I'd like to try an experiment - building a world here, adding a new (hopefully interconnected) piece each week. I don't have a story in mind - that's sort of the point, seeing what emerges just from the creation of the world itself. At some point this is going to need a name, but that feels rather premature for the moment.

Planetary plans

So our planet is orbiting around our super-hot, super-short-lived, super-deadly blue star once ever 120 years, far enough out that it's largely made of ice. Or it would be, assuming there's water. There doesn't have to be. But we're getting ahead of ourselves.

How big is this planet? What's the gravity like? Do we have huge creatures loping gracefully through a moon-like bound, or short, squalid inhabitants hugging the surface? Are there metals? How thick is the atmosphere?

Some of these are going to be decided for us by the fact that we're orbiting a blue star. Blue star radiation isn't just deadly to DNA, it also breaks molecular bonds in a process called photodissociation - blue stars steal your planet's free oxygen from its upper atmosphere. Without free oxygen, there's no life-as-we-know-it, no fire or civilised technology. So we're going to have to do something about that. All stars will photodissociate an atmosphere to some extent, even ours (though less so than a blue star). The Earth, however, is semi-protected by our magnetic field, which deflects the radiation around the planet, away from the atmosphere and the inhabitants. That magnetic field is created by the Earth's iron core, which rotates at a speed slightly faster than the rest of the planet.

So if we want inhabitants on our planet, as well as something for them to breathe, we're going to need our own iron core - and probably a bigger one than the Earth's to counter the increased radiation.

Size of a...

That dictates a lot about our planet's size - a core that dense gives the planet a lot of mass, which puts a limit on how large it can be before life-as-we-know-it becomes impossible due to gravity. It's been theorised that a sensible range would be .4 to 2.35 Earth masses, and that human habitation can stand up to 1.5 G's. Now, here's a maths warning again, but it'll be over quickly, I promise.

The gravity of a body is equal to its mass divided by the square of its radius. Let's work in terms relative to the Earth again to keep things simple. We need a heavy core to help protect against gamma radiation, so we'll make our planet a little heavier than the earth, and a little denser - say 1.8 Earth masses, and a gravity of 1.3. Now, if gravite equals mass divided by radius squared, then radius must equal the square root of mass-times-gravity. So 1.8 * 1.3 = 2.34. The square root of 2.34 is 1.53.

So our planet is 1.53 times the radius of the Earth. The mean radius of the Earth is 6,371Km. 6371*1.53 = 9747, so our planet's radius is just under 10,000 kilometers, giving it a circumference of 61,434 Km.

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Comments (2)

• |2010-07-28 03:38:53 Mujtaba Hussain  - hehe
  Stop listening to Jesse regarding whatever he has read up on planets/terraforming etc :P
  Reply

• |2010-07-28 04:30:27 Sofie
  It's from my own research, actually - I spent a while reading up on astrophysics and various planetary sciences last year.
  Reply

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