far out at all. I usually neglect the radius of the two bodies when talking about stars, since usually it is nowhere near, but man, not so here.

I want the combined radius to be much smaller than the distance. So if we peg the distance at radius * 100 = 20,000,000 km = ~0.14 AU.


The star is going to have to be a lot brighter it looks.


Let's just calculate a luminosity. Assume we want Earth like solar flux at that distance; 1.4 kW / m^2. The sphere at .14 AU has a surface area of... 4 pi r^2 = 5.5e21 m^2. Multiply that by our flux to get a total solar output of 7.7e24 watts.


Wolf 359's output is ~ 4e23 W. So to make this work, the star has to be about 20x brighter. (That makes it 2% that of our sun.) Know any real life candidate star with similar output that we can yank some numbers from?



Let's go back to how bright this would be in the amillian sky. We'll just use the power output divided by the surface area of a sphere with radius of 30 AU: 0.03 W / m^2. 1/47000th that of the sun. That works out to apparent magnitude of -15.04; over 6x brighter than the full moon. Wow.

What about at the far point of its orbit, at 60 AU? Inverse square means it will be 1/4 that, since distance doubled. Incredible, it is still brighter than the full moon! You'd only get truly dark nights during the times of the year when the suns are lined up (so the red star is only up during the day).

Maybe we should move it farther out. It'd make a mission there even longer, but at over 30 years one way (with Hohmann orbits), we're already looking at something much bigger than a 5 year mission, so it is all about the long haul as it is.


Since cranking up the brightness probably means cranking up the mass, moving it out helps stability of the inner system too, so probably a double good move.




I went looking for a binary star to look at distance, and came across Struve 2398 B on Wikipedia. It's luminosity is in the right ballpark. Its mass is .3 sol and its radius is .54 sol. Average distance of about 56 astronomical units,[citation needed] and the eccentricity of the orbit is 0.70. It says it gives out x-ray flares, mang (i guess wolf 359 does too). But at least here's some ballpark numbers to work with. It is also a red dwarf.

So 72 AU far out and 40 AU near. Magnitude varying from -14.4 to -13.1. So between about that of the full moon to almost 3x the full moon. Not too bad.



But yeah, this result gives some more interesting fun in that dwarf's orbit, so yay. I'll adjust my simulator for the new numbers and see what happens tomorrow.

Could you even get a stable orbit that close? I tried running a 750,000 km orbit on a .1 Sol mass star on gravity simulator and it wasn't stable. Then again, I didn't adjust the sun's diameter.
Mine agrees, for what it's worth. The new numbers above should give more fun to play with; just hopefully, it won't throw Saturn out of the system too now!
Maybe a tide-locked Mercury-like world. The nightside might be a significant source of volatiles in the form of frozen volatiles left behind by impactors in craters - since it that side of the planet never faces the sun such ice could easily exist despite the close proximity to the sun.
Me like.
Would imply higher tides, unless it was farther away from the planet. Just something to consider.
Right - that's part of why it is there. The tides will be rather mean to it. I'll have to crunch some numbers and see how mean.
I'm thinking a Mars-like world.
Could do it. I'm thinking about moving the asteroid belt inward too, but it could. See, if I can pull it off, I'd like to move the first jovian a bit in, and it can shove the asteroid belt inward with it. This makes them more easily accessible for manned missions :P
I'm thinking maybe a smaller world, more like Uranus/Neptune sized or somewhat bigger (30-40 Earth masses), or maybe even a large icy superterrestrial (< 8 Earth masses) instead of a gas giant. I'm thinking that the gravitational effects of the binary may have stunted the planet's growth somewhat, in the same way Jupiter might have done for Mars. Also it makes the system seem a bit less like a complete mirror image of ours, which I think makes it more interesting.
Yes, certainly. Let's do the superearth out there.
I also think a nice idea would be to have an outer icy asteroid belt beyond the final gas giant, where the gravity of the companion star may have prevented a planet from conglomerating, again in something of a mirror of the effect the gas giant has on the inner system.
win. It can have a nice moon system too perhaps.
Suggestion: a hot tidelocked superterrestrial planet inside the orbit of the hot Jupiter. This fits with a theory that a hot Jupiter may push material in front of itself as it spirals in, leading to the formation of hot inner terrestrial planet from that material. And it's just an idea I find appealing. Especially since that close to the star the hot Jupiter may not be able to retain any moons, so it gives something interesting in the vicinity besides the gas giant itself.
I also think the hot Jupiter should be moved further in if we want it to actually be hot. .3 AU isn't going to be particularly hot even for a large red dwarf. The M2 (relatively big and bright) red dwarf Lalande 21185 has a habitable zone at .22 AU according to Solstation. I'd make its orbit < .1 AU.
Hell, I asked for a candidate star above, and you have one here that is decent. Mass and radius are both 0.46 sol, luminosity 0.025 sol, so a bit more than my one above, but not by much.

As a hot Jupiter migrates inward it may drag in large quantities of volatile-rich material, leading to quite wet terrestrial planets. I imagine the red dwarf might have several icy terrestrial planets, and beyond that there might be a family of small gas giants (maybe Uranus/Neptune sized). With up to 3 AU to form in and such a dim star this might be quite an extensive family. If we go with the outer planet stunting model, there might also be an icy superterrestrial planet close to the edge of the system, inward of the outer asteroid belt. So perhaps:
With a brighter star, they'll be even more fun room. sqrt(20) = 4.4, so it could probably go out further. Maybe; depends on what the other sun does.


[snip]
This all sounds really good though. a little more tweaking to the star and we'll have something out here.
Also: clarification: do you want a true hot Jupiter or more of a hot Neptune or maybe a hot Saturn? Personally I lean toward the latter; they might be more common in the universe.
I'm open to anything cool. I've been assuming Neptune in my calcs, but anything works.
If you want something a bit more interesting than a Mercury-like world, perhaps the innermost planet could be a tidelocked Venus-like world. Alternately we could have a terrestrial planet that had suffered a truly massive impact, knocking off its outer layers, so it could indeed be like Mercury but closer to the size of Mars or larger, perhaps large enough to retain some sort of atmosphere. A Mars-sized or larger metal-rich and dense tidelocked world with a thin atmosphere would be nicely unlike anything in our solar system.
That sounds cool.
I don't think swapping the planets should be necessary so you can have rings - I don't see why a Jupiter-sized planet couldn't have a Saturn-like ring system.
Right; hell, Saturn is pretty close to Jupiter sized itself.


So in the morning, I'll see about updating my simulation and collecting it all again. This is so cool.
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Re: Possibility: killing FTL via slowboat colonists

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I'm focused on placing the habitats too. I've been saying 0.9 AU again and again here, but it is based on intuition, which is wrong.

So I'm crunching the numbers for travel out there. For something close to Earth, the travel time can be somewhat small, but the launch windows are infrequent.

The farther away you get, the more frequently you get launch windows.

See, in the close case, the target and destination move at about the same speed. When they are close together, this is good since you can get to it. But it isn't exactly the same speed - that gap will open up. Then since they are going close to the same speed, it takes a long time for that gap to close again.

Doing a Hohmann to something nearby would also be slow; it depends on using a half-orbit, which is bigger for the farther out you go. For something inside, this means it is actually faster to get to something farther away than close by!

For example, hohmann to Mercury is faster than hohmann to Venus! Because the orbit moves faster the closer in you are.

So for the nearby case, the window opens up rarely and the hohmann is slooow, but it is a small enough distance that you can do a burn / flip / burn for a reasonable delta-v and make good speed on it.


But, with the Hohmann result, I'm thinking about putting the human colonies closer to the sun. Then, they'd get super abundant solar power (but they really get that anywhere in the inner system) and the travel time isn't so bad.


I figure their final position will depend on just how the solar system takes shape, but just submitting this before I forget its there.
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Re: Possibility: killing FTL via slowboat colonists

Post by Somes J »

Hmm, suggestion:

Amillian sun system:
Tidelocked .2-.3 Earth mass iron-rich world with thin CO2 atmosphere.
Amillia and large moon
Inner asteroid belt
Jovian
Superterrestrial
Outer asteroid belt

Red dwarf:
Tidelocked inner superterrestrial
Hot neptune
Volatile-rich terrestrial planets (number?)
Small gas giants (number?)
Outer superterrestrial
Asteroid belt

I like it: it's in some ways similar to our solar system but different enough to seem alien.

Some possibilities:

Doing a calculation of the mass of Titan, a Jupiter with most of the mass of its satellite system concentrated in a single satellite might have a satellite up to ~.07 Earth masses - that's creeping toward the ballpark of Mars-sized. The lack of a Mars in the Amillian system suggests its gas giant may be a superjovian, in which case maximum moon mass might be larger still. One wonders what a Mars-sized Jovian moon might look like.

Large superterrestrials may possibly have small moons formed in the same way gas giant moons do. Again going off the mass of Titan a 7 Earth mass superterrestrial (close to the hydrogen retaining threshold which is 8 Earth masses IIRC) has a maximum moon mass almost 3 times the mass of Uranus's largest moon.
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Re: Possibility: killing FTL via slowboat colonists

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In my brightness in the sky calculation, I realized I made a big mistake: I did it from the star's total output, but neglected that much of that will be in wavelengths not visible to the human eye.

Adjusting for that, at 40 AU, the red dwarf will be closer to -11.6 magnitude; back to about 1/2.5 that of the full moon. So this is good. If I use the star in your post, it works out to -13.1; a bit brighter than the full moon. But we're in the right area now.

If you could see infrared light too, it'd be more like 6x the full moon, but humans can't so it is more like that of the half moon.

Big difference there, I can't believe I forgot about it yesterday!


Native life, hunting at night, might be adapted to use infrared. When the companion star is far out it won't help much, but given that it may be helpful for seeing the heat of your target, it isn't necessarily useless there anyway.
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Re: Possibility: killing FTL via slowboat colonists

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I used this lengthy equation in google to figure where the ice line would be at the red dwarf:

((3.846×10^26 W * .02) / (4*pi*(0.7 AU)^2)) / (2* 5.67 * 10^-8 W m^-2 K^-4 ))^(1/4) in kelvin

More simplified, it is energy absorbed / 4th root of (2 * stefan boltzmann constant)

Energy absorbed = (luminosity of sol * percentage of our sun) / (surface area of sphere at the given distance)

And it gives temperature in kelvin.


Surface area actually cancels out here. Power in = power out. Power in depends on surface area, and power out depends on surface area... it cancels out. All that remains is a factor of two (which is where 2x the stefan boltzmann constant came from), since only half it points at the sun.

This assumes spheres that are ideal conductors. good enough for this.



Anyway, the point is a 0.7 AU from that little star, the ideal sphere there goes to 150 k, which is about where the ice line is; methane, etc. freezes. So we'll say gas giants formed around it beyond that line.


This leads to my current version of the solar system:


Sun at the center, copy/paste of our own

Inner most planet: .4 AU, 0.2 earth masses

Earth: 1 AU, 1.1 earth mass (give or take). has a big moon like us.

asteroid belt, between about 1.5 and 3.5 AU. It holds fairly stable orbits; a few outer ones got ripped off by the jovian, but most were stable enough for an asteroid belt.

Large jovian, 3.9 AU, 1/1000 mass of the sun (that's 1.05 the mass of Jupiter). Has an impressive ring system and shitloads of moons.

Super earth, 10 AU, 7x earth masses. has shitloads of moons.

Large ice belt, has a bunch of objects on very elliptical orbits (the dwarf star skews them all to an eccentricity more similar to its own in my simulation). The simulation says an ice belt starting at 12 AU will be shepheaded a bit by the big rock, and shoved a fair amount by the dwarf, but not quite out of it... it put the ice objects generally on elliptical orbits from 9 - 15 AU or so. Yes, they overlap the superterrestrial a bit. This seems important - without it there to stabilize them into a kind of ring, they fall out of the system or into the sun.

If you much beyond that, you become comet material, either for the red dwarf (it sometimes captures them for itself, highly elliptical, usually ejected after one revolution around the main sun), or for the main sun as the dwarf steals their speed and they fall in. The ones that fall in tend to be screwed again when the dwarf returns; their orbits are pretty random.


RED DWARF, between 45 and 180 AU eccentric orbit. 0.3 solar masses, 2% solar luminosity
orbiting it:

Tidelocked planet: 0.07 AU, 4x Earth mass

Hop Neptune: 0.14 AU, 1/10000 mass of sun (about twice that of our Neptune)

Rich rocky planet: 0.4 AU, 0.6 earth masses

Another rocky planet: 0.6 AU, 0.1 earth masses

Outer gas giant (formed here naturally): 1.7 AU, 1/4 Jupiter mass, has a fair number of moons.

Beyond this: another ice belt. I'd peg it at about 4 AU out to 7 AU or so just because it is a pretty number. And the simulation shows it to be pretty stable, so cool.


Back to the main sun: 250 AU out: the oort cloud (or whatever it'd be called). It has to be this far out to avoid the red dwarf from destroying it. Each revolution of the dwarf, it captures and shoves a few comets in, and shoves some right out of the system, but things are slow enough moving out there, and not so dense that it can do this for a long time.

Wow, this is weird to watch. Some of them have their orbits reversed by the dwarf; it pulls them in, then slingshots them into retrograde orbits.


It might have to be even further out, since 250 AU has them just scattered throughout the system in the long run.

wooooooow some of these comets are now around 1000 AU out and still coming back. I didn't think my program would have enough precision for that! But some are most certainly ejected entirely, having escape velocity calculated.

But starting at 250 au you have a chance of bring a survivor object in the long run, so we can call that the start of the outer cloud anyway.
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Re: Possibility: killing FTL via slowboat colonists

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btw, imagine a planetarium like 100 meters or more wide, and a full sphere. It uses digital projection; the sphere is a giant lcd like screen, so there's no projector in the middle.

The sphere does not rotate, so inside it is zero g. You can rotate about however you want. Using little fan boosters, you can move around freely without handholds, though a little grid of some sort might be good, to guide the audience into their "seats".


This thing could let you go anywhere imaginable. If it has a floor option, you could even project vast scenes of exotic planet and moon surfaces. Since it is all background, it needn't be 3d; it is a planetarium not a holodeck, easy to build today (if you have several million bucks laying around, though the real world one wouldn't let you float freely, of course.).


But it'd be so cool.

The "real" equivalent would be just a glass sphere and an outer glass sphere, with water in between to act as radiation protection. You enter it and see the celestial sphere all around you. The space station in low orbit of amillia has one of these.

The real thing has the danger of the sun shining in on you though, potentially leading to burns and destroyed eyes if you look at it.

The real thing also can't show the variety the projection can do.


Both would be awesome.
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Re: Possibility: killing FTL via slowboat colonists

Post by Somes J »

The asteroid belt around the dwarf seems a bit broad just as a gut feeling, but it looks good.

I think I'll try a more detailed write-up of some of the planets.

Amillian Sun - First Planet:

This is a small terrestrial planet about .2-.3 Earth masses. The planet is unusually dense, and it seems likely that early in its history it experienced a truly titanic collision that knocked off much of the mass of a previously much larger (perhaps roughly Earth-sized) terrestrial planet, and the present planet is the stripped core of that older planet. The planet orbits close enough to the sun that it is tidally locked: one side experiences eternal day, the other eternal night. The planet has a thin atmosphere mostly composed of carbon dioxide. The dayside of the planet is mostly an extremely hot desert. The nightside is mostly deeply cold, well below the freezing point of water. Since the planet is very dry and generally volatile-poor the night side is mostly desert as well, only freezing cold instead of burning hot. The planet is believed to be approaching the end of the lifespan of this atmosphere; soon geologic activity and magnetism will die down enough that the atmosphere will thin to the point that the greenhouse effect on the night side is insufficient to maintain a temperature above the freezing point of CO2, and at that point the atmosphere will rapidly collapse. However, at the present time the planet retains a level of geologic activity, mostly in the form of hot spot volcanoes.
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Re: Possibility: killing FTL via slowboat colonists

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I'd peg its main element of composition to be iron - that fits the density and the formation.

A potential use of it for people is skimming CO2 out of its atmosphere, but this might be more trouble than it is worth. Landing on the surface though doesn't sound particularly useful. Nothing there they can't get more easily from asteroids.
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Re: Possibility: killing FTL via slowboat colonists

Post by Somes J »

That innermost superterrestrial planet of the red dwarf might be interesting. If it's really the stripped core of a hot Neptune then it would have started out very rich in volatiles. I wonder if we might be looking at a carbon planet here.

Checking the internets, graphite has a pretty high melting/boiling point, around 3600 C (reference). I'm not sure what temperature would be necessary to boil off the outer layers of a Neptunian planet, but Orion's Arm has a world like this (Sisyphos) and they give a day side temperature of 2000 C and a night side temperature of 500 C. I'm not sure what carbon compounds could exist under those conditions but graphite should exist. So possibly we could indeed be looking at a burning hellish desert with a surface of baked carbon-based crud. The place may possibly be rich in diamonds too, might even have a diamond mantle. Another thing I'm wondering about is whether low-temperature volatiles like water might be able to survive underground, would there be cooler subterraenean layers or would the whole thing be hot? Also, what sort of atmosphere would the place have? Thick carbon dioxide atmosphere, like Venus? Or maybe with the fierce heat and solar winds the atmosphere would be thin or the surface might be exposed to vacuum. If the atmosphere is extremely thin or nonexistant the night side might be deeply cold, which could be real interesting given the volatile-rich composition.

The only thing I'm worried about is the red dwarf is a fairly dim star, how close would a small gas giant have to get for the gas layers to be stripped off? According to the Carpentry Tips section of Planetocopia temperature increases by the fourth root with sunlight, i.e. to get 2X the temperature you need 16X the light. So at ~2 million km from a star with an Earthlike energy flux at ~.2 AU you're only going to have about 4 times Earth's temperature, which would be 1152.6 K or 897.45 C.

If a stripped gas giant core is impractical, the good alternative is a super-Earth formed from material pushed inward by the inward-migrating hot Neptune; I've read speculations that this would happen. I'm not sure what the composition of such a world would be like but offhand I imagine it'd probably be quite volatile-poor. Maybe a super-sized tidelocked Mercury.
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Re: Possibility: killing FTL via slowboat colonists

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I have to think and do some reading to answer your questions, but I'm glancing through your links and some things are coming to mind:
Goldilocks Zone = M2. A dim red star with 1/4 Sol's mass will want Earths only 1/4 x 1/4 = 1/16 as far out! That's only ten or twelve solar diameters away, so the sun will look dim but huge;
Imagine that - the sky being dominated by the massive, but dim sun. Surely an interesting sight.

On a related note:
(and it was even worse early on; the moon orbited ancient Earth MUCH more closely).
This reminds me of something my elementary school science teacher said: back in the day, the moon was "IN YOUR FACE" (he had a way of saying it that sticks in my brain). That'd be another interesting picture.


Speaking of interesting pictures, this is one I got years back when trolling for sci-fi art. Sikon on SDN might have posted it, or I might have gotten it from a google myself; I don't recall. But check out this sky:

Image

A couple fairly bright stars, a gas giant eclipsing the main star, and some moons all visible. Pretty cool pic. I have a handful of cool gas giant moon art. Here's another one:

Image

Again, original artist unknown to me. Just a cool pic, with an IN YOUR FACE moon.



Anyway, that Planetocopia looks like a fun site.
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Re: Possibility: killing FTL via slowboat colonists

Post by Destructionator »

I need time to get a more thinky answer, but what the hell, I'll let my gut blab too.
Somes J wrote:I wonder if we might be looking at a carbon planet here.
That would be pretty win, though pity it would be practically inaccessible for mining. Carbon is one amazing element; it can seemingly do everything. Of course, there's probably more than enough in the inner system anyway, so it isn't like inability to mine this planet matters much.
Checking the internets, graphite has a pretty high melting/boiling point, around 3600 C (reference). I'm not sure what temperature would be necessary to boil off the outer layers of a Neptunian planet, but Orion's Arm has a world like this (Sisyphos) and they give a day side temperature of 2000 C and a night side temperature of 500 C.
Let's look up Neptune. The bulk of its atmosphere is hydrogen. It is already a gas, so what we'd be looking at is when the average energy of a molecule exceeds that needed for escape velocity. Then, over time, it will fly off on its own, as the edges at escape velocity run off.

There was a formula given in some SDN mars thread a while ago. I don't recall it. But, looking over wikipedia and remembering my chemistry class, this one sounds right:

v^2_rms = 3 RT / molar mass

Where: v is the root mean square velocity of the molecules
R is the gas constant
T is the temperature
and molar mass is obviously the molar mass

Let's plug in hydrogen.

R = 8.3 J / (K mol)
T = what we're solving for
molar mass = 2 grams / mole (assuming H2)

v = the escape velocity of the planet. Let's use the same scaling as before for earth. 4 earth masses means about cube root 7 * earth radius, so let's call it 1.6x.

Escape velocity is is sqrt(2GM / r), where G is the gravitational constant, m is mass, and r is distance from the center. We'll use the surface for this approximation. So 17 km / s.

Back to the formula
(17 km / s)^2 = 3 * (8.3 J/ (K*mol)) * T / (2 grams / mol)

Solving for T:

(17 km / s)^2 * (2 grams / mol) / 3 / (8.3 J /(K*mol)) = T

T = 23000 kelvin


Damn son, that seems extremely high. So I either fucked up my math something nasty, have the entirely wrong principle here, or this planet is going to have an atmosphere.

I'm thinking option b, though watered down: this means all the atmosphere is blown off; every molecule has that kind of speed. That isn't necessary to blow it off. It could do a few molecules at a time, multiplied by billions of years. We just need some percentage fast enough to escape.

The odds of having a higher energy than that mean drops off exponentially though. If you double the energy, the odds are 1/e^2 that a particle will have it. You get down to about 1% odds are 4x the energy.


Blargh, it seems inevitable that it will have an atmosphere. There must be other factors at work than just temperature screwing over gas giants in the inner system; maybe the solar wind is to blame. That, or I'm fucking this up really really badly. According to my calculation, Earth should be holding on to a big hydrogen atmosphere, and that hasn't happened.

This super plant is big though, 3x bigger than Neptune's core or so. Maybe it needs to be scaled down in mass to help kill (the bulk of) its atmosphere?

I'm not sure what carbon compounds could exist under those conditions but graphite should exist. So possibly we could indeed be looking at a burning hellish desert with a surface of baked carbon-based crud. The place may possibly be rich in diamonds too, might even have a diamond mantle.
Incredible.
Another thing I'm wondering about is whether low-temperature volatiles like water might be able to survive underground, would there be cooler subterraenean layers or would the whole thing be hot?
Yeah, there probably would be. Graphite is a thermal insulator, isn't it? (at least in one direction - I'm pretty sure heat spreads out across graphite, which along with its high melting point makes it an excellent radiator material, but I don't think heat spreads so well into its depths).

If I'm remembering its properties correctly, the deeper you get into it, the cooler things will get. It will insulate the innards and the surface will radiate heat quickly, since it is so much hotter than the lower layers.

Hence, you'd have nice, cool caves.
Also, what sort of atmosphere would the place have? Thick carbon dioxide atmosphere, like Venus? Or maybe with the fierce heat and solar winds the atmosphere would be thin or the surface might be exposed to vacuum. If the atmosphere is extremely thin or nonexistant the night side might be deeply cold, which could be real interesting given the volatile-rich composition.
Given its large mass, it looks to me that it just must have an atmosphere, but I'm not completely sure about my math there.

Carbon dioxide sounds fun. If it is cool enough - or perhaps if oxygen is rare enough - we might get methane too.
So at ~2 million km from a star with an Earthlike energy flux at ~.2 AU you're only going to have about 4 times Earth's temperature, which would be 1152.6 K or 897.45 C.
Right. This might be enough, but might not. I don't know.
If a stripped gas giant core is impractical, the good alternative is a super-Earth formed from material pushed inward by the inward-migrating hot Neptune; I've read speculations that this would happen. I'm not sure what the composition of such a world would be like but offhand I imagine it'd probably be quite volatile-poor. Maybe a super-sized tidelocked Mercury.
Eh, I like the carbon idea a lot. Perhaps if all else fails, wave our hands and say maybe solar flares over billions of years eroded the atmosphere away or something.
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Re: Possibility: killing FTL via slowboat colonists

Post by Somes J »

I tried looking up the temperature of an actual disintegrating gas giant: HD 209458 b is ~1100 C (reference), and apparently...
New Scientist wrote:The team estimates that the comet-like planet has a trillion years before the entire planet disappears.
I think for a red dwarf planet it's safer to go with the rocky-iron superterrestrial.
Destructionator wrote:Eh, I like the carbon idea a lot. Perhaps if all else fails, wave our hands and say maybe solar flares over billions of years eroded the atmosphere away or something.
If we absolutely must have a carbon-rich planet ... how about this:

The Amillian star system isn't binary, it's triple. Plenty of those in the universe. The third star is another red dwarf, but it has no planets. I'm thinking that it formed two roughly equally sized gas giants that ejected each other from the solar system, with any other planets that might have formed there becoming collateral damage. However, at some point a large KBO got kicked into an eccentric orbit that takes it close enough to the dwarf that its surface temperature reaches well above the boiling point of water. Lacking enough gravity to retain the evaporating volatiles the KBO became essentially a giant comet, and now has long since turned into a giant dead comet, having lost perhaps 30-50% of its original mass to evaporation and now being perhaps 1/4-1/8 the mass of Earth's moon.
Destructionator wrote:This reminds me of something my elementary school science teacher said: back in the day, the moon was "IN YOUR FACE" (he had a way of saying it that sticks in my brain). That'd be another interesting picture.
That reminds me. You mentioned that Amillia's moon is larger than ours, and may be a collision-captured body rather than one formed like Earth's moon. Perhaps it is closer in nature to the impactor that collided with Earth than Luna? If so, it might be as large as Mars. Which would make it possibly big enough to retain an atmosphere, and perhaps to have had liquid water oceans for a limited period of time in its early history.

The Amillian system does seem to be missing a Mars-equivalent, although the innermost planet is in some ways like a mix of Mars and Mercury. A Mars-sized moon would be pretty interesting. Although it would be 10 times the size of Earth's moon, with proportionately brutal tides, and might have tidally locked the planet to itself by now. Maybe make it farther away. An orbit twice as far out as Earth's moon would leave Amillian tides only 2.5 times stronger than Earth tides.
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Re: Possibility: killing FTL via slowboat colonists

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Somes J wrote:If we absolutely must have a carbon-rich planet ... how about this:
There's no "absolutely must", it is just that I like variety where it fits. If it doesn't, no big deal.
The Amillian star system isn't binary, it's triple. Plenty of those in the universe.
That'll complicate the gravity situation again though; probably easier to just go with a super Mercury.
A Mars-sized moon would be pretty interesting. Although it would be 10 times the size of Earth's moon, with proportionately brutal tides, and might have tidally locked the planet to itself by now. Maybe make it farther away. An orbit twice as far out as Earth's moon would leave Amillian tides only 2.5 times stronger than Earth tides.
Hmm, it is interesting. I'll have to rethink the early days of their space program though. The way it worked was like an Apollo, but meant to be easily reusable. Then, moon mining started, using a mass driver - the starfleet is mostly built from space materials.

A lunar mass driver just wouldn't work with something that big. (Well, it might be possible, but it won't likely be cost effective) But the rest might work out.... it might even be more favorable, since it makes it harder and more different!


If the slowing of the day is proportional to the tide strength, which makes sense, we're looking at an initial day length, at the planet's birth, of about seven hours, slowed down by the moon over the years. That's a rapid rotation, but big ass Jupiter is only slightly longer, so it strikes me as believable.

(wikipedia: "Day length has increased by about 2 hours in the last 600 million years." Doing the difference in tangential velocity, and we get 40 m / s / 600 million years = 2e-15 m/s^2 rate of change. Assuming it is constant. and proportional, we now have 5e-15 m/s^2. Say today we want 22 hours. That's 500 m/s with Earth size. If Earth age, it was 1000 m/s faster in the past. That implies a 7 hour day back then.)


I think it could make this work, and it'd be different without being too different. This system has all the fun stuff, right within reach!
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Re: Possibility: killing FTL via slowboat colonists

Post by Somes J »

I think for the innermost planet of the red dwarf a super-sized tidelocked Mercury would indeed be the safer direction to go in.

I wonder whether it would have a significant atmosphere. It orbits at .07 AU as I recall. Assuming the habitable zone is at .2 AU at .1 AU it would be getting 4X Earth's sunlight and at .05 AU it would be getting 16X Earth's sunlight. It might be getting sunlight comparable with Mercury (I'm too lazy to figure it out exactly at the moment). It's big, which means it should still have a relatively high level of geologic activity and volcanism. Tides from the outer hot Neptune may be warming its interior too; a superterrestrial doesn't need it to retain geologic activity for billions of years, but it might make the level of volcanism even higher. There may be a substantial magnetic field too, which would offer some protection for the atmosphere against stripping by solar wind, although being tidelocked it might rotate rather slowly so maybe not (what would its year be anyway?).

I think we've got two possible models here. One is a superterrestrial planet formed in the inner system, and the other is a failed gas giant core that spiralled inward, as Gliese 581 c may be. Personally I prefer the second option; I don't feel quite up to figuring out what a superterrestrial formed from pushed-in material would be like, and we already have a super-Mercury around the Amillian sun so being an inspiralled failed gas giant core would make it more distinctive.

If we go with the failed gas giant core model I imagine we'd probably be looking at a very volatile rich planet here. Maybe a sort of transitional world between a terrestrial planet and a true ice giant. I imagine we'd likely be looking at a planet with a very thick, hot, cloudy atmosphere here, maybe gradually transitioning to a truly massive ocean of hot high pressure water as you go down. If the composition is similar to that of outer system bodies in our own solar system this planet may be 30-50% volatiles by mass! Which would make it for all practical purposes almost more like a miniature gas giant than a terrestrial planet - the main difference between this thing and a true gas giant is it's too small to hold on to molecular hydrogen.

One issue is whether we'd be looking at a water planet or a sulfuric acid planet like Venus. Photodisassociation happens most easily with high energy photons, which a red dwarf emits proportionately less of. This immensely volatile-rich terrestrial planet would also have a lot more water than primordial Venus probably did - possibly as much as a third or half its mass might be volatiles, much of which would be water. I think it would probably be safer to go with a water planet.

I like the failed gas giant core idea. It'd be quite unlike anything in our solar system.
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