Earlier this week, NASA announced that its scientists had discovered a seven-planet solar system orbiting a star named TRAPPIST-1. While “Earth-sized” doesn’t necessarily mean “able to support life,” it’s a good indicator that there are other systems out there that host multiple planets that may be habitable.

For science fiction authors, this is affirmation of a long-standing trope. For decades, authors have imagined solar systems with multiple Earth-like planets that allow for human settlement, providing plenty of space for stories in television and literature. Here are the three big ways writers tell those stories:

Planetary Nation States

The first, major way that inhabited solar systems have been used is as a way to realistically depict a space-faring civilization. Barring some handwaving in the form of faster-than-light-travel, inhabited star systems allow for authors to create a region of space in which they can operate, without having to worry too much about breaking the laws of physics. Characters can board a ship and jet off to the next inhabited planet in the system, which conveniently has its own society and cultures. In many ways, it’s a blown-up version of Earth, only with hundreds of countries.

There are plenty of notable examples from science fiction literature here: K.B. Wager’s The Indranan War trilogy features systems with multiple planets, as do Iain M. Banks’ Culture novel Against A Dark Background, Gene Wolfe’s Book of the Short Sun, several of David Weber’s Honor Harrington novels, and Brian Daley's Fall of the White Ship Avatar. If you want to squint a little, Kameron Hurley’s excellent novel The Stars are Legion is set in a system of artificial, organic worlds that orbit a central, star-like object.

This isn’t necessarily limited to literature. There’s a handful of television shows that use the same model. Battlestar Galactica’s Twelve Colonies are in a complicated system of 12 planets. According to one of the show’s writers, Jane Espenson, the system in question is a double binary system, with the 12 planets orbiting the four stars.

Joss Whedon’s science fiction show Firefly is famously set in a solar system that contains hundreds of habitable planets and moons, in the same arrangement as Battlestar Galactica: several stars orbiting one another, each with numerous planets.

This arrangement allows for a massive sandbox in which to stage a story: plenty of planets and moons for characters to visit, but all within a somewhat plausible way.

Cold War Setting

Creators have also gone beyond just using multiple planets as a convenient setting to drop their story: the multiple planets in the solar system become part of the narrative by creating divisions within society.

A great example of this is Ursula K. Le Guin’s The Dispossessed, set on Anarres and Urras, two worlds orbiting the star Tau Ceti. The two worlds are distinctly different: Urras is the larger of the two, and is split into several nation states, while Anarres is settled by separatists from Urras. The two planets have strictly limited contact, and when one scientist leaves Urras for Anarres, he threatens the delicate balance between the two worlds.

Star Trek presents a number of excellent examples as well. Take Romulus and Remus, the homeworlds of the Romulans and the Remans. The Romulans enslaved the Remans, forcing them to work in mines or as soldiers in their empire. Eventually, the Remans used a clone of Jean-Luc Picard to help overthrow their masters in Star Trek: Nemesis.

There’s yet another example of this in Stargate SG-1’s first season. In “Enigma,” SG-1 discovers the Tollans, a technologically advanced race of humans who lived in a system with another inhabited planet, Serita. Serita ultimately destroyed itself after accepting advanced technology from the Tollans.

What these instances do is go beyond just setting, instead using the multiple planets as a way to highlight inequality between two divisions in society, whether they be economic, political, or racial. Each planet becomes a proxy for the larger issue that each division of society represents.

Our own Solar System, The Expanse, Mars Trilogy, etc.

Finally, there are depictions of our own Solar System. While we don’t presently have colonies or habitats on any of our neighbors, there are plenty of science fiction authors who have imagined terraformed and habitable versions of Venus, Mars, Mercury, or the rocky moons of our gas giants.

The example that first springs to mind is James S.A. Corey’s Expanse series, which depicts a system-wide civilization. This isn’t the only one: Kim Stanley Robinson’s 2312 imagines a terraformed solar system, and his Red Mars novels depict the early stages of the transformation of Mars. Hell, Andy Weir’s The Martian technically qualifies, although Mark Watney hardly sets up a self-sustaining presence.

There’s still other examples that go further back: Edgar Rice Burroughs helped kick off a subgenre known as Planetary Romance with his Barsoom / John Carter novels, which saw planets like Venus and Mars hosting their own vibrant civilizations. Authors such as as Leigh Brackett, C.L. Moore, and Ray Bradbury all tapping into the tradition for their own works.

Many of these stories pick up on the same ideas as other creators: the proximity of the planets are well within our reach, and they allow for the creation of irreconcilable differences between populations to bring about conflict. Most of all, they’re the planets that we’re most familiar with, and it’s easy for creators to figure out what’s needed to set some colonists down on their surfaces.

There’s a long way to go before we discover if any of TRAPPIST-1’s planets could support human life. Even if one or more of them do, they’re too far out there for us to ever practically visit. Fortunately, we’ll have plenty of writers to imagine what adventures might be in store for us if we do make it out there someday.