GURPS Space - assistance

[Whill]

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,

[ericthered]

Welcome!

Ask your question. This is the place.

[Whill]

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.

[David Johnston2]

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.

[Whill]

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).

[Anthony]

Originally Posted by Whill

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?.

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.

[RyanW]

Originally Posted by Whill

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).

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.

Originally Posted by Whill

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.

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.

[David Johnston2]

Originally Posted by Whill

I'm not talking about the Inner Limit Radius and Outer Limit Radius.

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

[Whill]

Thank you all!

Originally Posted by RyanW

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.

Yeah, I prefer very low eccentricity in my orbits.

Originally Posted by RyanW

The outer edge is the distance at which objects can maintain a stable orbit around the pair as though it were one object.

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.

Originally Posted by RyanW

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.

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?

[RyanW]

Originally Posted by Whill

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?

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.