World Building with the Celestial Objects of our Solar System

by Melanie Marttila
published in Writing

In the past, I’ve addressed topics like artificial intelligence and time travel as both inspiration for story and as integral parts of setting. I think the best stories can work in just about any setting, but science is at the core of the science fiction genre.

This time, I’m going all in on setting. You may be interested in writing near, or near-ish future science fiction about establishing a colony on the moon, or on Mars. You may want to, like James S.A. Corey in The Expanse, go farther afield and establish mining colonies in the asteroid belt or settlements on some of the other potentially habitable moons in the Solar System.

Or, you may want to go all out and imagine new worlds, but what do you base them on? There is enough variety in our Solar System to help you build worlds beyond imagining.

Here are five places, close to home, that will help to get you started.

Inner Planets

The four inner planets are so called because they are the closest to the sun. All four are rocky planets composed of silicates and metals like iron and nickel. Three of the planets have enough atmosphere to create weather and all are potentially capable of supporting life. Except for our planet, what kind of life might exist remains to be seen.

Mercury is the closest, smallest, and densest of the inner planets. While it has an internal magnetic field, it has so little atmosphere that surface temperatures range from 430 degrees Celsius during the day to -187 degrees Celsius at night. Any life on Mercury would have to endure such extremes.

Venus is roughly the size of Earth, but the atmosphere is thick and composed primarily of carbon dioxide. It’s the hottest planet in the Solar System—talk about global warming! The clouds on Venus are made of sulphuric acid and other corrosive compounds. They’ve eaten the probes we’ve sent into the atmosphere and only the largest asteroids survive to make impact. The surface may not have many craters, but it sure has a lot of active volcanoes. Venus may look heavenly from afar but living on the planet would be hellish.

Earth, well, you know about our planet. We live here. Earth is the only one of the inner planets to have liquid water and enough atmosphere to support life as we know it. Our moon influences the tides and our magnetic field helps to protect us from some of the more volatile particles zinging around space.

We call Mars the “red planet” because of its rust-colored surface—literally oxidized iron. It’s a little smaller than Earth and has a magnetic field, but a very thin atmosphere. Mars has some of the most interesting geologic features, like Olympus Mons, the largest mountain in the Solar System at just over 13 miles high. Mars also features Valles Marineris, a canyon 2,500 miles long and up to 3.5 miles deep. NASA is currently examining the polar ice caps to prove definitively that there is liquid water beneath the surface—that might mean life!

Gas Giants

As astronomers learn more about the planets around other stars, they see that planets like our gas giants often orbit much closer to their stars than Jupiter, Saturn, Uranus, and Neptune do. Also, there are a lot of gas giants out there. This may be because they’re easier to find, but that’s what we’ve learned so far. It’s theorized that the giants have rocky, metallic cores, like the inner planets, but that during the Solar System’s formation, they were able to siphon off gasses from the sun’s formation rather than having most of those gases blown away by the same process (like the inner planets).

Jupiter is eleven times the size of Earth and is composed of hydrogen and helium. It’s so big that the hydrogen is condensed into liquid metallic hydrogen, which produces Jupiter’s massive magnetic field. Jupiter’s magnetic and gravitational forces are so strong, they deform its moons. The atmosphere is not only hydrogen and helium, but also ammonia and methane. Jupiter features the Solar System’s largest storm, the Great Red Spot, which has raged for most of Jupiter’s existence and could engulf the Earth.

Saturn is Jupiter’s partner, and is slightly smaller, at only nine times Earth’s size. The composition of the planet and atmosphere is similar to Jupiter’s, but Saturn’s distinguishing feature is its rings. Astronomers aren’t quite sure how they formed, yet, but they sure are beautiful collections of ice and rock.

Just as Jupiter and Saturn are grouped together, so are Uranus and Neptune. Both of the outer giants are about four times the size of Earth and both have atmospheres consisting of hydrogen, helium, and methane. Uranus is distinguished because it’s tilted on its side, relative to the rest of the planets, and rotates in the opposite direction.

What would it take to survive the intense gravity, magnetic fields, and atmosphere of the gas giants? What kind of life might be found when we finally penetrate the dense and volatile atmosphere? Could the gasses of the giants be harvested for use as fuel or for weaponry?


The Solar System has a plethora of moons, but not all the planets have them. Mercury and Venus don’t have any. Earth has one, and Mars two. When we get out to Jupiter, though, the biggest planet, it has 67 moons, not all of which have been confirmed and named. Saturn has 62 moons. Uranus has 27 moons and Neptune 14.

A good portion of the moons are rocky and dormant, like our moon. Some, like Io, are volcanically active, spewing molten silicate out into space. There are icy moons like Europa and Enceladus, which scientists are investigating for signs of life. Europa’s sub-surface sea may contain more water than Earth and Enceladus’s geysers show evidence of water, food (in the form of methane) and heat, all the ingredients for life. Neptune’s Triton has nitrogen ice as its surface.

Ganymede, the biggest moon in the Solar System, is bigger than Mercury, and it’s the only moon that generates its own magnetic field. Some of the bigger moons may be suitable for establishing research stations or even colonization. They might become waystations en route to other destinations beyond the Solar System. The Star Wars movies have famously used moons as locations for the Rebellion’s various bases.

The Asteroid and Kuiper Belts

The Asteroid Belt sits between the orbits of Mars and Jupiter and encircles the sun in its own orbit. It’s what was left after the formation of the planets in our solar system. Roughly half the mass of the belt is in the four largest asteroids, Ceres, Pallas, Vesta and Hygiea, but the objects range in size down to dust. There are three kinds of asteroids: carbonaceous, silicate, and metal-rich.

Though the movies make asteroid fields look like potentially fatal obstacle courses, the objects in the Asteroid Belt are spaced fairly far apart and all the spacecraft that have passed through it have done so intact. They still collide occasionally and may form clusters if their gravitational fields are strong enough.

The Kuiper Belt is a disc-shaped region beyond the orbit of Neptune that extends 30 to 55 times the distance from the sun to Earth (an astronomical unit, or AU). This vast region is home to Pluto, hundreds of thousands of icy bodies, and over a trillion comets, several of which orbit the sun.

Asteroids and comets can be both hazards to life on colonies, Earth, and spacecraft in transit, as well as potential resources.

Dwarf Planets, Planetesimals, and Other Celestial Objects

Pluto, way out in the Kuiper Belt, was demoted from planet to dwarf planet in 2006. Dwarf planets, also called minor planets, or planetoids, are large asteroids, some of which have their own moons, like Pluto’s Charon. Other dwarf planets include Ceres, Haumea, Makemake, and Eris. What distinguishes a dwarf planet from other celestial objects is their ability to maintain a spherical shape. Dwarf planets, like their large asteroid cousins, could be suitable for research or military bases, space stations, or even small colonies.

Planetesimals are accretions of smaller asteroids and dust. The theory is that these objects were on their way to becoming planets before the process was halted, somehow. Phobos and Deimos, Mars’s moons, are thought to be planetesimals, as well as several of Jupiter’s moons.

There are other groups of asteroids, the Trojans, which pass through the orbits of various planets. The Oort Cloud, some 186 billion miles from the sun, is a vast group of icy bodies and the source of long-range comets, many of which won’t pass Earth in thousands, or even millions, of years.

Taking it to the page

Our Solar System is home to a variety of celestial objects. Any fictional system should have a similar composition, or, there should be plausible reasons for the differences. A lot of world building never makes it into the novel, but knowing the details of your fictional universe will allow you to insert telling details and achieve verisimilitude.

It’s much like creating rounded characters—the author knows every character’s back story, but only a few key details will be used to evoke the three-dimensional presence of a real person on the page. Setting can be a character in itself or, if seen through the eyes of your protagonist or other point-of-view characters, could reveal key pieces of character or plot development.

I’ve included a list of resources at the end of this column to deepen your research.

Until next time, keep speculating and see where it leads you!


Melanie Marttila creates worlds from whole cloth. She’s a dreamsinger, an ink alchemist, and an unabashed learning mutt. Her speculative short fiction has appeared in Bastion Science Fiction Magazine, On Spec Magazine, and Sudbury Ink. She lives and writes in Sudbury, Ontario, Canada, where she spends her days working as a corporate trainer. She blogs at and you can find her on Facebook and Twitter.

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