Now that we have a star chosen, we need to decide the properties of the planet. There’s a large variety of planet types to choose from, but we need to make sure of one thing, it has to be in a habitable region in the system, or the Goldilocks Zone. But that’s not all. There’s a lot to consider.
Creating Your Planet
First, let’s figure out where the planet is. It needs to be far enough from the star so that it isn’t incredibly hot, and it has to be close enough so it isn’t completely frozen. This area is the Goldilocks Zone. I wrote about creating a calendar for a hypothetical planet in this post recently, and it uses the same equation.
D = (Ts^2Rs/2Tp^2)((1-a)/(1-τ/2))^1/2
D is the distance of the planet from the star. Ts is the temperature of the star (you can figure this out with a Wikipedia search of your chosen star). Rs is the radius of the star (again, use Wikipedia). The other two variables are what you assign yourself. First is a, or the albedo of the planet. An albedo of 1 means that all light is reflected, such as an ice planet. An albedo of 0 means no light is reflected, which is pretty much impossible. The Earth is around 0.37. You can fudge with that number, but the higher you go, the colder it is, the lower you go, the warmer it is. A forest can be anywhere from 0.09 to 0.18. Grass is 0.25. Desert sand is 0.40. Now, if there were no atmosphere, a desert would be colder than a forest, but that’s not the way it works on Earth. That’s where the other term comes in, τ. This is the optical depth of the atmosphere. If it’s a higher number, it’s hotter, if it’s a lower number, its colder. Basically, a high number implies a stronger greenhouse effect. Venus has a very high optical depth. The Earth’s is about 0.6. Finally, we have Tp, or the temperature of the planet. This is the key number you want to set. If you want an Earth-like world, set it at about 288 Kelvin (or about 15 degrees Celsius). If you want a hot planet, maybe 313 Kelvin is fine. A cold planet could be 273 Kelvin. With these numbers, you can choose a range of orbits for your planet.
Now, putting aside the mathematical part, we have to choose a size for the planet.
The above chart shows what planets have been discovered by Kepler. It’s apparent that Super Earths and Earth-size planets are very common. Obviously, an Earth-sized planet is ideal if you want an Earth-like world to create. But the Super Earths create an interesting possibility. However, there’ll be higher gravity, and you’ll possibly have a thicker atmosphere. But that’s not necessarily true. What you’ll also get with a larger planet is much more land to play with. Imagine the exploration possibilities. There are some other effects of having a Super Earth, though. Mountains will be smaller, ocean waves will be smaller, animals will likely have thicker and stronger legs, people will become stockier. A planet smaller than the Earth will have opposite effects. Mountains are taller, though plate tectonics is less likely. Possibly no earthquakes or volcanic activity. This can be a problem with maintaining an atmosphere that can trap heat, unless there’s a healthy plant cover. The core of the planet will cool more quickly, and a smaller magnetic field could make it a less habitable planet. People would also have problems with maintaining calcium in the bones, and may be unable to return to Earth. But if it’s just a bit smaller, it should be no problem.
The surface of the planet is another thing to consider. How much land will you have? Earth is around 25% land. Do you want more land and less water? Sure, go ahead. It’ll likely be a more arid planet with more extreme summers and winters. Or do you want more water? Then you’ll have a more moderate climate, but expect plenty of tropical cyclones.
But what if your planet is a moon of a gas giant, like Pandora in Avatar? Well, you can go ahead and do that, but if you want scientific accuracy, keep in mind that the tides on the moon will be quite high if it’s close to the planet. This will mean very strong tides, making any coastal regions very dangerous to live in. Also, even if the moon is farther from the planet, it’s likely to be tidally locked. This means one side of the moon will always face the planet. A larger orbit means longer days. You could have a world with incredibly long days and nights. The Earth’s moon has days and nights of 14 days long. For a habitable world, this would create an interesting way of life, I would think.
You can take as much or as little of this advice as you like. Remember, it’s fiction. For a fantasy world, you don’t have to think about this part, unless you want to.
What I did
Ariadne turned out to be in a very favourable position around Beta Comae Berenices. I took a temperature of 288 Kelvin, an albedo of 0.37, and an optical depth of 0.6 to produce a very Earth-like planet. It’s slightly larger than Earth at 1.028 its mass. It has a roughly Earth-like ratio of land to water, consisting of one very large continent and three smaller continents. It has polar ice caps. Thanks to the world being in a rather humid period, there are only two major deserts and a lot of tropical rainforests. It’s a geologically active world with plenty of volcanoes and earthquakes. The volcanism is a bit more than Earth due to its younger age. Although I didn’t touch on moons of planets in the above section, Ariadne has two moons, one closer and larger than our Moon, and the other farther and much smaller. There will be significant tides, of course.
Next time, we’ll take a much closer look at the surface of the world.