[Space] Exotic star system features, and their storytelling role

[Michael Thayne]

Most of the features a star system generated with the system in GURPS Space are things we're relatively familiar with. Most of the world types are based on either planets or moons that exist in our solar system. But there are exceptions: such as ammonia worlds, chthonian worlds, and large worlds. I have two questions: are there other things like this that Space left out, i.e. "scientifically plausible but found nowhere in the Sol system?" And what's the storytelling significance of these weirder solar system features, both the ones in Space and any the book left out?

Possible features not mentioned in Space:

• Moons in the same size category as their planet—though possibly at opposite ends of their size category (e.g. a planet with a radius of 1.05 Earth radii, whose moon has a radius of 0.55 Earth radii).
• In Space, any Small world too hot to be an Ice world is automatically a Rock world, with only trace atmosphere. But as far as I can tell this isn't required by physics, and in principle we could discover planets that resemble a naturally occurring, half-terraformed Mars.
• Worlds that have a particular chemical element in weirdly abundant quantities. Maybe the planet's crust has extremely high levels of lithium (this was a plot point in one of the Expanse novels. Or a planet that shouldn't have a significant atmosphere is covered in a relatively think blanket of krypton.

Storytelling significance of unusual worlds:

• Maybe Large worlds provide a source of Helium-3 that's easier to mine than gas giant Helium-3?
• Space mentions that a planet with ammonia-based could effectively have naturally-occurring high explosive, raising the possibility of a planet whose economy is based on HE farming.
• Worlds with an unusually high abundance of any rare element are obviously economically valuable.

But I'm as a non-astronomer I'm just sort of guessing at these things. I'd be really curious to get input from more informed folks.

[mr beer]

Originally Posted by Michael Thayne

[*] Space mentions that a planet with ammonia-based could effectively have naturally-occurring high explosive, raising the possibility of a planet whose economy is based on HE farming.

Wouldn't naturally occurring HE have already exploded though?

I hesitate to think of a planet which is ready to just ignite like a bad plot idea from an L Ron Hubbard novel.

[Michael Thayne]

Originally Posted by mr beer

Wouldn't naturally occurring HE have already exploded though?

I hesitate to think of a planet which is ready to just ignite like a bad plot idea from an L Ron Hubbard novel.

No—as explained in Space, this only works on very cold planets. Such life forms would die from over heating well before they exploded (just like a human will die from heat exposure long before catching fire).

[AlexanderHowl]

We have already located planets beyond the capabilities of GURPS Space to quantify. For example, there is a planet with a density over three times that of iron, meaning that it would be a 4.0 by the scale of GURPS Space (which means that it is denser than any substance on Earth, though a planet of pure platinum group metals might compress to its density). In addition, most of the star systems seem to laugh at the limited orbits of GURPS Space and blithely form any mathematically stable resonance.

[dataweaver]

I may be wrong, but I seem to recall seeing somewhere that our study of exoplanets has basically trashed GURPS Space's system design assumptions. One trend in particular seems to be that in other star systems, things tend to be a lot more uniform: you get a bunch of gas giants, or a bunch of large terrestrials, or a bunch of small terrestrials, etc.; but not a mix of a bunch of different types like we have in our own star system.

As well, orbital resonances like the 1:2:4 arrangement of Io, Europa, and Ganymede are far more common than was previously assumed, not some freak accident. Taken together with the aforementioned uniformity of planet type, and it's surprisingly likely that you'll find a number of worlds of the same type closely packed together: this means that if a system has one Earth-like world in the habitable zone, it stands a surprisingly good chance of having another one.

[Calvin]

Originally Posted by Michael Thayne

Moons in the same size category as their planet—though possibly at opposite ends of their size category (e.g. a planet with a radius of 1.05 Earth radii, whose moon has a radius of 0.55 Earth radii).

These are usually referred to as double planets. And they are awesome.

I strongly prefer hard Sci-Fi, but in hard sci fi it's normally difficult to come up with a decent way to have multiple unique civilizations interacting with each other without requiring some decidedly soft science FTL. Our sun for example is really only going to have one habitable planet at a time, whether that's Venus, Earth, or Mars.

The solution then is to pack in as many habitable planets as possible, and here are the tricks I've put together so far:

1. The double planets you mention. These are absolutely a real thing (In theory, probably) By some broad definitions of the term, the Earth-Moon system is even a double planet. Just make sure that your own numbers leave both habitable.

2. Hot/Warm gas giants. Giants can migrate inwards, into the habitable zone. Combine this with their abundant moons and the possibility that they picked up any properly sized planets on their way nearer the star and you could have a gas giant that's well into the habitable zone with one moderate G world and several low G worlds . Or even several moderate G worlds, depending on how you can fudge the numbers.

3. Rings. Venus is technically, barely, within the habitable zone. You'd need to start by ditching the runaway greenhouse effect, but a saturn like ring system with a 45° inclination could block a ton of sunlight, keeping the temperature to something more moderate. (I might be misremembering this. It might not strictly be the ring's own inclination, but there's some way things work out so that they provide shade) This trick let's you sneak in another habitable planet, just inside the habitable zone.

4. Ocean and or Jungle worlds. I'm not quite referring the the Star Wars style single biome planets, but a planet with a lot of water or vegetation would be better at absorbing solar radiation. This could give you a habitable planet roughly where Mars is.

5. Bigger stars. Simply making the star larger will also widen the habitable zone. You don't want to go too far, otherwise the stars stop being able to support habitable planets, either because they're generally unpleasant to be around, or because they burn out before intelligent life can develop, but bigger can be better. I think I went for F series, maybe F4 stars, last time I did this.

6. Binary stars. Why settle for one habitable zone when you can have two? GURPS Space already provides rules for this. You'd want them far enough away that the gravity from the other star doesn't destabilize the habitable zone planets, but beyond that I suggest setting the stars distance based on how long you want the journey from star to star to take.

7. Trinary Stars. This is pushing the limits of hard sci fi, but arguably you could have a third and much smaller star orbiting the barycentre of the larger stars at a very great distance. This works better the closer together the two larger stars are. This would maybe be some captured red dwarf, etc. The habitable zone would be smaller, but some clever resonant orbits might give you two or three habitable planets still.

8. Trojan planets. Jupiter already has a significant number of trojan asteroids. In theory, if you had a super-jovian planet in a habitable zone with a ton of habitable moons, you could have an additional habitable moon/planet at the L4/L5 point where otherwise you'd have your trojan asteroids.

Each option lends itself to creating one political alliance or another. Double planets and habitable gas giants could result in colony-parent relationships between the relevant worlds, western style political alliances, Roman style conquering, Soviet style central command, or we could go right back around to British style empire building. There could even be a combination of the above, particularly in the case of a Gas Giant system. Imagine the Cold War, but played out between a system of planets.

You'd have a further level of this between the planets of two or three stars, with alliances growing naturally between 'neighbors' simply due to the distances involved

[Anthony]

Originally Posted by Michael Thayne

In Space, any Small world too hot to be an Ice world is automatically a Rock world, with only trace atmosphere. But as far as I can tell this isn't required by physics, and in principle we could discover planets that resemble a naturally occurring, half-terraformed Mars.

Well, the way Small worlds are defined is 'too small to hold atmosphere for geological time'. If you came upon a small world with a considerable atmosphere, it would be because of something recent (though 'recent' might be a hundred million years or something).

Originally Posted by Michael Thayne

Worlds that have a particular chemical element in weirdly abundant quantities

There's no clear cause for anything like that. There's a couple processes of nucleosynthesis so the ratios can be somewhat different, but that would just change the ratio of light (below iron) to heavy (above iron) and, depending on age, the amount of radioactives.

[AlexanderHowl]

Technically, there should really be no real interest in mining terrestrial planets if you have affordable space travel. It is much easier to mine asteroids and even unimpressive asteroids can supply a star system with all of its metal and silicon needs for tens of thousands of years. For example, 8 Flora could supply metals and silicon to human civilization for fifty thousand years before we used five percent of its potential.

When it comes to He-3, any civilization that can fuse He-3 can produce He-3 as a byproduct of DD fusion. Every 8 grams of DD fused produces 3 grams of He-3, so a civilization that produced 10 TW of electricity through DD fusion would produce around 10 kilograms of He-3 every second as a byproduct (an average of 300,000 metric tons per year). In effect, no civilization with fusion energy ever needs to mine He-3, they probably have more than enough f9r their needs as a byproduct of electricity production.

[ericthered]

Originally Posted by AlexanderHowl

Technically, there should really be no real interest in mining terrestrial planets if you have affordable space travel. It is much easier to mine asteroids and even unimpressive asteroids can supply a star system with all of its metal and silicon needs for tens of thousands of years.

On the other hand, planets are much better at holding onto volitiles like nitrogen, phosphorus, and sulfur (oxygen and carbon are more plentiful). If you're building large artificial habitats, you'll need a good source.

Of course, earth-sized room temperature planets are a bad option for that. I've heard that Titan is the best source in the solar system for extra nitrogen. It also has the advantage that you can use a steel cable for your space elevators there rather than a hyper-advanced currently theoretical material.

[Michael Thayne]

Originally Posted by AlexanderHowl

We have already located planets beyond the capabilities of GURPS Space to quantify. For example, there is a planet with a density over three times that of iron, meaning that it would be a 4.0 by the scale of GURPS Space (which means that it is denser than any substance on Earth, though a planet of pure platinum group metals might compress to its density). In addition, most of the star systems seem to laugh at the limited orbits of GURPS Space and blithely form any mathematically stable resonance.

How large is this planet in terms of absolute mass? Depending on the radii you could get a planet with Earth-like surface gravity but lower escape velocity, a la Kerbin, which could be interesting. (Though less extreme than Kerbin—Kerbin has a density 10 times that of Earth, which I don't think you can explain without invoking alien tech.) Also, a planet of solid platinum group metals could be insanely valuable economically. Is that considered the most likely explanation, or could it be something else?

Originally Posted by dataweaver

I may be wrong, but I seem to recall seeing somewhere that our study of exoplanets has basically trashed GURPS Space's system design assumptions. One trend in particular seems to be that in other star systems, things tend to be a lot more uniform: you get a bunch of gas giants, or a bunch of large terrestrials, or a bunch of small terrestrials, etc.; but not a mix of a bunch of different types like we have in our own star system.

As well, orbital resonances like the 1:2:4 arrangement of Io, Europa, and Ganymede are far more common than was previously assumed, not some freak accident. Taken together with the aforementioned uniformity of planet type, and it's surprisingly likely that you'll find a number of worlds of the same type closely packed together: this means that if a system has one Earth-like world in the habitable zone, it stands a surprisingly good chance of having another one.

Isn't our ability to detect terrestrial planets pretty limited right now? How confident can we be that a system that seem to be only gas giants doesn't have some terrestrial planets in there somewhere? Also, a 1 : 2 : 4 arrangement is wider spacing than Space assumes I think. To maximize the chances of multiple habitable planets, wouldn't you want like 1 : 1.4 : 2? Has that also been found?