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Old 09-16-2018, 10:34 AM   #1
Join Date: Aug 2004
Location: Buffalo, New York
Default World Size formulas and GURPS SPACE

Hello Folks,
A while back, I took the GURPS PLANETARY RECORD pdf and made it so that it was capable of taking data in fields for the worksheet. As I started to work with it, a thought occurred to me and I thought I'd ask here...

Step 6 on the worksheet talks about World Size, and gives you two alternative methods for determining world size.

Both methods utilize the Blackbody temperature in such a way that I have to ask...

Why does temperature have anything to do with planetary diameters?

The formula is:

(Blackbody/Density)^.5 as a constant. This constant is then multiplied by the smallest diameter possible, and the largest diameter possible. If you want to randomly determine the diameter, you roll against 2d6-2 (giving a number 0 to 10) and multiply it by the difference of Largest diameter minus smallest diameter. This is then added to the smallest diameter value (giving a range from smallest diameter to largest).

My question is "Why does this work?"
hal is offline   Reply With Quote
Old 09-16-2018, 01:25 PM   #2
Join Date: Mar 2013
Default Re: World Size formulas and GURPS SPACE

The short of it is that GURPS Space bases world size on the gases the body can retain in its atmosphere, which is a function of the body's mass, density, and temperature.

Large worlds are heavy enough to retain helium, but not heavy enough to retain hydrogen. Standard worlds, water but not helium. Small, nitrogen but not water. Tiny worlds are too small to even retain nitrogen.

A consequence of this classification scheme is that worlds of a given type get less massive as you move away from the star, since their temperature goes down and therefore they need less mass to retain the same gases.

The box on page 86 explains the reasoning in more detail.

As for why the random formula works, the types of gases that can be retained are proportional to surface gravity, which happens to be linear with diameter and a constant density. The constants in the Size Constraints Table give the minimum and maximum diameter that a world can have and still be of the appropriate type.
For GURPS reviews and Psi-Wars inspired content, check out my blog at Libris Ludorum!
Nemoricus is offline   Reply With Quote
Old 09-16-2018, 01:42 PM   #3
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: World Size formulas and GURPS SPACE

Thanks Nemoricus.

I was using a random number generator in to try and get the random elements filled in (I want to try and detail star systems in THE SPINWARD MARCHES for GURPS TRAVELLER) using elements of GURPS SPACE along with elements of Jon Zeigler's new formulas he posted online subsequent to the publishing of GURPS SPACE.

Part of me simply wants to ignore the stellar data provided with THE SPINWARD MARCHES campaign and start from scratch, part of me wants to try and use as best as possible, the material already there. Worlds with diameters too small to retain water vapor should not have surface water, let alone (in my opinion) free oxygen (if there is no photosynthesis going on to break up the water, the free oxygen combines with other elements and falls out of the atmosphere as no longer being free).

So, I was trying to find a way to explain why worlds of a given atmosphere mass will have a given pressure rating, and reverse engineer worlds in Traveller based on their atmosphere description, etc.

But knowing WHY things are the way they are in GURPS SPACE or GURPS TRAVELLER FIRST IN helps me to understand what it is I'm playing with. ;)

So, thank you.

Addenda: I almost forgot! I was trying to confirm Surface Gravity and World Mass values based on what GURPS SPACE was generating in my code with what GURPS TRAVELLER FIRST IN said they should be. It took a bit of fiddling with the data to make it work out properly, but the two methods in the two books do more or less mach each other.

Last edited by hal; 09-16-2018 at 01:44 PM. Reason: Addenda
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