Asked to design my own planet, the first thing that came to mind was to design a planet with life that I can control and observe from the outside. It would be something similar to having my own personal aquarium or pet zoo of sorts. As tempting as it were, I thought of designing a planet that I would be able to inhabit with my friends and family when I wanted to. Like a home away from home, a planet away from this planet. This meant that I would have to have a planet that would be able to sustain life in a manner similar to Earth. While it would be fun to have it in a different manner, it wouldn’t be so much more fun to relax entire weekends in a space suit. For this to happen, we would need a planet with a similar atmosphere.

I quickly rubbished the idea that any size of a planet could have an atmosphere. I mean look at the Moon. While not a planet, it is however a body smaller than Earth. It doesn’t have enough gravitational force to hold an atmosphere. A planet can only have an atmosphere if the escape velocity is larger than the atmospheric gasses’ molecules. The escape velocity of Earth is 11.2 km/s, which means that something must be travelling at least that fast to escape the gravitational field.

Now Escape Velocity is calculated by the equation Ve=sqrt (2GM/R)
where G = Gravitational constant, M=Mass, and R=radius of the planet

For a sustainable atmosphere, we would need the Gas Velocity to be less than this Ve.

Gas Velocity Vg=sqrt (3kT/m)
Where k = Boltzmann constant, T is the minimum temperature in Kelvin, and m is the mass of the gas molecule.

Since I want a habitable planet I believe that a difference of ten times would be sufficient.

On comparing 10*Vg=Ve to calculate, we have to unknowns M & R. Since I want to have a planet similar to Earth, I can give a value to M. A quick reminder of high school science: Density = Mass/volume. We know the average density of Earth is 5.52 gm/cm cubed. We also that that the volume of a sphere is (4/3)*Pi*R^3. This gives us a value for M as 23122.12*R^3.

We can now compare Vg and Ve to get an approximate value of 2845 kms. This would mean that my planet would need to be at least of a radius of this size to have a sustainable atmosphere.

Now that I have a lower limit on my planet size, I began to think of a higher limit. If I had a larger radius, I would have more mass. This would mean more gravity. Not only would more gravity make it more difficult to walk and move around, it would affect my future generations should I choose to move to the planet for good. You see, the more gravitational force a planet has, the shorter will be the beings born on it. It makes sense from an evolutionary perspective, as the heart would have to face lesser resistance against the gravity to pump blood to the top most part of the being. It would cause the great grandkids of my great grand kids (I really don’t know how many generations would have to be born for it to make a noticeable change) to be short and stocky.

Now that the planet size has been finalized. I do want my planet to be different from Earth in one particular way. The days and years on Earth are defined based on Earth’s rotation on its own axis, and its revolution around the sun. Instead of having just one sun, I want two. I would have 1 large sun around which a smaller sun revolves, and my planet would revolve around this smaller sun. Quite like how the Moon revolves around the Earth which revolves around the Sun.

Earth is habitable also on account of its magnetosphere. The molten metal core of the Earth generates a magnetic field that protects us from much of the harmful particles of the Solar wind. Since I will have two suns, the planet would need a magnetic core sufficient to create a protective layer against both. Of course this would need to take into account the size of both the suns, the distance of the smaller sun from the larger, the distance of my planet from both, their orbits, and what is the safest habitable zone for my planet from them.

As much as I did read upon it, I couldn’t calculate these aspects and ended up with a headache. Sigh, my planet design will have to be a work in progress for now.

Written for the “Interplanet Janet” Open Prompt at Project 365.

Thanks to Robert Frost’s answer at Quora for solving the equations, without which I would spend a good five more minutes doing the math.