Planets and Moons

In this post we’ll discuss the creation of planets within a solar system, establishing their orbits, determining habitable zones, and looking at why Jupiter is the guardian of our solar system.

Hey everyone, my name is Matthew, at least while I’m still in orbit, and this video is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time we created the stars of our solar system, Flavus and Rufus, which gave us all the information we’ll need to move forward with making the rest of the planetary system.

For today’s discussion, we’ll be placing our habitable planet into orbit, as well as building it some neighbours that it will share the solar system with, from small terrestrial cousins to huge gas giants that will keep all the other planets safe during their stay.

Let’s start by placing Locus into orbit within the habitable zone of the solar system. The Circumstellar Habitable Zone, or more commonly just, ‘the habitable zone’, is the range around a star (or stars) that is suitable for a planetary surface to support liquid water, assuming sufficient atmospheric pressure. Due to the importance of liquid water for life, it is generally translated that being within a solar system’s habitable zone is a requirement for life to emerge and survive.

In our last discussion where we built Flavus and Rufus, our two stars, we established that the habitable zone of the solar system is between 1.199 AU and 1.727 AU. Locus is going to have to settle within this habitable zone to sustain life as we know it. Let’s have Locus settle in at an average distance of 1.357 AU, with an eccentricity of 0.109. I haven’t just pulled this eccentricity number from nowhere, but I’ll explain more on that in just a little bit.

From here, measured in AU, we can work out the maximum and minimum distance that Locus will be from Flavus and Rufus, making sure that both values still fall within the habitable zone of the solar system. We can also work out the orbital period compared to earth, and therefore how long a year will be in earth days, what its orbital velocity is and therefore it’s orbital velocity in kilometres per second. Perhaps most importantly, we can also determine how much solar heat it receives compared to earth, which tells us what it’s average surface temperature will be. Finally, by giving Locus a rotational speed, we can establish its day length in hours, and then convert all of our previously given values in earth days to Locus days!

Importantly, we can see that our average temperature on Celsius is around 10 degrees, which, while a little colder than earth at 14 degrees, is still well within the safe range for life to be supported. We can also see that the year length is about 1.3 times the length of an earth year, which while not extreme does pose some challenges for seasonal life. To compensate, let’s give the planet a lower axial tilt to make the seasonal variations less extreme. Between 1 and 89 degrees, the lower the axial tilt, the less variation in seasons, and the higher the tilt, the greater the variation in seasons. The opposite is true for tilts between 91 and 179 degrees. For reference, earth has an axial tilt of 23.4 degrees, so let’s give Locus an axial tilt of just under 15 degrees. Therefore, while the seasons will be longer than earth, they won’t be as severe.

Now, remember I said that I didn’t just pull Locus’ eccentricity number out of nowhere? Well, it turns out there’s an equation for what a planet’s orbital eccentricity will be based on the number of planets within the solar system. You can see based on this table that the eccentricity we’ve chosen for our planet is the expected eccentricity for a system with 4 planets. Which means we’ve got 3 more planets to make! Thankfully, the process is very straightforward, and we can use the same process for all our planets. But where to put these planets? Well, considering Locus is a rogue planet, we want them to be reasonably far from where Locus will settle, especially any bigger planets, to avoid their gravity affecting Locus while it’s settling into the solar system. So, let’s put the bigger two in the outer of the system, and the third smaller planet in the inner solar system as Locus’ immediate neighbour.

To begin with, it’s strongly expected that a habitable planet will have a gas giant present within its solar system, so that most minor celestial objects will get caught within its gravitational pull, keeping the habitable planet safer. In our real-life solar system, these gas giants are Jupiter and Saturn. Let’s call our fictional gas giant Vigil, Latin for ‘guard’ or ‘sentinel’. The process for creating Vigil will be almost identical to the process used for creating Locus, except that we’re creating it with values compared to Jupiter, instead of values compared to Earth. Let’s make Vigil 2.719 the mass of Jupiter, and 0.976 times it’s radius. We also want to place it just past the frostline of the solar system, between 1 and 1.5 AU away. Let’s put it at a distance of 7.205AU from Flavus and Rufus, which is 1.107AU past the frostline. It’s eccentricity (and the eccentricity of all planets in the system) are likely to be very similar, so for simplicity let’s keep it exactly the same as Locus at 0.109. From here, we can determine all the other values that we could for Locus, though remembering that values are relative to Jupiter rather than Earth. Specifically, let’s pay attention to the surface temperature, which is 201 kelvin, or -72 degrees Celsius.

Based on the Sudarsky Classification system, which organises gas giants based on their temperatures, we can identify Vigil as a class 2 gas giant, which exist below about 250 kelvin (-23 Celsius), are dominated by water vapour clouds. This will give Vigil a whitish grey appearance, with wind bands similar to Jupiter.

Now that we’ve established our gas giant, we can determine all other stable orbits within the solar system. The equation is very easy: for planets further out than Vigil, we simply take its orbital distance and multiply it by around 1.5, and then we can take that new number and multiply THAT by 1.5, and so on until we reach the edge of the solar system. The reverse is true for planets further in than Vigil; we take its orbital distance and divide it by around 1.5, and then that number, and so on. We can see by doing this that Locus’ orbital distance is reasonably close to the value we’ve gotten, which means its orbit is likely to be stable.

Let’s quickly make two more planets, one icy terrestrial, and one icy gas dwarf. Both are going to have their values compared to earth, which we’ll choose now and... voila! Let’s call the small icy terrestrial Catula, meaning “puppy” or “kitten”, and the icy gas dwarf Solus, meaning “lonely”, given how far away it is from every other planet in the system.

The solar system is looking a little more complete now! Note that because we have two stars in our system, less planets are likely to form because most of the material in the area is taken up by the stars. There aren’t any hard rules to this, but generally speaking it’s unlikely that a single star would have more than 8-10 planets, a binary system would likely have no more than 5, a trinary no more than 3, and a quaternary probably maxes out at a single planet. Any more stars than this and the chances of a planet existing at all in a stable orbit is very unlikely, not even to mention the unlikelihood that a system with more than four stars would exist in a stable manner in the first place. But for our binary system, three native planets and one adopted planet works just fine.

Now, moons! We could really go overboard and establish moons for every single planet, and there’s a pretty high likelihood in our system that every planet would have moons. Given that the likelihood for moons increases the bigger a planet gets and the further out into the system we go, every planet except maybe Catula is likely to have at least one moon, and Vigil is likely to have MANY moons. In our real-life solar system, Jupiter has 79 moons and Saturn has 82! And Vigil doesn’t have to compete with another gas giant for its moons, so it will probably end up with a huge amount. Instead of painstakingly creating every single moon, let’s focus on Locus and instead say that Catula has one moon, Vigil has 217 moons, and Solus has 13 moons.

For Locus, a large moon is important, because without a moon there aren’t really any significant tides. With no tides, there’s no intertidal zone which makes any life form’s transition from water to land exceptionally difficult. Large moons also stabilise the tilt of a planet, so without one the axis we gave Locus of around 15 would fluctuate heavily. For perspective, earth’s tilt oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle, a 2.4-degree variation at its extremes. By comparison, Mars does not have a large moon, and its axial tilt varies by tens of degrees on a 100,000-year cycle, which cause seasons on Mars to be wildly inconsistent across geological time periods.

Before we make the moon, we need to determine the hill sphere of Locus, which is the space around a planet where the gravity of the planet will keep something in orbit around it. The output of this equation is measured in earth radii.

Now that we’ve got that value, let’s give Locus a single major moon to make sure Locus’ axial tilt will be stabilised and start to gather some information about it. Major moons are generally located within the inner half of a planet’s hill sphere, so let’s put it at just under 80 earth radii away from Locus. For reference, earth’s moon is 60 earth radii away from earth.

While a moon can technically follow the same mathematical creation process as a planet, there’s an extra step we can easily take due to the numbers involved being much smaller. Let’s determine what the moon is going to be composed of, and then determine its density from that. I really like the idea of a coloured moon, so I’ve googled the composition of Sodalite, a compound that is royal blue in colour. Let’s give the moon enough iron to form a core, and then split its remaining makeup to form a dense layer of sodalite visible on the surface, with some spare in materials like silicon. Simply put, so long as there isn’t an abundance of a material less dense than sodalite, then the denser materials are likely to settle deeper below the surface, giving the moon a striking blue colour to those observing it. In fact, lets call this moon Caeruleus, meaning “blue” in Latin. From here we can determine all the other relevant information about the moon, paying particular attention to its orbital length in days, and its perspective size. Caeruleus is going to orbit Locus every 29.7 Locus days, and it’s tidally locked, meaning that the same side of Caeruleus is going to be always facing Locus, just like our moon and earth. Its perspective size is how big it appears compared to how big the moon appears on earth, which we can see is around 20% larger.

Before we finish though, now that we’ve got a blue moon in orbit, I really love the idea of a complimenting red moon. But it’s unlikely that a terrestrial planet like Locus would have two major moons, in fact it’s not super common for terrestrial planets to even have one major moon, though it is needed for life as we know it. Let’s give Locus a second moon, but much, much smaller than Caeruleus. As a minor moon, it’s likely to not be a spherical shape but rather quite irregular, so we can get creative with how this red moon looks. To achieve the red look, let’s give it a composition of mostly iron, so that iron oxide or rust can form. This red is perfect for what I have in mind, because it's a very dark almost sinister red. Let’s even call this moon Malus, meaning “evil”. But it’s SO much smaller than Caeruleus that if we put it at the same distance, it would be barely visible from Locus, so instead let’s have it orbit much closer to Locus. At around 41 earth radii away, this has Malus orbiting at exactly half the distance that Caeruleus is, but by perspective it’s around 0.8 times the perceived size of our moon in real-life. This is perfect for what I had in mind, and these two opposingly coloured moons will dance independently of each other around Locus, giving spectacular worldbuilding potential for cultures down the line.

So, to recap, Locus has been pulled into a solar system by the binary stars Flavus and Rufus. It’s settled into a stable orbit with a year slightly longer than that of earth, and a temperature slightly colder than earth’s but only by 4 degrees. We’ve created the gas giant Vigil, as well as two other icy planets Catula and Solus. And finally, we’ve ensured that Locus has a major moon called Caeruleus and thrown in a minor moon called Malus for flavour.

Join me next time where Locus is going to change from an inhospitable hellscape into a springboard for life, establishing its early geography. And until next time… stay awesome!