Featured Essays astronomy No Two Worlds Are the Same: Planetary Diversity in SF There is no cookie cutter when it comes to celestial bodies… By James Davis Nicoll | Published on August 21, 2025 Credit: NASA/JPL-Caltech Comment 0 Share New Share Credit: NASA/JPL-Caltech I was poking through my old reviews of Reginald Bretnor symposia and encountered this firmly worded assertion: “There’s no reason fictional planets should be any less diverse than fictional characters.” I didn’t mean “should” in the sense of some moral directive—rather, it is an expression of doubt that any fictional planet plausibly comes from the same cookie cutter, any more than humans do. Consider the Galilean moons of Jupiter. At a first glance, they do seem pretty similar. They range from about 3100 km to 5300 km diameter, large enough to be considered planets if they didn’t orbit another world. Three of them are icy worlds, and all of them have surface conditions that would kill an unprotected human1. On a closer look, the Galilean moons have as many differences as they do similarities. Io is the most striking example. Because of the tidal flexing to which it has been subjected, Io has many active volcanos. If it had ever had as much ice as Europa, Callisto, and Ganymede, Io would have been rendered down to a silicate moon2. However, the other three moons also display great disparities. In fact, it is trivial to tell them apart with a glance3: The Galilean moons (Click to enlarge) Why would this be? Each moon accreted from slightly different materials, each has a different mass, a different orbit (and thus different orbital speed), the degree of tidal flexing over billions of years has been different, and so on. Basically, they had their own individual geological histories, resulting in a different geological present. We see much the same situation around the Solar System. It’s possible to lump planets into groups (the gas giants, the ice giants, minor debris) but within sets, the differences are obvious. Nobody would confuse Saturn with Jupiter, or Mars with Mercury4. Even if we suppose that a focus on shirt-sleeve-habitable worlds narrowed down fictional settings to unusually Earth-like worlds (such as in Asimov’s Empire, where only a few million of the hundred billion star systems in the Milky Way were colonized), it’s intuitively obvious that “extremely Earth-like” covers a lot ground. Earth, for example, is as Earth-like a planet as it is possible for a planet to be. Global conditions on Earth have varied from the icy cold of the Cryogenian to the sweltering heat of the Paleocene–Eocene Thermal Maximum, to pick one obvious parameter. Even on specific planets in a single specific period, there’s considerable variation from location to location. You might think “Well, Calgary and Point Nemo are both on Earth, which means they’ve experienced a far more similar sequence of events than, say, Calgary and Mons Olympus.” In fact, even the least observant of tourists would spot dramatic differences between Calgary and Point Nemo as soon as they stepped out of their vehicles. What we see in on Earth in particular and the Solar System in general should be repeated on a vaster and more detailed scale throughout the Milky Way (and universe, for that matter). Each star has its own unique parameters (mass, age, original composition, orbital path through the galaxy and so on). Each world orbiting those stars will likewise. Like snowflakes, all of the worlds are subjected to the same essential forces, but the outcomes vary wildly. And why should all of that matter to a hard-working SF author whose only desire is to sell stories for money? To begin with, because taking into account that every planet is a wonderfully varied body with its own unique history might well suggest interesting story ideas. Moreover, diverse worlds provide a more textured reading experience than planets that might as well be a series of Paramount backlots. Finally, as Poul Anderson and Hal Clement’s careers made clear, detailed worldbuilding can be very fun, at least for a certain kind of author5. Maybe you aren’t one, but you probably can find one who will be thrilled to put their expertise at your disposal. Nobody would write a mystery set in “City” and nobody would write a romance focused on “Person A” and “Person B.” So why settle for bland, interchangeable planets?[end-mark] Much to the disappointment of Stanley G. Weinbaum fans. ︎I did try to determine if anyone has measured the hydrogen to deuterium ratios on Io because that would give a hint as to how much water was lost. No luck. ︎On that note, if sci fi movie makers who want to provide their alien planets with several large moons, it would awesome if it wasn’t clear that the SFX folks had just photoshopped two identical images of Earth’s moon into the sky. ︎Leaving aside the Babylon 5 tie-in novel author who knew that Mars was somehow different from Earth and guessed that it was much hotter. ︎And some tabletop roleplaying fans. Here, let me show you hundreds of Traveller subsectors I rolled up and will never use… ︎The post No Two Worlds Are the Same: Planetary Diversity in SF appeared first on Reactor.