(A table of contents will be available at the end of the series. In the meantime, you can find previous installments in the space building tag and other original posts in the posts by pear tag.)
Part Six: Types of Planets
I wish I could say what we’re going to talk about next is a complete and perfect list of the types of planets that could form in your created systems. Unfortunately, we just don’t know enough about our galaxy and universe to be able to say that for certain. What I can say is that this is our current best guess.
As I said in Part Four, planets form out of the spinning cloud of debris around the star. As they pick up whatever happens to be in their path, they gain mass, and spin together into planets with specific elements creating their cores. From what we’ve observed so far, every planet has some kind of highly dense core, even the gas giants who boast thick gaseous envelopes around their cores. There’s a correlation between a planet’s mass and radius that helps us determine what a planet is most likely to be composed of. Check it out:
R in the above graph means radius, as measured in Earth’s radius, and M is a planet’s mass, as measured by Earth’s mass. The solid lines note homogeneous planets, that is, those that are comprised of a single material: Hydrogen, water, silicates, and iron. The dotted lines denote planets formed from mixes of materials, like Jupiter and Saturn as mixed hydrogen and helium planets, and water worlds composed mostly of water with silicates and iron in various percentages. These types of planets are not considered capable of sustaining life.
One chief characteristic of a habitable planet is that they’re thought to be considered “terrestrial,” and in order to be terrestrial, a planet has to be considered rocky and composed mostly of carbon, silicate, and/or metals. Those planets are marked on the above graph using the red-orange and green colored lines, varying from pure silicate worlds, Earth-like silicate-dominant worlds with iron cores, planets with silicate mantles and more massive iron cores, and those comprised of a pure iron core.
Take a look at at it another way:
These types of planets are classified by composition as:
Gas giants (or gas dwarfs, depending on their mass) – planets composed primarily of hydrogen and helium. Includes Jupiter and Saturn.
Eccentric Jupiters are gas giants with orbits that are not circular. All non-circular orbits are considered “eccentric” and can either be elliptical, parabolic, or hyperbolic
Hot Jupiters are one variety of gas giant that orbits close to the planet’s sun, causing the surface to be very high. These are close enough that their gases are being burned off, leaving a trail of material in their wake. Because of the required circumstances of forming a gas giant, they’re thought to have formed away from the star and migrated inward.
Hot Saturns are also called puffy planets, with densities similar or lower than Saturn but with an extra large radius.
Hot Neptunes are similar to the concept of Hot Jupiters in that they’re similar in mass to Neptune and Uranus and orbit close to their star.
Once these migrated gas giants–the Hot planets–have had their gaseous atmospheres stripped, their cores remain and they become Chthonian planets.
Ice giants – planets composed of substances heavier than hydrogen and helium, including water, methane, and ammonia. Includes Neptune and Uranus.
Terrestrial planets – planets composed of carbon, silicate, and/or metals, including: carbon planets, silicate planets, and iron planets according to what they’re made of. Includes all the inner planets: Mercury, Venus, Earth, and Mars.
The water planets mentioned in the above charts are considered a theoretical type of planet called an ocean planet. There’s also theoretical desert planets and lava planets depending on what their surfaces are comprised of.
Dwarf planets are a step below true “planet” in terms of labeling, and are characterized by the following: They orbit a sun, are not a moon, are nearly round in shape (which requires a certain level of mass), and, most importantly, have not cleared the debris from their orbit. Includes Pluto. (Of course, these days, dwarf planets orbiting our sun beyond Neptune are considered “plutoids,” but that’s just a nicety and wouldn’t apply to any dwarf planets outside our own solar system.)
Some other ways of classifying planets are names that apply specifically to our own solar system:
Exoplanets or extrasolar planets orbit other stars, but not our sun.
Extragalactic planets are outside the Milky Way.
Inferior planets orbit our sun within the Earth’s orbit.
Superior planets orbit out sun outside Earth’s orbit.
Inner planets orbit our sun within the asteroid belt.
Outer planets orbit our sun outside the asteroid belt.
Finally, you can also classify planets based on what they’re orbiting:
Circumbinary planets orbit two stars.
Double planets, or binary planets, are two planetary masses orbiting each other.
Aliquid de Magis 