Arnâron by the Numbers

GURPS Space provides a handy collection of rules, guidelines and formula to calculate all characteristics of a planet. I am not a sucker for 100% scientific accuracy, but I like to have at least some plausibility on my side when it comes to these things – I’ll at least try to avoid pulling off a Niven. Doing all these calculations was way too complicated (and quite honestly GURPS Space could use a lot of editing to make it easier to use in that chapter) so I am sure I made a mistake here or there. I won’t go through the whole process in this post, it would be boring as hell.

The Sun

Spectral class: G0V
Solar masses: 1.1 Sol
Age: 3.9 billion years
Luminosity: 1.66 Sol


Orbital Distance: 1.26 AU
Orbital period: 1.35 Earth years (492.38 Earth days)

Rotational period: 25h (Sidereal), 23.09 hours (apparent length in standard hours)
Year: 492,38 standard days, 511.81 local days
Axial tilt: 17°

Diameter: 8919.4 km (0.7x Earth)
Circumference: 28021.12 km
Surface Area: 249931591,24 km^2
Hydrographic coverage: <10%

Mean density: 6.62 g/ccm
Gravity: 0.84 Earth

Average Temperature: 314 Kelvin (40.85° Celsius)

Atmospheric pressure: 0.92 atm

Number of moons: 2

Moon 1: Orbital distance: 240,000 km, Diameter 1129km, Apparent orbit 1.26 local days

Moon 2: Orbital distance: 410,000 km, Diameter 2113km, Apparent orbit 1,09 local days


Arnâron is a world significantly smaller than Earth – only about 70% its diameter – and consequently has a lower gravity than Earth, and a thinner atmosphere. Both are perfectly within the human comfort zone, but the inhabitants of Arnâron will have more efficient lungs (or simply require less oxygen to function, or have much lower stamina…) and they’ll be somewhat weaker and more slender than Earth humans.

The sun is slightly larger and hotter than our Sun; Arnâron orbits much further out from its primary than Earth and is still much, much warmer. Luckily, the thinner atmosphere is able to retain less heat. The lack of temperating oceans means that the differences in climate between the poles and the equator will be much more severe. This was different in the past, where Arnârons now dead oceans played a much greater role in keeping certain areas habitable than they do on Earth.

The local day of 23 standard hours worked out pretty well, it won’t require much adjustment from players, readers, or displaced Terrans. The longer local year will influence local culture and customs, but won’t pose a problem to the human rhythm of life.

Arnâron will have normal, but slightly less tectonic activity than Earth (less water means less movement of the plates), and roughly the same amount of volcanism.

The moons will be very visible from the surface: With angular sizes of 16.2 minutes and 17.7 minutes, respectively, they are each seen as about half the diameter of Luna from Earth. This worked out to slightly smaller than I had intended, but if I make them much larger or closer, tidal effects will slow down the rotation of Arnâron too much – I really like to have a day that’s relatively close to 24 standard hours.

Another concern I have is that the world may be too hot. This could easily be changed by making the sun dimmer or by changing the orbit, without much effect on the overall design, so I’ll keep the current value for now.

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