Note: What I write here was covered in Poul Anderson’s 1960-something essay, “How to build a planet”, plus other sources. However, it’s worth restating.
This is the first in a series of posts for non-scientists who want to use science in their science fiction stories effectively. I’m going to start with a venerable one, how to design a planet suitable for Earth-like life. There’s a simple rule which makes it very easy to begin, although you have to tweak it (nothing’s ever too simple.) Imagine you have a planet circling a star. Let’s say that the star is L times as bright as our sun, and the planet is D times as far away from the star on average as Earth is from our sun. For example, since Mars is about 140 million miles on average from the sun, and Earth is about 93 million, D for Mars is about 1.5. (I’m rounding) In order for the planet to be roughly the same temperature as the Earth (which is to say, suitable for Earth-like life) there is a relation between D and L: L should be approximately equal to D*D (that is, D-squared.) That is, if D=1, L=1*1=1. If D=2, L=2*2=4; if D=1/3, L=(1/3)*(1/3) = 1/9.
People usually start the other way, by picking a star and then calculating the distance, but doing it this way makes the math something you can do in your head. The reason for this rule is that the light from the star (the main source of energy warming the planet) spreads out in all directions, and decreases as the square of the distance from the star. If you have two planets in orbit around the same star, and one is twice as far from the star as the other, then the illumination of the second is 2*2=4 times less than the illumination that the first one gets. By using the rule that L=D*D, we compensate for that.
This is a good place to start when you’re thinking about where to put your planet, but a bad place to stop. There are many caveats to the simple rule. First of all, it assumes that the orbit is roughly circular – the orbits of many planets aren’t. Secondly, it ignores planetary albedo, the average amount of light which the planet reflects back into space. Earth’s averge albedo is about 30%, meaning that it absorbs about 70% of the light from the sun. Planets with higher albedos need to be moved closer to compensate.
Finally, it ignores the effect of the atmosphere. Earth’s atmosphere traps heat due to a mild greenhouse effect due to a number of gases in the atmosphere; it warms up the planet by about 30 degrees Celsius on average. Venus has a runaway greenhouse effect because its atmosphere is 95% carbon dioxide; its surface temperature is about 450 degrees Celsius, far, far warmer than it would be if position is all we needed to worry about. Take the advice given to any starting writer: pay attention to your story’s atmosphere!
In the second post in this series we’ll discuss how L determines the type of star which the planet circles.