Gravity
First and foremost, consider the gravity of your planet. Do you want a heavy-gravity planet, a low-gravity world, or an Earth-normal planet? A planet’s gravity will determine a number of other important parameters. The gravity of an object is described by the equation G=M/r^2—that is, gravity is equal to an object’s mass divided by its radius squared. Stephen Dole, who wrote the book Habitable Planets for Man, suggests that a habitable planet’s mass should fall between 0.4 and 2.35 Earth masses. It’s a balancing act; too much gravity will crush any possible life, and too little gravity will make it difficult for the planet to retain a protecting atmosphere. You don’t have to adhere to Dole’s theories entirely, but be careful when playing with the radius, as the mass of your planet is related to its density.
Density, in turn, is affected by the relative proportions of elements that make up your world. Heavier elements such as uranium and gold are denser than lighter elements such as carbon and silicon. Jack Vance used this density concept to create the story “Big Planet,” set on a world fixed in the stone age because heavier metals are extremely rare on the planet. In a similar vein is Charles Harness’ novel, Redworld, which is about a planet which has only 23 of the known elements. Another example of a story set on a heavy-gravity planet is Hal Clement’s Close to Critical, which is about humans attempting to contact the inhabitants of the heavy gravity world Tenebra. Another idea that can be used for heavy-gravity worlds (and has even been hypothesized for the atmosphere of Jupiter) is that of airborne life-forms which glide the atmospheric currents, without experiencing the crushing forces below.
Rotation
The rotation of your planet will determine the length of its day. Though not carved in any mathematical stone, rotation of a planet seems to be somewhat related to mass of the planet. Massive planets such as Jupiter and Saturn have relatively short days when compared to Earth, Venus, and Mars. However, you do have a lot of leeway when determining the rotational period of your planet. Think about the rotation time of your planet and its effects on your planetary ecosystem; it may be kind of neat to have a planet which has a day many hundreds of times longer than Earth’s, but the temperature extremes between the day and night sides of such a planet would be so great that life as we know it would be an unlikely concept. A shorter rotational period would result in a more even temperature distribution.
Dole suggests that for Earth-type life to evolve, the maximum rotational period should be 96 hours. However, not all planets need to be stocked with Earth-type life forms. In fact, science fiction thrives on this idea. Hal Clement’s Mission of Gravity is set on a planet with a rotational period of 18 minutes. This quick rotation creates another consideration: as a planet rotates on its axis, it tends to flatten out. With a rotation time of 24 hours, the flattening out of the Earth is barely noticeable, though there is a slight flattening at the poles and a bulging out at the equator. Mesklin, the planet in Mission of Gravity, is extremely flattened out, so that there is a significant variation in gravity at the poles and the equator; the equator, because it is further from the center of the planet, has less gravity, whereas the poles, which are closer to the planet center, have very high gravity.
Axial Tilt
The 23.5 degree tilt of the Earth on its axis gives us the seasons. If there were no tilt, there would be little or no seasonal variation (the Earth’s elliptical orbit means it’s not always the same distance from the sun, but the difference is relatively minor), and if there were a greater tilt, for example at 90 degrees, one hemisphere would be completely in shadow in the winter and the equator would only receive a feeble light year-round. The only way to create seasons without such oddities of axial tilt is to alter the orbit of your planet, making it extremely eccentric rather than almost circular as Earth’s is.
Satellites
The closest alien world to Earth is the Moon. Early science fiction is often set on and under the surface of the Moon. With this intimate connection to our nearest neighbor, many science fiction writers give their planets moons as well. If one moon inspires our species, maybe many moons will inspire another race. Don’t do it! Many moons will create such complex tidal forces that the planet that you are creating will be impossible to describe plausibly. At least one author has tried to describe the effects of a multiple moon system; Michael Coney’s story “Syzygy” centers around a planet with six moons. When the moons reach a certain alignment, tidal forces and general madness are the result.
Moons must also be set outside what is known as the Roche Limit, which for Earth-type worlds is equal to 2.5 Earth radii from the centre of the planet. If a moon were to wander inside that limit, it would be crushed by the tidal forces of the planet, creating a ring of dust and particles (you could conceivably have a ring around your planet, much like Saturn or Uranus). Your moon or moons will also have phases, as our Moon does. A moon waxes when it circles the side of the planet furthest from its sun and wanes when it circles the side of the planet closest to its sun. The Moon also has its effects on Earth, most significantly on tidal forces. The most dramatic of these tidal effects is seen off of New Brunswick’s eastern coast in the Bay of Fundy.
Magnetic Field
Does your world have a magnetic field? Does it need one? How strong is it? Our planet has a magnetic field, which has allowed life to develop. It protects the Earth from the devastating effects of solar winds and cosmic rays. The source of our magnetic field is unknown, but the most widely accepted theory is the Dynamo Theory postulated by two British physicists, W. Elasser and Sir Edward Bullard. They theorized that the magnetic field is created by the currents flowing in the fluid outer core of the Earth; in other words, mechanical energy is being converted into electromagnetic energy. The magnetic field is not a constant across the planet; it’s greatest at the magnetic poles, and weakest at the magnetic equator. These points are constantly moving over time. In fact, there is evidence that there have been complete reversals in the distribution of the magnetic field on Earth. How can you alter the planet’s magnetic field? Change its rotation rate and its density. A low density, slowly rotating planet will have a weak or no magnetic field—another reason to avoid those tempting slow-rotation worlds as the setting for your story.
Plate Tectonics
Take a cross-section of the Earth and you’ll find that it’s layered like a cake. There is an inner solid core surrounded by an outer liquid core (as noted, it is the motion of the liquid outer core that is believed to generate the magnetic field of Earth). Around the core is the mantle, a layer about 2900 kilometers thick, which is the dominant layer of our planet. Over the mantle is the lithosphere, which is the outermost shell of the Earth. The lithosphere is not one solid piece of material; rather, it is divided into a number of plates. These plates are hardly static—they’re in constant motion, the energy for which is provided by the radioactive decay of material in the Earth’s interior. It is best to think of the motion of the plates as a conveyor belt. The action of convection (the transfer of heat from an area of greater concentration to an area of lesser concentration) causes an upwelling of hot molten material at the mid-oceanic ridges and rift valleys. Following the direction of convection flow, the material then moves along a horizontal pattern. As it approaches the continents, it cools, thereby becoming denser and then sinking back into subduction zones in the mantle.
Plate tectonic activity is also responsible for mountain building (as two plates collide with one another), the creation of oceanic trenches, and earthquakes. Earthquakes are a result of the motion of plates relative to one another; they either collide or slide past one another. It is at plate boundaries, therefore that earthquakes are most common. Here is where, again, it’s important to determine the gravity of your world. A low-gravity world would experience greater amounts of tectonic activity than a high-gravity world, so the process of mountain building and decay would be a lot faster. Therefore, a low-gravity world would appear rougher, with higher mountains, and the high-gravity world would appear smoother, with lower mountains.
Atmosphere
Our planet is made up of 78% nitrogen, 21% oxygen and small percentages of several other gases. If you are creating Earth-like life forms, do not alter this formula greatly. Increasing the oxygen may sound like a good idea (with the thought that if a little is good a lot would be so much better), but unless you want to set your planet ablaze, this is not advisable. If life is to evolve in a high oxygen environment, it can perhaps evolve fire-retardant chemicals, so do not be entirely turned away from a high oxygen environment. Even if altering the oxygen content is not your idea for an interesting planet, there are a number of other atmospheric effects that you can use.