GURPS Space - assistance
 

Hello. I here am new. I love GURPS Space 4e! I submitted an erratum that was added to the official errata for this book a few years ago after getting it, but I am just now getting around to generating my first star system, the central system for my next campaign setting. I wanted to make it somewhat disctint and unusual, but still somewhat plausible. So of course I didn't start out with an easy one.

Would this be a good place to ask for a little assistance and discuss the worldbuilding / star system generation process? (If not please move the thread as needed.) Thanks,

Re: GURPS Space - assistance
 

Welcome!

Ask your question. This is the place.

Re: GURPS Space - assistance
 

Thank you. Welcome to the "Primordial System"...

I envision a binary system, but the twist is that a G2 V star ("Astra") more luminous than our sun but the same mass, is a companion to a black hole ("Thanatos"), with the planetary system around the companion. I want them to be the maximum distance apart possible for the dice roll, which I calculated to be 600 AUs (12x50).

I don't want the star to just orbit around the black hole - I instead want them both to technically orbit around a gravitation barycenter that is of course closer to Thanatos due to minimum black hole mass being higher than Astra's. I think I consulted an outside equation for this step but I worked out a barycenter to be 10 AUs from Thanatos, which has a mass of 59 solar masses (likely to be a conglomeration of multiple black holes).

Anyway, in calculating the forbidden planetary orbit range, I got an Inner Edge of 200 AUs and an Outer Edge of 1800 AUs! My first question is what the Outer Edge actually means here? According to this system, does that mean that no planets could form around Astra because it is between 200 and 1800 AUs away from Thanatos? Or does that just mean no planets could orbit Thanatos in that range, but the companion star's own planetary orbit stability would be uneffected? Of course Thantos doesn't need planets, just Astra.

Re: GURPS Space - assistance
 

That's the zone where planets are allowed, not the zone where no planets are allowed. Those stars are so far apart that they can't interfere with each other's planet formation at all.

Re: GURPS Space - assistance
 

I'm not talking about the Inner Limit Radius and Outer Limit Radius. That is the range that stable planetary orbits can form around each star (p. 106). This is the Forbidden orbit zone for systems with more than one star (p. 107).

Re: GURPS Space - assistance
 

The latter. The exclusion zone is based on distance from the object being orbited, so an object cannot orbit Astra at 200-1,800 AU, or Thanatos as 200-1,800 AU, but a 1 AU orbit around Astra (even though it's 600 AU from Thanatos) is entirely legal.

Re: GURPS Space - assistance
 

Technically, all celestial pairings orbit a barycenter. It's just that most such pairings have enough size difference that the barycenter is within the larger body. The average distance from the barycenter should be the average orbital radius times the mass of the other body, divided by the sum of the masses of the bodies. So they will indeed orbit a point 10 AUs from the black hole.

Other than a few orbital and observational tricks, an external barycenter is nothing more than a scientific curiosity.
That's the ranges at which stable planets cannot form. Any object farther than the inner edge from its primary will be perturbed to the point that the orbit is unstable. The outer edge is the distance at which objects can maintain a stable orbit around the pair as though it were one object.

The distances involved suggest that there is no need to worry. There is no chance of objects at that kind of distance anyway, assuming the roughly circular orbit these numbers imply.

ETA: If I remember the numbers in Space correctly, I realized that there is a possibility of a planet out in the far fringe of the system, beyond the outer edge of the forbidden zone.

Re: GURPS Space - assistance
 

I know what you are talking about. My answer remains the same.

Re: GURPS Space - assistance
 

Thank you all!

Yeah, I prefer very low eccentricity in my orbits.

It seems that I have a misunderstanding of forbidden zones, but this is the key I was missing about the outer edge. Math was my major in college, but English was only my minor. :-) I had no plans for anything orbiting Thanatos-Astra as a pair, so I'm good here.

Yes, external barycenter is what I meant. On a much smaller scale, in our solar system Pluto-Charon have a barycenter outside of Pluto.

Which brings me to an astronomical question. Before I get back to system generation questions, what would it be like at the barycenter? I thought of making the space near the barycenter a sacred site for a local species. If you could park your ship there (not moving with respect to the Thanatos-Astra system), would it just be completely weightless with no gravitational forces pulling in either direction?

Re: GURPS Space - assistance
 

That sounds more like a Lagrange point, which is closer to the smaller object and is the point where the forces pulling you toward a large body and a smaller one orbiting it are balanced.

Re: GURPS Space - assistance
 

Uninteresting; you won't get star motion due to orbiting, but that's not human-visible to start with.
No, you want lagrange points.

Re: GURPS Space - "desert planet"
 

I'm really liking this forum.

Even though I don't roll for the aspects most important to me, I try to keep my choices within the ranges possible from rolling. For a Standard Garden world, the minimum hydrographical coverage is 45% (min roll with optional variation of the result by 5%). Is this worldbuilding system indicating that desert planets like Arrakis or Tatoonie (with maybe single-digit hydro) are just not possible?

What if the planet had 45% at one time, leading to plant life, oxygen, animal life and conditions suitable for human colonization, but then the oceans, seas and lakes were somehow drained into massive underground reservoirs within the planet? Step 4 on p.81 mentions worlds possibly having extensive underground water supplies but that doesn't count towards its hydro rating. Could something like that make a 5% hydro desert planet with polar seas at all reasonable?

Before the loss of the oceans, if there were already large deserts on the planet with lifeforms adapted for that environment and the change happened slow enough, I'm thinking a desert planet with a smattering of plant and animal life of its own is possible. As unoriginal as it may be in sci-fi, I really want desert planet in the Astral System!

Re: GURPS Space - "desert planet"
 

They probably aren't possible; the hydrological cycle is too important to keeping a planet habitable. On the other hand, you can simply ignore realism.

Re: GURPS Space - desert planet
 

That's not what I wanted to hear, but I truly do appreciate your reply. I really am trying to make my star systems as plausible as possible, but if I absolutely have to whip out the cosmic hand and waive something into existence, I will. Perhaps a long time ago, a very advanced species reverse-terraformed the planet to degree for some unknown purpose, and solving that ancient mystery as well as setting things in motion towards a return to its more garden-like past will be plot point in the campaign. Yeah, that's the ticket.

Re: GURPS Space - gas giant arrangement
 

In an eccentric gas giant arrangement where a gas giant formed outside of the snow line and migrated sunward before stablizing into a new orbit, shouldn't one of the outer orbits be left empty of any gas giants to represent the inner gas giant's original orbit before migration?

Or is that not necessary because a cause for the migration could be that the original gas giant orbit was not stable in the long term, which is what may have caused the close encounter that sent the eccentric gas giant inward in the first place?

Re: GURPS Space - gas giant arrangement
 

Not necessarily. Migrating gas giants don't necessarily move alone, but dragging things in from outer orbits behind them. In our system Jupiter formed at about 3.5 AU, migrated in to 1.5 AU by interactions with the disk of planetismals, then got into a resonance with Saturn that first dragged Saturn inwards and then pulled both of them outwards. It ended up further out than it formed, but there was an era in which Jupiter had migrated inwards from the distance that it formed at and had pulled Saturn in to a distance less than that which Jupiter formed at.

Re: GURPS Space - "desert planet"
 

This is one of the reasons that I am intensely skeptical about the habitable tide-locked worlds that the GURPS Space 4th ed. generator produces in such profusion.

Re: GURPS Space - assistance
 

Even Lagrange points have substantial gravity at them. They are characterised as the "points" in the synchronous rotating frame of reference at which the gravitation is exactly right to permit an object of negligible mass to participate in a circular orbit around the barycentre of the system that has a period equal to the period of the orbits of the principal components of the system.

The equipoise, the point at which the planet's and moon's (or whatever) gravity is equal and opposite is on the line between the planet and moon rather close to the moon, whereas the barycentre is closer to the planet. And the problem with the equipoise is that it doesn't stay still (as the barycentre does), nor does any possible trajectory at the point give you a circular orbit with co-incident period. "Park" your ship at the equipoise and what happens is that the equipoise moves away following the moon in its orbit. And no possible velocity that you give your hip at the equipoise will make it keep up with the equipoise as it moves.

Re: GURPS Space - "desert planet"
 

Wouldn't that just make a very different but possibly still life sustaining water cycle?

Re: GURPS Space - assistance
 

Sure, but the key thing is that you can be in free fall without your path causing you to eventually get closer or further from either object.

Re: GURPS Space - assistance
 

You have not yet mentioned any accretion disk around your black hole and that is almost certainly its' most important characteristic in this scenario. You need for there to be virtually no disk to limit hard radiation to acceptable lavels.

You also need for these two bodies to have formed at realtively distant places in space and tiime. Co-evolution is not really an option. There was a large supernova (and maybe even a hypernova/gamma ray buster) at that black hole's location just slightly before its' formation. It may have eaten a star or two afterward as well.

All very interesting as long as it takes place at a safe distance but a safe distance is probably rather more than 600 AU.

The capture of Astra's solar system by Thantos needs to have happened well after planetary formation and never brought Astra's planets too close to Thanatos. Otherwise you get accretion disk and radiation problems and might well have problems with a highly elliptical orbit long afterwards.

I suspect that there is some gentle long range capture scenario that meets your needs but it will be another low probabitliy occurence.

The good news is that the requirements for post-planetery capture does away will all considerations about how Thanatos would have affected Astra's developing solar system. There was no effect at all because it wasn't there at the time.

If Thanatos and Astra had started forming around the same time at a distance of 600 AU, Thanatos would have gone boom before Astra really got started and the supernova would have blown away too much of the gas and dust Astra needed to make a solar system.

Recent (in astronomical terms) supernovas of the correct type that we know of tend to have largish accretion disks and intense radiation too. Mnay of them even form pulsars. You really don't want to be 600 aU from a pulsar.

Thanatos has had a violent past and that all needs to have taken place more thaa 600 AU from Astra. Probably quite a long time in the past as well. Even now Thantos would be doing nasty things to Astra's Oort Cloud.

Re: GURPS Space - "desert planet"
 

Possibly. If you start with a nice thick atmosphere to convect plenty of heat to the cold side, and if you start out with so much water that there is still plenty on the sunny side after the formation on the shady side of an ice cap fifteen times the size of Antarctica. I suspect that the rule for synchronously-rotating planets on Space p.125 may be generous in the matter of the effect on hydrographics. I'm pretty sure that it does not take into account that the ocean will freeze solid on the shadyside, so final hydrographics figures should be cut in half after the present adjustment.

Re: GURPS Space - "desert planet"
 

Does this take into account "superrotation" of the atmosphere? Most of the habitable tide-locked planet scenarios I've seen appear to require somethiong like a planetwide tradewind blowign at 30 mph all day every day (for Earth-standard values of "day").

They allso seem to require that the superroatation just fall into a "Goldilocks" range and stay there without any as yet proposed regulation method.

All sounds unlikley to me but proponents emphasize that it might be _possible_.

Re: GURPS Space - assistance
 

Life will likely pop up anywhere you have water and some energy gradient.
Getting it to be even vaguely earth like in weather is a bit less likely.
But with how many red dwarfs there are, the unlikely becomes a near certainty somewhere.

I still like the concept so much, I would have to put at least one colonized/studied tidal lock world for fun and weird life.

Re: GURPS Space - "desert planet"
 

I wouldn't say "profusion". They can exist, but with the inner limit size penalty and the narrow habitable zones of red dwarfs, I don't see a lot of them.

Re: GURPS Space - "desert planet"
 

I'm going by Merlis, T.M. and Schneider, T., 2010 Atmospheric dynamics of Earth-like tidally locked aquaplanets. Detailed results depend on how fast the orbit is, because in a small orbit (as around a late star) the tidally-locked world is still rotating fast enough to have significant Coriolis effects. Merlis & Schneider consist (as extremes) the 24-hour and the 8760-hour orbits. I would be interested in similar investigations of intermediate values if any were available.

The 8760-hour orbit gives a weather pattern that is roughly symmetrical about the subsolar point, with winds blowing directly from sunnyside to shadyside (i.e. no super-rotation). Shadyside is pretty uniformly between 240 K and 250K.

The 24-hour orbit gives a super-rotating jet at the equator, up to 45 m/s (100 mph), with much more structure to the temperature pattern over shadyside. But still ends up with the temperature on the shady side well below 260 K. Even taking into account the rotation of the planet and the ciculation of the atmosphere, on shadyside water is a rock-forming mineral and snow is a form of sand.

Re: GURPS Space - "desert planet"
 

I see a great many. In my 10,000-star run they came in as about half of all "habitable" planets.

Re: GURPS Space - assistance
 

So much cool discussion here.

I had considered Thanatos becoming a black hole and swallowing other stars far away from and much earlier than the Astra system forming, and then they both just happened to come into gravitational contact with each other when migrating through the galaxy from different locations and directions if necessary. Low probability is remotely possible and rare. That works for me.

As far as Thanatos still doing nasty things to Astra's oort cloud, I was hoping that would be the case. I wanted locals and vistors to the Astral system to have a general sense of fear and dread regarding Thanatos being in relatively close proximity. Oort clouds are already so dark and spooky even without a massive black hole beyond. Interstellar navigators, take the long way around.

Re: GURPS Space - gas giant arrangement
 

I hadn't read all of that early migration in and out of Jupiter and Saturn. Cool. But does this mean that the book's formula for calculating snow lines is incorrect or out-of-date? On p.106 it says R = 4.85 x square root of L. According to the chart on p.103 has the L-Min for a G2 as .68. So according to the book, Sol's snow line would be (4.85 x square root of .68), about 4 AU.

Re: GURPS Space - "desert planet"
 

http://www.astrobio.net/exclusive/41...‘dune’-planets-

I thought this was an interesting line of speculation regarding the likelihood and habitability of "land class" planets.

Re: GURPS Space - "desert planet"
 

wow, you're getting less of those than I am. Though I am counting ALL red dwarf planets with life on them.

I like to interpret "Garden World" as meaning "organic life lives here". That doesn't actually mean much. It includes snow ball earth, several alternate biologies, and tidally locked worlds will certainly have issues. but this isn't reflected in habitability score.

Re: GURPS Space - "desert planet"
 

I'm counting only worlds with Habitability of 5 or higher as habitable. Also, I am not counting spin:orbit resonant worlds as tide-locked. And finally, I have fiddled with the procedures for initial spin and tidal braking as discussed in the errata thread.

Re: GURPS Space - "desert planet"
 

It's interesting, but I wonder how quickly the oxygen atmosphere can develop without oceans and without forests.

Re: GURPS Space - "desert planet"
 

Oceans are actually a big barrier to oxygenation because they contain so much dissolved material that oxidizes. The real question (assuming that initial life can come from offworld) is how you put enough lichen down when most of the world only gets rain as a freakish occurence.

Re: GURPS Space - "desert planet"
 

Interesting. What happens to all that reduced material if it doesn't dissolve? Or do the small oceans become saturated and reduced compounds precipitate out to form thick layers of anoxic ooze?
Well, that's what I was trying to gets at about forests. If the oceans are tiny that means the great majority of the planet is arid and won't support either photosynthesis or the accumulation of fixed carbon, then the oxygen catastrophe is going to be delayed.

Re: GURPS Space - "desert planet"
 

I'm not sure I understand the question. Most of the stuff that could become solutes doesn't because much of the planet rarely experiences rain. Instead it just remains buried underneath the oxidised surface. There's still got to be a minimum amount of surface water to maintain an oxygen atmosphere, but I'm not at all sure what percentage that minimum is.

Re: GURPS Space - assistance
 

Just so. And that isn't true at either the barycentre or the equipoise.

It is very common for people to confuse the Lagrange points, the barycentre, and the equipoise, and especially to give one of those that characterisation of one of the others. It's worth keeping them straight, I think.

Re: GURPS Space - assistance
 

Would it be possible for an Earth-mass world to stay in a Lagrange point of a star and a very massive gas giant?

Re: GURPS Space - assistance
 

L4 or L5, yes. L1, L2, or L3, no. The gas giant has to be at least about 25 times the mass of the terrestrial.

In the context of a planet orbiting a star, these trajectories are also called the leading and trailing Trojan points.

Re: GURPS Space - assistance
 

Thanks. Would there be any severe radiation or other habitability complications with a terrestrial planet at L4 or L5 when the gas giant is 4000 earth masses?

Re: GURPS Space - assistance
 

No. A gaseous body of 4,000 Earth-masses is getting on for being a brown dwarf rather than a gas giant, but L4 and L5 are just as far from the planet as they are from the star, so any radiation hazard from the planet will be dwarfed by that from the star.

Re: GURPS Space - assistance
 

Sorry, there is a limit but that's not the right one. That's the limit on the ratio of the masses of the star and gas giant.

Re: GURPS Space - assistance
 

Yeah, you've got an extremely large super-Jupiter but even if its' magnetosphere is in proportion to its; mass (c.12x Jupiter) L4 or 5 would be about 1 AU away. The radiation belts won't stretch that far.

The Earth-type planet might still need a largish moon of its' own to avoid tidal locking or at least length of day issues.