1. Member
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Nov 2018
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## Planetary Info

Thank you guys for all the help you've offered with my roleplay setting's astronomy thus far. I really appreciate it.

As I've stated a few times in my topic on Orbital Periods, the setting's home star system has multiple habitable planets. While the topic specifies three—as was my original intention, my co-GM inserted a fourth, which only has subsurface habitation, between the stars and the then-inner-most planet. But, this post relates to the Empire's home planet, which is noted as E1. Its mass (mE1), radius (rE1), density (DE1), and surface gravity (gE1) are 9.4621824 × 1024 kg (1.5844 m), 7,645.2 km (1.2 r), 5.57 g/cm³ (1.011 D), and 10.804 m/s2 (1.1 g), respectively. Is DE1 proportional to mE1, rE1, and gE1? I used this calculator to figure out gE1, so I know it's proportional to mE1 and rE1.
Last edited by TurkeySloth; 2019-Oct-29 at 03:39 PM.

2. The density of a spherical planet will, in general, change from the surface to the center; in most cases, it will become denser closer to the center.

But for rough purposes, we might pretend that a planet has a uniform density. In that case, the density is a simple function of mass and radius:

density = mass / volume = mass / ( [4*pi/3] * radius^3 )

3. Member
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Okay. The planet's density shows as, approximately, 5.0552 g/cm³, here, making it a water world and surface-uninhabitable for the most part. How do I back-figure mE1, rE1, and gE1 knowing that DE1 is 5.57 g/cm³?

4. Originally Posted by TurkeySloth
Okay. The planet's density shows as, approximately, 5.0552 g/cm³, here, making it a water world and surface-uninhabitable for the most part. How do I back-figure mE1, rE1, and gE1 knowing that DE1 is 5.57 g/cm³?
That is only a few percent less than Earth's mean density, and it does not necessarily follow that there must be any particular amount of water and dry land. If Earth's lithosphere was a perfect gravitational/centrifugal geoid the surface would be all water.

5. Member
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Nov 2018
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Ah... okay. I let my search for as close to 1.1 g as possible get in the way of my original calculations. The final values for mE1, rE1, DE1, and gE1 are 9.4621824 × 1024 kg (1.5844 m), 7,400 km (1.16 r), 5.575 g/cm³ (1.0118 D), and 11.532 m/s2 (1.1741 g), respectively.
Last edited by TurkeySloth; 2019-Oct-30 at 06:22 PM.

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