A Habitable, Terrestrial World

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The dominant race is going to arise with the S-type star system I created earlier.   The planet in question will be a bit larger than Earth, and possess a higher but still human-tolerable gravity.  It’s less dense than earth, however, which implies that it either has less iron or more water / ice in its makeup.

It will be orbiting the primary star of the system, and as I calculated earlier, it will be orbiting a star 1.2 times the mass of ours at 1.59 AU.  That puts it at the fifth rock from the sun, assuming I decide to use all the orbits.

Property Calculation Full Calculation Unit
Radius 1.49 9,492.790 Kilometers
Mass 3 1.792E+25 Kilograms
Gravity 1.351 13.252 Meters/Second^2
Density 0.907 4.671 Grams/Centimeter^3
Escape Velocity 1.419 15.892 Kilometers/Second
Circumference 9.362 59,644.959 Kilometers
Surface Area 27.899 1.13E+09 Kilometers^2
Volume 13.856 3.58E+12 Kilometers^3

As you can see, that 50% increase in the planet’s radius gave us a much bigger surface to work with than is possessed by Earth, even if we do dedicate a lot of that to oceans.

All we know about the planet at this point is that it’s big and wet.  Yay?  However, we can extrapolate a few things.  This is the home planet of our dominant race, thus it will have possessed resources sufficient to have developed a space-faring culture.  The world will be very highly developed, perhaps at this point in its history it is getting low on natural resources and must import.  Or perhaps the culture upon it strives to preserve its homeworld, and thus conducts most of dirty industry on bases on the neighboring planets or its moon.

Speaking of which, since this is a stable, habitable planet, it needs a moon.  Rings would be cool, but rings make heading into space difficult.  A single large moon will be sufficient.  However, I’m going to make this moon different from Earth’s by messing with its makeup a bit.  It has a solid iron core, but much like it’s planet, it also has a lot of water for a moon.  Thus, it is actually slightly less dense than its host.

Composition Percent Density Calculation Unit
Silicate Rock 70% 3.25               2.28 g/cm^3
Water Ice 12% 0.93               0.11 g/cm^3
Iron 18% 7.87               1.42 g/cm^3
Total               3.80 g/cm^3

For the record, the moon has about 70% the density of Earth, and the planet about 90%.   It’s a good sized moon, bigger than our own and possessing a higher gravity.  As the moon does have a lot of water, it will also look different in the sky.  It may even have a bit of atmosphere.

Property Calculation Unit
Mass 0.020 Earth Mass
Radius 0.300 Earth Radius
Gravity 0.222

Time to figure out the orbit of our moon.   We need the sun’s mass (1.21 solar masses) and the planet’s mass (3 earth masses) as well as the semi-major axis of the planet’s orbit (1.59 AU).  With those numbers, we can calculate the Hill Sphere and the Roche Limit.  Drop in on Artifexian if you need an explanation for those numbers.

Hill Sphere 505.72
Roche Limit 2.25

So the moon must orbit between 2.25 and 505.72 planetary radii from the planet.  I’ll also need to know the inclination and the eccentricity.   I want the orbit fairly circular, so I’m going with a low eccentricity.  Since I also want the moon to stay near the equator, I’m going with a small incline.  Yes, boring.

Orbit 112 Earth Radius
Inclination 2 Degrees
Eccentricity 0.002
Orbital Period 40.11

However, this gives us fairly close to a nice, even, 40 day orbit.  Perhaps 40 is a superstitious number then, to this race.  I’m going to do some work on the planet’s orbit while I’m here.  I am leaving out some data because it isn’t as important and is somewhat arbitrary.

Property Calculation Value Notes
Mass 3.00 Solar Mass (in Solar Masses)
Semi-Major Axis 1.59
Eccentricity 0.03
Orbital Period 1.16 422.50 Earth days
Orbital Velocity 1.37 40.91 kilometers/second
Inclination 0.02
Axial Tilt 20.00
Tropic 20.00
Arctic 70.00

The eccentricity makes the orbit around the sun closer to the circular side, and the low axial tilt also serves to make the planet’s seasons stable.  These guys have it pretty good.


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