# World Size

Traveller system generation starts with determining the size of the main world. Roll 2d6-2. The result is read in 1000s of miles, though of course we use metric units around here. Like so:

 Digit Diameter (km) Surface Gravity (g) Examples 0 800 Negligible Asteroids, small moons 1 1600 0.05 Ceres, Orcus, Makemake 2 3200 0.15 Europa 3 4800 0.25 Mercury 4 6400 0.35 Mars 5 8000 0.45 6 9600 0.7 7 11200 0.9 8 12800 1 Venus, Earth, Kepler 20f 9 14400 1.25 10 (A) 16000 1.4

Worthless Rocks Revisited

As we found out in the last post on the subject, almost 42% of the worlds we generate this way will be size 4 or lower and thus fall into the special “hard science” rules that almost ensure that their atmosphere is “none” or “traces”. They all fall into the gaping huge category of “worthless rocks” that will make up 40% of the systems.

One of the ideas of mitigating this is to tamper with the size rolls. We can’t really add any modifiers to the roll since the size roll is the first datum we generate about our new system.

Instead, we could use 3d6-3. The probabilities of getting certain world sizes change quite a bit, and results go up to 15:

With this method, only 16.2% of all main worlds will be size 4 or less. However, another 16.2% will be larger than size 10. Does this open up a new can of worms?

Just how big are Exoplanets anyway?

Although we know a lot more about exoplanets than we did in 1977 (we didn’t know anything, then, after all) our data is still severely limited because detecting exoplanets is no trivial task.

However, on the upper end it seems that planet sizes run smoothly all the way up to Jupiter and beyond. This doesn’t say anything about probability, just whether or not such worlds exist.

We don’t know much about the makeup of exoplanets. However, we have been able to determine both mass and radius of some exoplanets. From this data, it seems that worlds with a mass over 1.5 Earths will not get rid of the hydrogen they collected during planetary formation, and are likely mini gas giants.

Potential 3d6-3 results

An extended Traveller world size table up to 15 (F) would yield:

 Digit Diameter (km) Diameter (Earths) Examples 11 17600 1.375 Kepler 10b 12 19200 1.5 Kepler-33b 13 20800 1.625 Kepler-21b, Kepler-9d 14 22400 1.75 15 24000 1.875 Kepler-20b, Kepler-23b

Kepler 10b is an earthlike world (just too close to its primary); 33b has a high surface gravity (3.6g) and is thus useless for colonization; 21b is hot enough to melt iron; 9d is another insanely hot world; 20b is the “largest planet with an earth-like density”; and not much is known about 23b.

Count size 11 and 12 as superearths, which may be unpleasant but still basically habitable. Size 13-15 are mini-gas giants. If they are inhabited at all, people will live in floating cities akin to Cloud City on Bespin.

Conclusion

Barring playtest results to the contrary, it is feasible to use this method. We have effectively reduced the amount of fringe-y, useless worlds from 42% to 32% and we have split the group itself into half, achieving the goal of “more variety”.

There will of course be side effects because world size feeds back into Atmosphere and Hydrographics, but I think this is not a problem. If we can have blanket filters to set worlds to vacuum on the low end of the spectrum, we can do so at the high end as well.

I will deal with atmospheres in the next post on World Generation matters.

Update: 3d6-4

After some trial runs, it became apparent that 3d6-4 is a better fit for what I want. Not much of a difference, right? But the probabilities do shift a bit.

## One thought on “World Size”

1. I really like the charts illustrating the various sizes and likelihood of planets. I vote for the 3d6-3 rolls.